text:
6.1 ControllersA controller handles requests and creates or prepares the response and is request-scoped. In other words a new instance is created for each request. A controller can generate the response or delegate to a view. To create a controller simply create a class whose name ends with
Controller and place it within the
grails-app/controllers directory.The default URL Mapping setup ensures that the first part of your controller name is mapped to a URI and each action defined within your controller maps to URI within the controller name URI.
6.1.1 Understanding Controllers and Actions
Creating a controllerControllers can be created with the create-controller target. For example try running the following command from the root of a Grails project:
The command will result in the creation of a controller at the location
grails create-controller book
class BookController { … }
BookController by default maps to the /book URI (relative to your application root).
create-controller command is merely for convenience and you can just as easily create controllers using your favorite text editor or IDE
Creating ActionsA controller can have multiple properties that are each assigned a block of code. Each of these properties maps to a URI:
This example maps to the
class BookController { def list = { // do controller logic // create model return model } }
/book/list URI by default thanks to the property being named
The Default ActionA controller has the concept of a default URI that maps to the root URI of the controller. By default the default URI in this case is
/book. The default URI is dictated by the following rules:
- If only one action is present the default URI for a controller maps to that action.
- If you define an
indexaction which is the action that handles requests when no action is specified in the URI
- Alternatively you can set it explicitly with the
static defaultAction = "list"
6.1.2 Controllers and Scopes
Available ScopesScopes are essentially hash like objects that allow you to store variables. The following scopes are available to controllers:
- servletContext - Also known as application scope, this scope allows you to share state across the entire web application. The servletContext is an instance of javax.servlet.ServletContext
- session - The session allows associating state with a given user and typically uses cookies to associate a session with a client. The session object is an instance of HttpSession
- request - The request object allows the storage of objects for the current request only. The request object is an instance of HttpServletRequest
- params - Mutable map of incoming request (CGI) parameters
- flash - See below.
Accessing ScopesScopes can be accessed using the variable names above in combination with Groovy's array index operator even on classes provided by the Servlet API such as the HttpServletRequest:
You can even access values within scopes using the de-reference operator making the syntax even clearer:
class BookController { def find = { def findBy = params["findBy"] def appContext = request["foo"] def loggedUser = session["logged_user"] } }
This is one of the ways that Grails unifies access to the different scopes.
class BookController { def find = { def findBy = params.findBy def appContext = request.foo def loggedUser = session.logged_user } }
Using Flash ScopeGrails supports the concept of flash scope is a temporary store for attributes which need to be available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirection, for example:
def delete = { def b = Book.get( params.id ) if(!b) { flash.message = "User not found for id ${params.id}" redirect(action:list) } … // remaining code }
6.1.3 Models and Views
Returning the ModelA model is essentially a map that the view uses when rendering. The keys within that map translate to variable names accessible by the view. There are a couple of ways to return a model. First, you can explicitly return a map instance:
def show = { [ book : Book.get( params.id ) ] }
The above does not reflect what you should use with the scaffolding views - see the scaffolding section for more details.If no explicit model is returned the controller's properties will be used as the model thus allowing you to write code like this:
class BookController { List books List authors def list = { books = Book.list() authors = Author.list() } }
This is possible due to the fact that controllers are prototype scoped. In other words a new controller is created for each request. Otherwise code such as the above would not be thread safe.In the above example the
books and
authors properties will be available in the view.A more advanced approach is to return an instance of the Spring ModelAndView class:
One thing to bear in mind is that certain variable names can not be used in your model:
import org.springframework.web.servlet.ModelAndViewdef index = { def favoriteBooks = … // get some books just for the index page, perhaps your favorites // forward to the list view to show them return new ModelAndView("/book/list", [ bookList : favoriteBooks ]) }
Selecting the ViewIn both of the previous two examples there was no code that specified which view to render. So how does Grails know which view to pick? The answer lies in the conventions. For the action:
Grails will automatically look for a view at the location
class BookController { def show = { [ book : Book.get( params.id ) ] } }
grails-app/views/book/show.gsp (actually Grails will try to look for a JSP first, as Grails can equally be used with JSP).If you wish to render another view, then the render method there to help:
In this case Grails will attempt to render a view at the location
def show = {
def map = [ book : Book.get( params.id ) ]
render(view:"display", model:map)
grails-app/views/book/display.gsp. Notice that Grails automatically qualifies the view location with the
book folder of the
grails-app/views directory. This is convenient, but if you have some shared views you need to access instead use:
In this case Grails will attempt to render a view at the location
def show = {
def map = [ book : Book.get( params.id ) ]
render(view:"/shared/display", model:map)
Rendering a ResponseSometimes its easier (typically with Ajax applications) to render snippets of text or code to the response directly from the controller. For this, the highly flexible
render method can be used:
The above code writes the text "Hello World!" to the response, other examples include:
render "Hello World!"
If you plan on using Groovy's MarkupBuilder to generate html for use with the render method becareful of naming clashes between html elements and Grails tags. e.g.
// write some markup render { for(b in books) { div(id:b.id, b.title) } } // render a specific view render(view:'show') // render a template for each item in a collection render(template:'book_template', collection:Book.list()) // render some text with encoding and content type render(text:"<xml>some xml</xml>",contentType:"text/xml",encoding:"UTF-8")
Will actually call the form tag (which will return some text that will be ignored by the MarkupBuilder). To correctly output a <form> elemement, use the following:
def login = { StringWriter w = new StringWriter() def builder = new groovy.xml.MarkupBuilder(w) builder.html{ head{ title 'Log in' } body{ h1 'Hello' form{ } } } def html = w.toString() render html }
def login = { // … body{ h1 'Hello' builder.form{ } } // … }
6.1.4 Redirects and Chaining
RedirectsActions can be redirected using the redirect method present in all controllers:
Internally the redirect method uses the HttpServletResonse object's
class OverviewController {
def login = {} def find = {
sendRedirect method.The
redirect method expects either:
- Another closure within the same controller class:
// Call the login action within the same class redirect(action:login)
- The name of a controller and action:
// Also redirects to the index action in the home controller redirect(controller:'home',action:'index')
- A URI for a resource relative the application context path:
// Redirect to an explicit URI
- Or a full URL:
Parameters can be optionally passed from one action to the next using the
// Redirect to a URL
params argument of the method:
These parameters are made available through the params dynamic property that also accesses request parameters. If a parameter is specified with the same name as a request parameter the request parameter is overridden and the controller parameter used.Since the
redirect(action:myaction, params:[myparam:"myvalue"])
params object is also a map, you can use it to pass the current request parameters from one action to the next:
Finally, you can also include a fragment in the target URI:
redirect(action:"next", params:params)
will (depending on the URL mappings) redirect to something like "/myapp/test/show#profile".
redirect(controller: "test", action: "show", fragment: "profile")
ChainingActions can also be chained. Chaining allows the model to be retained from one action to the next. For example calling the
first action in the below action:
Results in the model:
class ExampleChainController { def first = { chain(action:second,model:[one:1]) } def second = { chain(action:third,model:[two:2]) } def third = { [three:3]) } }
The model can be accessed in subsequent controller actions in the chain via the
[one:1, two:2, three:3]
chainModel map. This dynamic property only exists in actions following the call to the
chain method:
Like the
class ChainController { def nextInChain = { def model = chainModel.myModel … } }
redirect method you can also pass parameters to the
chain method:
chain(action:"action1", model:[one:1], params:[myparam:"param1"])
6.1.5 Controller InterceptorsOften it is useful to intercept processing based on either request, session or application state. This can be achieved via action interceptors. There are currently 2 types of interceptors: before and after.
If your interceptor is likely to apply to more than one controller, you are almost certainly better off writing a Filter. Filters can be applied to multiple controllers or URIs, without the need to change the logic of each controller
Before InterceptionThe
beforeInterceptor intercepts processing before the action is executed. If it returns
false then the intercepted action will not be executed. The interceptor can be defined for all actions in a controller as follows:
The above is declared inside the body of the controller definition. It will be executed before all actions and does not interfere with processing. A common use case is however for authentication:
def beforeInterceptor = {
println "Tracing action ${actionUri}"
The above code defines a method called
def beforeInterceptor = [action:this.&auth,except:'login'] // defined as a regular method so its private def auth() { if(!session.user) { redirect(action:'login') return false } } def login = { // display login page }
auth. A method is used so that it is not exposed as an action to the outside world (i.e. it is private). The
beforeInterceptor then defines an interceptor that is used on all actions 'except' the login action and is told to execute the 'auth' method. The 'auth' method is referenced using Groovy's method pointer syntax, within the method itself it detects whether there is a user in the session otherwise it redirects to the login action and returns false, instruction the intercepted action not to be processed.
After InterceptionTo define an interceptor that is executed after an action use the
afterInterceptor property:
The after interceptor takes the resulting model as an argument and can hence perform post manipulation of the model or response.An after interceptor may also modify the Spring MVC ModelAndView object prior to rendering. In this case, the above example becomes:
def afterInterceptor = { model ->
println "Tracing action ${actionUri}"
This allows the view to be changed based on the model returned by the current action. Note that the
def afterInterceptor = { model, modelAndView -> println "Current view is ${modelAndView.viewName}" if(model.someVar) modelAndView.viewName = "/mycontroller/someotherview" println "View is now ${modelAndView.viewName}" }
modelAndView may be
null if the action being intercepted called redirect or render.
Interception ConditionsRails users will be familiar with the authentication example and how the 'except' condition was used when executing the interceptor (interceptors are called 'filters' in Rails, this terminology conflicts with the servlet filter terminology in Java land):
This executes the interceptor for all actions except the specified action. A list of actions can also be defined as follows:
def beforeInterceptor = [action:this.&auth,except:'login']
The other supported condition is 'only', this executes the interceptor for only the specified actions:
def beforeInterceptor = [action:this.&auth,except:['login','register']]
def beforeInterceptor = [action:this.&auth,only:['secure']]
6.1.6 Data BindingData binding is the act of "binding" incoming request parameters onto the properties of an object or an entire graph of objects. Data binding should deal with all necessary type conversion since request parameters, which are typically delivered via a form submission, are always strings whilst the properties of a Groovy or Java object may well not be.Grails uses Spring's underlying data binding capability to perform data binding.
Binding Request Data to the ModelThere are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' implicit constructor:
The data binding happens within the code
def save = {
def b = new Book(params)
new Book(params). By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:
Then the
title and
author request parameters would automatically get set on the domain class. If you need to perform data binding onto an existing instance then you can use the properties property:
This has exactly the same effect as using the implicit constructor.
def save = { def b = Book.get(params.id) b.properties = params b.save() }
Data binding and Single-ended AssociationsIf you have a
one-to-one or
many-to-one association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:
Grails will automatically detect the
.id suffix on the request parameter and look-up the
Author instance for the given id when doing data binding such as:
An association property can be set to
def b = new Book(params)
null by passing the literal
String "null". For example:
Data Binding and Many-ended AssociationsIf you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type.If you have a
Set based association (default for a
hasMany) then the simplest way to populate an association is to simply send a list of identifiers. For example consider the usage of
<g:select> below:
This produces a select box that allows you to select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the
<g:select name="books"
size="5" multiple="yes" optionKey="id"
value="${author?.books}" />
books association.However, if you have a scenario where you want to update the properties of the associated objects the this technique won't work. Instead you have to use the subscript operator:
However, with
<g:textField name="books[0].title" value="the Stand" /> <g:textField name="books[1].title" value="the Shining" />
Set based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a
Set has no concept of order, so although we're referring to
books0 and
books1 it is not guaranteed that the order of the association will be correct on the server side unless you apply some explicit sorting yourself.This is not a problem if you use
List based associations, since a
List has a defined order and an index you can refer to. This is also true of
Map based associations.Note also that if the association you are binding to has a size of 2 and you refer to an element that is outside the size of association:
Then Grails will automatically create a new instance for you at the defined position. If you "skipped" a few elements in the middle:
<g:textField name="books[0].title" value="the Stand" /> <g:textField name="books[1].title" value="the Shining" /> <g:textField name="books[2].title" value="Red Madder" />
Then Grails will automatically create instances in between. For example in the above case Grails will create 4 additional instances if the association being bound had a size of 2.You can bind existing instances of the associated type to a
<g:textField name="books[0].title" value="the Stand" /> <g:textField name="books[1].title" value="the Shining" /> <g:textField name="books[5].title" value="Red Madder" />
List using the same
.id syntax as you would use with a single-ended association. For example:
Would allow individual entries in the
<g:select name="books[0].id" from="${Book.list()}" value="${author?.books[0]?.id}" /> <g:select name="books[1].id" from="${Book.list()}" value="${author?.books[1]?.id}" /> <g:select name="books[2].id" from="${Book.list()}" value="${author?.books[2]?.id}" />
books List to be selected separately.Entries at particular indexes can be removed in the same way too. For example:
Will render a select box that will remove the association at
<g:select name="books[0].id" from="${Book.list()}" value="${author?.books[0]?.id}" noSelection="['null': '']"/>
books0 if the empty option is chosen.Binding to a
Map property works in exactly the same way except that the list index in the parameter name is replaced by the map key:
This would bind the selected image into the
<g:select name="images[cover].id" from="${Image.list()}" value="${book?.images[cover]?.id}" noSelection="['null': '']"/>
Map property
images under a key of
Data binding with Multiple domain classesIt is possible to bind data to multiple domain objects from the params object.For example so you have an incoming request to:
You'll notice the difference with the above request is that each parameter has a prefix such as
author. or
book. which is used to isolate which parameters belong to which type. Grails'
params object is like a multi-dimensional hash and you can index into to isolate only a subset of the parameters to bind.
Notice how we use the prefix before the first dot of the
def b = new Book(params['book'])
book.title parameter to isolate only parameters below this level to bind. We could do the same with an
Author domain class:
def a = new Author(params['author'])
Data binding and type conversion errorsSometimes when performing data binding it is not possible to convert a particular String into a particular target type. What you get is a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. Take this example:
Here we have a domain class
class Book { … URL publisherURL }
Book that uses the Java concrete type
java.net.URL to represent URLs. Now say we had an incoming request such as:
In this case it is not possible to bind the string
a-bad-url to the
publisherURL property os a type mismatch error occurs. You can check for these like this:
Although we have not yet covered error codes (for more information see the section on Validation), for type conversion errors you would want a message to use for the error inside the grails-app/i18n/messages.properties file. You can use a generic error message handler such as:
def b = new Book(params)if(b.hasErrors()) { println "The value ${b.errors.getFieldError('publisherURL').rejectedValue} is not a valid URL!" }
Or a more specific one:
typeMismatch.java.net.URL=The field {0} is not a valid URL
typeMismatch.Book.publisherURL=The publisher URL you specified is not a valid URL
Data Binding and Security concernsWhen batch updating properties from request parameters you need to be careful not to allow clients to bind malicious data to domain classes that end up being persisted to the database. You can limit what properties are bound to a given domain class using the subscript operator:
In this case only the
def p = Person.get(1)p.properties['firstName','lastName'] = params
firstName and
lastName properties will be bound.Another way to do this is instead of using domain classes as the target of data binding you could use Command Objects. Alternatively there is also the flexible bindData method.The
bindData method allows the same data binding capability, but to arbitrary objects:
However, the
def p = new Person()
bindData(p, params)
bindData method also allows you to exclude certain parameters that you don't want updated:
Or include only certain properties:
def p = new Person()
bindData(p, params, [exclude:'dateOfBirth'])
def p = new Person()
bindData(p, params, [include:['firstName','lastName]])
6.1.7 XML and JSON Responses
Using the render method to output XMLGrails' supports a few different ways to produce XML and JSON responses. The first one covered is via the render method.The
render method can be passed a block of code to do mark-up building in XML:
The result of this code would be something like:
def list = { def results = Book.list() render(contentType:"text/xml") { books { for(b in results) { book(title:b.title) } } } }
Note that you need to be careful to avoid naming conflicts when using mark-up building. For example this code would produce an error:
<books> <book title="The Stand" /> <book title="The Shining" /> </books>
The reason is that there is local variable
def list = { def books = Book.list() // naming conflict here render(contentType:"text/xml") { books { for(b in results) { book(title:b.title) } } } }
books which Groovy attempts to invoke as a method.
Using the render method to output JSONThe
render method can also be used to output JSON:
In this case the result would be something along the lines of:
def list = { def results = Book.list() render(contentType:"text/json") { books = array { for(b in results) { book title:b.title } } } }
Again the same dangers with naming conflicts apply to JSON building.
[ {title:"The Stand"}, {title:"The Shining"} ]
Automatic XML MarshallingGrails also supports automatic marshaling of domain classes to XML via special converters.To start off with import the
grails.converters package into your controller:
Now you can use the following highly readable syntax to automatically convert domain classes to XML:
import grails.converters.*
The resulting output would look something like the following::
render Book.list() as XML
An alternative to using the converters is to use the codecs feature of Grails. The codecs feature provides encodeAsXML and encodeAsJSON methods:
<?xml version="1.0" encoding="ISO-8859-1"?> <list> <book id="1"> <author>Stephen King</author> <title>The Stand</title> </book> <book id="2"> <author>Stephen King</author> <title>The Shining</title> </book> </list>
For more information on XML marshaling see the section on REST
def xml = Book.list().encodeAsXML() render xml
Automatic JSON MarshallingGrails also supports automatic marshaling to JSON via the same mechanism. Simply substitute
XML with
The resulting output would look something like the following:
render Book.list() as JSON
Again as an alternative you can use the
[ {"id":1, "class":"Book", "author":"Stephen King", "title":"The Stand"}, {"id":2, "class":"Book", "author":"Stephen King", "releaseDate":new Date(1194127343161), "title":"The Shining"} ]
encodeAsJSON to achieve the same effect.
6.1.8 More on JSONBuilderThe previous section on on XML and JSON responses covered simplistic examples of rendering XML and JSON responses. Whilst the XML builder used by Grails is the standard XmlSlurper found in Groovy, the JSON builder is a custom implementation specific to Grails.
JSONBuilder and Grails versionsJSONBuilder behaves different depending on the version of Grails you use. For version below 1.2 there deprecated grails.util.JSonBuilder class is used. This section covers the usage of the Grails 1.2 JSONBuilderFor backwards compatibility the old
JSonBuilder class is used with the
render method for older applications, if you want to use the newer/better
JSONBuilder class then you can do so by setting the following in
Rendering Simple ObjectsTo render a simple JSON object just set properties within the context of the closure:
The above will produce the JSON:
render(contentType:"text/json") { hello = "world" }
Rendering JSON ArraysTo render a list of objects simple assign a list:
This will produce:
render(contentType:"text/json") {
categories = ['a', 'b', 'c']
You can also render lists of complex objects, for example:
This will produce:
render(contentType:"text/json") { categories = [ { a = "A" }, { b = "B" } ] }
If you want to return a list as the root then you have to use the special
{"categories":[ {"a":"A"} , {"b":"B"}] }
element method:
The above code produces:
render(contentType:"text/json") {
element 1
element 2
element 3
Rendering Complex ObjectsRendering complex objects can be done with closures. For example:
The above will produce the JSON:
render(contentType:"text/json") { categories = ['a', 'b', 'c'] title ="Hello JSON" information = { pages = 10 } }
{"categories":["a","b","c"],"title":"Hello JSON","information":{"pages":10}}
Arrays of Complex ObjectsAs mentioned previously you can nest complex objects within arrays using closures:
However, if you need to build them up dynamically then you may want to use the
render(contentType:"text/json") { categories = [ { a = "A" }, { b = "B" } ] }
array method:
def results = Book.list() render(contentType:"text/json") { books = array { for(b in results) { book title:b.title } } }
Direct JSONBuilder API AccessIf you don't have access to the
render method, but still want to produce JSON you can use the API directly:
def builder = new JSONBuilder()def result = builder.build { categories = ['a', 'b', 'c'] title ="Hello JSON" information = { pages = 10 } }// prints the JSON text println result.toString()def sw = new StringWriter() result.render sw
6.1.9 Uploading Files
Programmatic File UploadsGrails supports file uploads via Spring's MultipartHttpServletRequest interface. To upload a file the first step is to create a multipart form like the one below:
There are then a number of ways to handle the file upload. The first way is to work with the Spring MultipartFile instance directly:
Upload Form: <br /> <g:form action="upload" method="post" enctype="multipart/form-data"> <input type="file" name="myFile" /> <input type="submit" /> </g:form>
This is clearly handy for doing transfers to other destinations and manipulating the file directly as you can obtain an InputStream and so on via the MultipartFile interface.
def upload = { def f = request.getFile('myFile') if(!f.empty) { f.transferTo( new File('/some/local/dir/myfile.txt') ) response.sendError(200,'Done'); } else { flash.message = 'file cannot be empty' render(view:'uploadForm') } }
File Uploads through Data BindingFile uploads can also be performed via data binding. For example say you have an
Image domain class as per the below example:
Now if you create an image and pass in the
class Image {
byte[] myFile
params object such as the below example, Grails will automatically bind the file's contents as a byte to the
myFile property:
It is also possible to set the contents of the file as a string by changing the type of the
def img = new Image(params)
myFile property on the image to a String type:
class Image {
String myFile
6.1.10 Command ObjectsGrails controllers support the concept of command objects. A command object is similar to a form bean in something like Struts and they are useful in circumstances when you want to populate a subset of the properties needed to update a domain class. Or where there is no domain class required for the interaction, but you need features such as data binding and validation.
Declaring Command ObjectsCommand objects are typically declared in the same source file as a controller directly below the controller class definition. For example:
As the previous example demonstrates you can supply constraints to command objects just as you can with domain classes.
class UserController { … } class LoginCommand { String username String password static constraints = { username(blank:false, minSize:6) password(blank:false, minSize:6) } }
Using Command ObjectsTo use command objects, controller actions may optionally specify any number of command object parameters. The parameter types must be supplied so that Grails knows what objects to create, populate and validate.Before the controller action is executed Grails will automatically create an instance of the command object class, populate the properties of the command object with request parameters having corresponding names and the command object will be validated. For Example:
class LoginController { def login = { LoginCommand cmd -> if(cmd.hasErrors()) { redirect(action:'loginForm') } else { // do something else } } }
Command Objects and Dependency InjectionCommand objects can participate in dependency injection. This is useful if your command object has some custom validation logic which may need to interact with Grails services:
In this example the command object interacts with a bean injected by name from the Spring
class LoginCommand { def loginService String username String password static constraints = { username validator: { val, obj -> obj.loginService.canLogin(obj.username, obj.password) } } }
6.1.11 Handling Duplicate Form SubmissionsGrails has built in support for handling duplicate form submissions using the "Synchronizer Token Pattern". To get started you need to define a token on the form tag:
Then in your controller code you can use the withForm method to handle valid and invalid requests:
<g:form useToken="true" ...>
If you only provide the withForm method and not the chained
withForm { // good request }.invalidToken { // bad request }
invalidToken method then by default Grails will store the invalid token in a
flash.invalidToken variable and redirect the request back to the original page. This can then be checked in the view:
<g:if test="${flash.invalidToken}"> Don't click the button twice! </g:if>
The withForm tag makes use of the session and hence requires session affinity if used in a cluster.
6.1.12 Simple Type Converters
Type Conversion MethodsIf you prefer to avoid the overhead of Data Binding and simply want to convert incoming parameters (typically Strings) into another more appropriate type the params object has a number of convenience methods for each type:
The above example uses the
def total = params.int('total')
int method, there are also methods for
short and so on. Each of these methods are null safe and safe from any parsing errors so you don't have to perform any addition checks on the parameters.These same type conversion methods are also available on the
attrs parameter of GSP tags.
Handling Multi ParametersA common use case is dealing with multiple request parameters of the same name. For example you could get a query string such as
?name=Bob&name=Judy.In this case dealing with 1 parameter and dealing with many has different semantics since Groovy's iteration mechanics for
String iterate over each character. To avoid this problem the params object provides a
list method that always returns a list:
for(name in params.list('name')) {
println name
6.2 Groovy Server PagesGroovy Servers Pages (or GSP for short) is Grails' view technology. It is designed to be familiar for users of technologies such as ASP and JSP, but to be far more flexible and intuitive.In Grails GSPs live in the
grails-app/views directory and are typically rendered automatically (by convention) or via the render method such as:
A GSP is typically a mix of mark-up and GSP tags which aid in view rendering.
Although it is possible to have Groovy logic embedded in your GSP and doing this will be covered in this document the practice is strongly discouraged. Mixing mark-up and code is a bad thing and most GSP pages contain no code and needn't do so.A GSP typically has a "model" which is a set of variables that are used for view rendering. The model is passed to the GSP view from a controller. For example consider the following controller action:
This action will look-up a
def show = { [book: Book.get(params.id)] }
Book instance and create a model that contains a key called
book. This key can then be reference within the GSP view using the name
6.2.1 GSP BasicsIn the next view sections we'll go through the basics of GSP and what is available to you. First off let's cover some basic syntax that users of JSP and ASP should be familiar with.GSP supports the usage of
<% %> blocks to embed Groovy code (again this is discouraged):
As well as this syntax you can also use the
<html> <body> <% out << "Hello GSP!" %> </body> </html>
<%= %> syntax to output values:
GSP also supports JSP-style server-side comments as the following example demonstrates:
<html> <body> <%="Hello GSP!" %> </body> </html>
<html> <body> <%-- This is my comment --%> <%="Hello GSP!" %> </body> </html>
126.96.36.199 Variables and ScopesWithin the
<% %> brackets you can of course declare variables:
And then re-use those variables further down the page:
<% now = new Date() %>
However, within the scope of a GSP there are a number of pre-defined variables including:
application- The javax.servlet.ServletContext instance
applicationContextThe Spring ApplicationContext instance
flash- The flash object
grailsApplication- The GrailsApplication instance
out- The response writer for writing to the output stream
params- The params object for retrieving request parameters
request- The HttpServletRequest instance
response- The HttpServletResponse instance
session- The HttpSession instance
webRequest- The GrailsWebRequest instance
18.104.22.168 Logic and IterationUsing the
<% %> syntax you can of course embed loops and so on using this syntax:
As well as logical branching:
<html> <body> <% [1,2,3,4].each { num -> %> <p><%="Hello ${num}!" %></p> <%}%> </body> </html>
<html> <body> <% if(params.hello == 'true' )%> <%="Hello!"%> <% else %> <%="Goodbye!"%> </body> </html>
184.108.40.206 Page DirectivesGSP also supports a few JSP-style page directives.The import directive allows you to import classes into the page. However, it is rarely needed due to Groovy's default imports and GSP Tags:
GSP also supports the contentType directive:
<%@ page import="java.awt.*" %>
The contentType directive allows using GSP to render other formats.
<%@ page contentType="text/json" %>
184.108.40.206 ExpressionsIn GSP the
<%= %> syntax introduced earlier is rarely used due to the support for GSP expressions. It is present mainly to allow ASP and JSP developers to feel at home using GSP. A GSP expression is similar to a JSP EL expression or a Groovy GString and takes the form
However, unlike JSP EL you can have any Groovy expression within the
<html> <body> Hello ${params.name} </body> </html>
${..} parenthesis. Variables within the
${..} are not escaped by default, so any HTML in the variable's string is output directly to the page. To reduce the risk of Cross-site-scripting (XSS) attacks, you can enable automatic HTML escaping via the
grails.views.default.codec setting in
Other possible values are 'none' (for no default encoding) and 'base64'.
6.2.2 GSP TagsNow that the less attractive JSP heritage has been set aside, the following sections cover GSP's built-in tags, which are the favored way to define GSP pages.
The section on Tag Libraries covers how to add your own custom tag libraries.All built-in GSP tags start with the prefix
g:. Unlike JSP, you don't need to specify any tag library imports. If a tag starts with
g: it is automatically assumed to be a GSP tag. An example GSP tag would look like:
GSP tags can also have a body such as:
<g:example />
Expressions can be passed into GSP tag attributes, if an expression is not used it will be assumed to be a String value:
<g:example> Hello world </g:example>
Maps can also be passed into GSP tag attributes, which are often used for a named parameter style syntax:
<g:example attr="${new Date()}"> Hello world </g:example>
Note that within the values of attributes you must use single quotes for Strings:
<g:example attr="${new Date()}" attr2="[one:1, two:2, three:3]"> Hello world </g:example>
With the basic syntax out the way, the next sections look at the tags that are built into Grails by default.
<g:example attr="${new Date()}" attr2="[one:'one', two:'two']"> Hello world </g:example>
184.108.40.206 Variables and ScopesVariables can be defined within a GSP using the set tag:
Here we assign a variable called
<g:set var="now" value="${new Date()}" />
now to the result of a GSP expression (which simply constructs a new
java.util.Date instance). You can also use the body of the
<g:set> tag to define a variable:
Variables can also be placed in one of the following scopes:
<g:set var="myHTML"> Some re-usable code on: ${new Date()} </g:set>
page- Scoped to the current page (default)
request- Scoped to the current request
flash- Placed within flash scope and hence available for the next request
session- Scoped for the user session
application- Application-wide scope.
scope attribute:
<g:set var="now" value="${new Date()}" scope="request" />
188.8.131.52 Logic and IterationGSP also supports logical and iterative tags out of the box. For logic there are if, else and elseif which support your typical branching scenarios:
For iteration GSP has the each and while tags:
<g:if test="${session.role == 'admin'}"> <%-- show administrative functions --%> </g:if> <g:else> <%-- show basic functions --%> </g:else>
<g:each in="${[1,2,3]}" var="num"> <p>Number ${num}</p> </g:each><g:set var="num" value="${1}" /> <g:while test="${num < 5 }"> <p>Number ${num++}</p> </g:while>
220.127.116.11 Search and FilteringIf you have collections of objects you often need to sort and filter them in some way. GSP supports the findAll and grep for this task:
Stephen King's Books: <g:findAll in="${books}" expr="it.author == 'Stephen King'"> <p>Title: ${it.title}</p> </g:findAll>
expr attribute contains a Groovy expression that can be used as a filter. Speaking of filters the grep tag does a similar job such as filter by class:
Or using a regular expression:
<g:grep in="${books}" filter="NonFictionBooks.class"> <p>Title: ${it.title}</p> </g:grep>
The above example is also interesting due to its usage of GPath. GPath is Groovy's equivalent to an XPath like language. Essentially the
<g:grep in="${books.title}" filter="~/.*?Groovy.*?/"> <p>Title: ${it}</p> </g:grep>
books collection is a collection of
Book instances. However assuming each
Book has a
title, you can obtain a list of Book titles using the expression
books.title. Groovy will auto-magically go through the list of Book instances, obtain each title, and return a new list!
22.214.171.124 Links and ResourcesGSP also features tags to help you manage linking to controllers and actions. The link tag allows you to specify controller and action name pairing and it will automatically work out the link based on the URL Mappings, even if you change them! Some examples of the link can be seen below:
<g:link action="show" id="1">Book 1</g:link> <g:link action="show" id="${currentBook.id}">${currentBook.name}</g:link> <g:link controller="book">Book Home</g:link> <g:link controller="book" action="list">Book List</g:link> <g:link url="[action:'list',controller:'book']">Book List</g:link> <g:link action="list" params="[sort:'title',order:'asc',author:currentBook.author]"> Book List </g:link>
22.214.171.124 Forms and Fields
Form BasicsGSP supports many different tags for aiding in dealing with HTML forms and fields, the most basic of which is the form tag. The
form tag is a controller/action aware version of the regular HTML form tag. The
url attribute allows you to specify which controller and action to map to:
In this case we create a form called
<g:form name="myForm" url="[controller:'book',action:'list']">...</g:form>
myForm that submits to the
list action. Beyond that all of the usual HTML attributes apply.
Form FieldsAs well as easy construction of forms GSP supports custom tags for dealing with different types of fields including:
- textField - For input fields of type 'text'
- checkBox - For input fields of type 'checkbox'
- radio - For input fields of type 'radio'
- hiddenField - For input fields of type 'hidden'
- select - For dealing with HTML select boxes
GSP also contains extended helper versions of the above tags such as radioGroup (for creating groups of radio tags), localeSelect, currencySelect and timeZoneSelect (for selecting locale's, currencies and time zone's respectively).
<g:textField name="myField" value="${myValue}" />
Multiple Submit ButtonsThe age old problem of dealing with multiple submit buttons is also handled elegantly with Grails via the actionSubmit tag. It is just like a regular submit, but allows you to specify an alternative action to submit to:
<g:actionSubmit value="Some update label" action="update" />
188.8.131.52 Tags as Method CallsOne major different between GSP tags and other tagging technologies is that GSP tags can be called as either regular tags or as method calls from either controllers, tag libraries or GSP views.
Tags as method calls from GSPsWhen called as methods tags return their results as a String instead of writing directly to the response. So for example the createLinkTo tag can equally be called as a method:
This is particularly useful when you need to use a tag within an attribute:
Static Resource: ${createLinkTo(dir:"images", file:"logo.jpg")}
In view technologies that don't support this feature you have to nest tags within tags, which becomes messy quickly and often has an adverse effect of WYSWIG tools such as Dreamweaver that attempt to render the mark-up as it is not well-formed:
<img src="${createLinkTo(dir:'images', file:'logo.jpg')}" />
<img src="<g:createLinkTo dir="images" file="logo.jpg" />" />
Tags as method calls from Controllers and Tag LibrariesYou can also invoke tags from controllers and tag libraries. Tags within the default
g: namespace can be invoked without the prefix and a String result is returned:
However, you can also prefix the namespace to avoid naming conflicts:
def imageLocation = createLinkTo(dir:"images", file:"logo.jpg")
If you have a custom namespace you can use that prefix instead (Example using the FCK Editor plugin):
def imageLocation = g.createLinkTo(dir:"images", file:"logo.jpg")
def editor = fck.editor()
6.2.3 Views and TemplatesAs well as views, Grails has the concept of templates. Templates are useful for separating out your views into maintainable chunks and combined with Layouts provide a highly re-usable mechanism for structure views.
Template BasicsGrails uses the convention of placing an underscore before the name of a view to identify it as a template. For example a you may have a template that deals with rendering Books located at
To render this template from one of the views in
<div class="book" id="${book?.id}"> <div>Title: ${book?.title}</div> <div>Author: ${book?.author?.name}</div> </div>
grails-app/views/book you can use the render tag:
Notice how we pass into a model to use using the
<g:render template="bookTemplate" model="[book:myBook]" />
model attribute of the render tag. If you have multiple
Book instances you can also render the template for each
Book using the render tag:
<g:render template="bookTemplate" var="book" collection="${bookList}" />
Shared TemplatesIn the previous example we had a template that was specific to the
BookController and its views at
grails-app/views/book. However, you may want to share templates across your application.In this case you can place them in the root views directory at grails-app/views or any subdirectory below that location and then with the template attribute use a
/ before the template name to indicate the relative template path. For example if you had a template called
grails-app/views/shared/_mySharedTemplate.gsp, you could reference it as follows:
You can also use this technique to reference templates in any directory from any view or controller:
<g:render template="/shared/mySharedTemplate" />
<g:render template="/book/bookTemplate" model="[book:myBook]" />
The Template NamespaceSince templates are used so frequently there is template namespace, called
tmpl, available that makes using templates easier. Consider for example the following usage pattern:
This can be expressed with the
<g:render template="bookTemplate" model="[book:myBook]" />
tmpl namespace as follows:
<tmpl:bookTemplate book="${myBook}" />
Templates in Controllers and Tag LibrariesYou can also render templates from controllers using the render method found within controllers, which is useful for Ajax applications:
The render method within controllers writes directly to the response, which is the most common behaviour. If you need to instead obtain the result of template as a String you can use the render tag:
def show = {
def b = Book.get(params.id)
render(template:"bookTemplate", model:[book:b])
Notice the usage of the
def show = { def b = Book.get(params.id) String content = g.render(template:"bookTemplate", model:[book:b]) render content }
g. namespace which tells Grails we want to use the tag as method call instead of the render method.
6.2.4 Layouts with Sitemesh
Creating LayoutsGrails leverages Sitemesh, a decorator engine, to support view layouts. Layouts are located in the
grails-app/views/layouts directory. A typical layout can be seen below:
The key elements are the layoutHead, layoutTitle and layoutBody tag usages, here is what they do:
<html> <head> <title><g:layoutTitle default="An example decorator" /></title> <g:layoutHead /> </head> <body onload="${pageProperty(name:'body.onload')}"> <div class="menu"><!--my common menu goes here--></menu> <div class="body"> <g:layoutBody /> </div> </div> </body> </html>
layoutTitle- outputs the target page's title
layoutHead- outputs the target pages head tag contents
layoutBody- outputs the target pages body tag contents
Triggering LayoutsThere are a few ways to trigger a layout. The simplest is to add a meta tag to the view:
In this case a layout called
<html> <head> <title>An Example Page</title> <meta name="layout" content="main"></meta> </head> <body>This is my content!</body> </html>
grails-app/views/layouts/main.gsp will be used to layout the page. If we were to use the layout from the previous section the output would resemble the below:
<html> <head> <title>An Example Page</title> </head> <body onload=""> <div class="menu"><!--my common menu goes here--></div> <div class="body"> This is my content! </div> </body> </html>
Specifying A Layout In A ControllerAnother way to specify a layout is to specify the name of the layout by assigning a value to the "layout" property in a controller. For example, if you have a controller such as:
You can create a layout called
class BookController {
static layout = 'customer' def list = { … }
grails-app/views/layouts/customer.gsp which will be applied to all views that the
BookController delegates to. The value of the "layout" property may contain a directory structure relative to the
grails-app/views/layouts/ directory. For example:
Views rendered from that controller would be decorated with the
class BookController {
static layout = 'custom/customer' def list = { … }
grails-app/views/layouts/custom/customer.gsp template.
Layout by ConventionAnother way to associate layouts is to use "layout by convention". For example, if you have a controller such as:
You can create a layout called
class BookController { def list = { … } }
grails-app/views/layouts/book.gsp, by convention, which will be applied to all views that the
BookController delegates to.Alternatively, you can create a layout called
grails-app/views/layouts/book/list.gsp which will only be applied to the
list action within the
BookController.If you have both the above mentioned layouts in place the layout specific to the action will take precedence when the list action is executed.If a layout may not be located using any of those conventions, the convention of last resort is to look for the application default layout which
grails-app/views/layouts/application.gsp. The name of the application default layout may be changed by defining a property
grails-app/conf/Config.groovy as follows:
With that property in place, the application default layout will be
// grails-app/conf/Config.groovy
Inline LayoutsGrails' also supports Sitemesh's concept of inline layouts with the applyLayout tag. The
applyLayout tag can be used to apply a layout to a template, URL or arbitrary section of content. Essentially, this allows to even further modularize your view structure by "decorating" your template includes.Some examples of usage can be seen below:
<g:applyLayout name="myLayout" template="bookTemplate" collection="${books}" /><g:applyLayout name="myLayout" url="http://www.google.com" /><g:applyLayout name="myLayout"> The content to apply a layout to </g:applyLayout>
Server-Side IncludesWhile the applyLayout tag is useful for applying layouts to external content, if you simply want to include external content in the current page you can do so with the include:
You can even combine the include tag and the applyLayout tag for added flexibility:
<g:include controller="book" action="list"></g:include>
Finally, you can also call the include tag from a controller or tag library as a method:
<g:applyLayout name="myLayout"> <g:include controller="book" action="list"></g:include> </g:applyLayout>
The resulting content will be provided via the return value of the include tag.
def content = include(controller:"book", action:"list")
6.2.5 Sitemesh Content BlocksAlthough it is useful to decorate an entire page sometimes you may find the need to decorate independent sections of your site. To do this you can use content blocks. To get started you need to divide the page to be decorate using the
<content> tag:
Then within the layout you can reference these components and apply individual layouts to each:
<content tag="navbar"> … draw the navbar here… </content> <content tag="header"> … draw the header here… </content> <content tag="footer"> … draw the footer here… </content> <content tag="body"> … draw the body here… </content>
<html> <body> <div id="header"> <g:applyLayout name="headerLayout"> <g:pageProperty name="page.header" /> </g:applyLayout> </div> <div id="nav"> <g:applyLayout name="navLayout"> <g:pageProperty name="page.navbar" /> </g:applyLayout> </div> <div id="body"> <g:applyLayout name="bodyLayout"> <g:pageProperty name="page.body" /> </g:applyLayout> </div> <div id="footer"> <g:applyLayout name="footerLayout"> <g:pageProperty name="page.footer" /> </g:applyLayout> </div> </body> </html>
6.2.6 Making Changes to a Deployed ApplicationOne of the main issues with deploying a Grails application (or typically any servlet-based one) is that any change to the views requires you to redeploy your whole application. If all you want to do is fix a typo on a page, or change an image link, it can seem like a lot of unnecessary work. For such simple requirements, Grails does have a solution: the
grails.gsp.view.dir configuration setting.How does this work? The first step is to decide where the GSP files should go. Let's say we want to keep them unpacked in a
/var/www/grails/my-app directory. We add these two lines to
grails-app/conf/Config.groovy :
The first line tells Grails that modified GSP files should be reloaded at runtime. If you don't have this setting, you can make as many changes as you like but they won't be reflected in the running application. The second line tells Grails where to load the views and layouts from.
grails.gsp.enable.reload = true grails.gsp.view.dir = "/var/www/grails/my-app/"
The trailing slash on the Setting "grails.gsp.view.dir" is optional. If it's not specified, you can update files directly to the application server's deployed war directory. Depending on the application server, these files might get overwritten when the server is restarted. Most application servers support "exploded war deployment" which is recommended in this case.With those settings in place, all you need to do is copy the views from your web application to the external directory. On a Unix-like system, this would look something like this:
grails.gsp.view.dir value is important! Without it, Grails will look for views in the parent directory.
The key point here is that you must retain the view directory structure, including the
mkdir -p /var/www/grails/my-app/grails-app/views cp -R grails-app/views/* /var/www/grails/my-app/grails-app/views
grails-app/views bit. So you end up with the path
/var/www/grails/my-app/grails-app/views/... .One thing to bear in mind with this technique is that every time you modify a GSP, it uses up permgen space. So at some point you will eventually hit "out of permgen space" errors unless you restart the server. So this technique is not recommended for frequent or large changes to the views.There are also some System properties to control GSP reloading:
|altervative system property for enabling the GSP reload mode without changing Config.groovy
|interval between checking the lastmodified time of the gsp source file, unit is milliseconds
|the number of milliseconds leeway to give before deciding a file is out of date. this is needed because different roundings usually cause a 1000ms difference in lastmodified times
6.2.7 GSP Debugging
Viewing the generated source code
- Adding "?showSource=true" or "&showSource=true" to the url shows the generated groovy source code for the view instead of rendering it. It won't show the source code of included templates. This only works in development mode
- The saving of all generated source code can be activated by setting the property "grails.views.gsp.keepgenerateddir" (in Config.groovy) . It should point to a directory that's existing and writable.
- During "grails war" gsp pre-compilation, the generated source code is stored in grails.project.work.dir/gspcompile (usually in ~/.grails/(grails_version)/projects/(project name)/gspcompile).
Debugging GSP code with a debugger
Viewing information about templates used to render a single urlGSP templates are re-used in large web applications by using the g:render taglib. A lot of small templates can be used to render a single page. It might be hard to find out what gsp template actually renders the html seen in the result. The debug templates -feature adds html comments to the output. The comments contain debug information about gsp templates used to render the page.Usage is simple: append "?debugTemplates" or "&debugTemplates" to the url and view the source of the result in your browser. "debugTemplates" is restricted to development mode. It won't work in production.Here is an example of comments added by debugTemplates :
Each comment block has a unique id so that you can find the start & end of each template call.
<!-- GSP #2 START template: /home/user/sampleapp/grails-app/views/_carousel.gsp precompiled: false lastmodified: 22.6.2010 10:45 -->
<!-- GSP #2 END template: /home/user/sampleapp/grails-app/views/_carousel.gsp rendering time: 115 ms -->
6.3 Tag LibrariesLike Java Server Pages (JSP), GSP supports the concept of custom tag libraries. Unlike JSP, Grails tag library mechanism is simply, elegant and completely reloadable at runtime.Quite simply, to create a tag library create a Groovy class that ends with the convention
TagLib and place it within the
grails-app/taglib directory:
Now to create a tag simply create property that is assigned a block of code that takes two arguments: The tag attributes and the body content:
class SimpleTagLib {}
class SimpleTagLib { def simple = { attrs, body -> } }
attrs argument is a simple map of the attributes of the tag, whilst the
body argument is another invokable block of code that returns the body content:
As demonstrated above there is an implicit
class SimpleTagLib { def emoticon = { attrs, body -> out << body() << (attrs.happy == 'true' ? " :-)" : " :-(") } }
out variable that refers to the output
Writer which you can use to append content to the response. Then you can simply reference the tag inside your GSP, no imports necessary:
<g:emoticon happy="true">Hi John</g:emoticon>
To help IDEs like SpringSource Tool Suite (STS) and others autocomplete tag attributes, you should add Javadoc comments to your tag closures with
@attrdescriptions. Since taglibs use Groovy code it can be difficult to reliably detect all usable attributes.For example:and any mandatory attributes should include the REQUIRED keyword, e.g.class SimpleTagLib { /** * Renders the body with an emoticon. * * @attr happy whether to show a happy emoticon ('true') or a sad emoticon ('false') */ def emoticon = { attrs, body -> out << body() << (attrs.happy == 'true' ? " :-)" : " :-(") } }class SimpleTagLib { /** * Creates a new password field. * * @attr name REQUIRED the field name * @attr value the field value */ def passwordField = { attrs -> attrs.type = "password" attrs.tagName = "passwordField" fieldImpl(out, attrs) } }
6.3.1 Variables and ScopesWithin the scope of a tag library there are a number of pre-defined variables including:
actionName- The currently executing action name
controllerName- The currently executing controller name
flash- The flash object
grailsApplication- The GrailsApplication instance
out- The response writer for writing to the output stream
pageScope- A reference to the pageScope object used for GSP rendering (i.e. the binding)
params- The params object for retrieving request parameters
pluginContextPath- The context path to the plugin that contains the tag library
request- The HttpServletRequest instance
response- The HttpServletResponse instance
servletContext- The javax.servlet.ServletContext instance
session- The HttpSession instance
6.3.2 Simple TagsAs demonstrated it the previous example it is trivial to write simple tags that have no body and merely output content. Another example is a
dateFormat style tag:
The above uses Java's
def dateFormat = { attrs, body ->
out << new java.text.SimpleDateFormat(attrs.format).format(attrs.date)
SimpleDateFormat class to format a date and then write it to the response. The tag can then be used within a GSP as follows:
With simple tags sometimes you need to write HTML mark-up to the response. One approach would be to embed the content directly:
<g:dateFormat format="dd-MM-yyyy" date="${new Date()}" />
Although this approach may be tempting it is not very clean. A better approach would be to re-use the render tag:
def formatBook = { attrs, body -> out << "<div id="${attrs.book.id}">" out << "Title : ${attrs.book.title}" out << "</div>" }
And then have a separate GSP template that does the actual rendering.
def formatBook = { attrs, body ->
out << render(template:"bookTemplate", model:[book:attrs.book])
6.3.3 Logical TagsYou can also create logical tags where the body of the tag is only output once a set of conditions have been met. An example of this may be a set of security tags:
The tag above checks if the user is an administrator and only outputs the body content if he/she has the correct set of access privileges:
def isAdmin = { attrs, body -> def user = attrs['user'] if(user != null && checkUserPrivs(user)) { out << body() } }
<g:isAdmin user="${myUser}"> // some restricted content </g:isAdmin>
6.3.4 Iterative TagsIterative tags are trivial too, since you can invoke the body multiple times:
In this example we check for a
def repeat = { attrs, body -> attrs.times?.toInteger().times { num -> out << body(num) } }
times attribute and if it exists convert it to a number then use Groovy's
times method to iterate by the number of times specified by the number:
Notice how in this example we use the implicit
<g:repeat times="3"> <p>Repeat this 3 times! Current repeat = ${it}</p> </g:repeat>
it variable to refer to the current number. This works because when we invoked the body we passed in the current value inside the iteration:
That value is then passed as the default variable
out << body(num)
it to the tag. However, if you have nested tags this can lead to conflicts, hence you should should instead name the variables that the body uses:
Here we check if there is a
def repeat = { attrs, body -> def var = attrs.var ? attrs.var : "num" attrs.times?.toInteger().times { num -> out << body((var):num) } }
var attribute and if there is use that as the name to pass into the body invocation on this line:
out << body((var):num)
Note the usage of the parenthesis around the variable name. If you omit these Groovy assumes you are using a String key and not referring to the variable itself.Now we can change the usage of the tag as follows:
Notice how we use the
<g:repeat times="3" var="j"> <p>Repeat this 3 times! Current repeat = ${j}</p> </g:repeat>
var attribute to define the name of the variable
j and then we are able to reference that variable within the body of the tag.
6.3.5 Tag NamespacesBy default, tags are added to the default Grails namespace and are used with the
g: prefix in GSP pages. However, you can specify a different namespace by adding a static property to your
TagLib class:
Here we have specified a
class SimpleTagLib { static namespace = "my" def example = { attrs -> … } }
namespace of
my and hence the tags in this tag lib must then be referenced from GSP pages like this:
Where the prefix is the same as the value of the static
<my:example name="..." />
namespace property. Namespaces are particularly useful for plugins.
Tags within namespaces can be invoked as methods using the namespace as a prefix to the method call:
This works from GSP, controllers or tag libraries
out << my.example(name:"foo")
6.3.6 Using JSP Tag LibrariesIn addition to the simplified tag library mechanism provided by GSP, you can also use JSP tags from GSP. To do so simply declare the JSP you want to use via the
taglib directive:
Then you can use it like any other tag:
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
With the added bonus that you can invoke JSP tags like methods:
<fmt:formatNumber value="${10}" pattern=".00"/>
${fmt.formatNumber(value:10, pattern:".00")}
6.3.7 Tag return valueSince Grails 1.2, a tag library call returns an instance of
org.codehaus.groovy.grails.web.util.StreamCharBuffer class by default.
This change improves performance by reducing object creation and optimizing buffering during request processing.
In earlier Grails versions, a
java.lang.String instance was returned.Tag libraries can also return direct object values to the caller since Grails 1.2..
Object returning tag names are listed in a static
returnObjectForTags property in the tag library class.Example:
class ObjectReturningTagLib { static namespace = "cms" static returnObjectForTags = ['content'] def content = { attrs, body -> CmsContent.findByCode(attrs.code)?.content } }
6.4 URL MappingsThroughout the documentation so far the convention used for URLs has been the default of
/controller/action/id. However, this convention is not hard wired into Grails and is in fact controlled by a URL Mappings class located at
UrlMappings class contains a single property called
mappings that has been assigned a block of code:
class UrlMappings {
static mappings = {
6.4.1 Mapping to Controllers and ActionsTo create a simple mapping simply use a relative URL as the method name and specify named parameters for the controller and action to map to:
In this case we've mapped the URL
"/product"(controller:"product", action:"list")
/product to the
list action of the
ProductController. You could of course omit the action definition to map to the default action of the controller:
An alternative syntax is to assign the controller and action to use within a block passed to the method:
Which syntax you use is largely dependent on personal preference. If you simply want to rewrite on URI onto another explicit URI (rather than a controller/action pair) this can be achieved with the following example:
"/product" { controller = "product" action = "list" }
Rewriting specific URIs is often useful when integrating with other frameworks.
6.4.2 Embedded Variables
Simple VariablesThe previous section demonstrated how to map trivial URLs with concrete "tokens". In URL mapping speak tokens are the sequence of characters between each slash / character. A concrete token is one which is well defined such as as
/product. However, in many circumstances you don't know what the value of a particular token will be until runtime. In this case you can use variable placeholders within the URL for example:
In this case by embedding a $id variable as the second token Grails will automatically map the second token into a parameter (available via the params object) called
static mappings = { "/product/$id"(controller:"product") }
id. For example given the URL
/product/MacBook, the following code will render "MacBook" to the response:
You can of course construct more complex examples of mappings. For example the traditional blog URL format could be mapped as follows:
class ProductController { def index = { render params.id } }
The above mapping would allow you to do things like:
static mappings = { "/$blog/$year/$month/$day/$id"(controller:"blog", action:"show") }
The individual tokens in the URL would again be mapped into the params object with values available for
id and so on.
Dynamic Controller and Action NamesVariables can also be used to dynamically construct the controller and action name. In fact the default Grails URL mappings use this technique:
Here the name of the controller, action and id are implicitly obtained from the variables
static mappings = { "/$controller/$action?/$id?"() }
action and
id embedded within the URL.You can also resolve the controller name and action name to execute dynamically using a closure:
static mappings = { "/$controller" { action = { params.goHere } } }
Optional VariablesAnother characteristic of the default mapping is the ability to append a
? at the end of a variable to make it an optional token. In a further example this technique could be applied to the blog URL mapping to have more flexible linking:
With this mapping all of the below URLs would match with only the relevant parameters being populated in the params object:
static mappings = { "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show") }
/graemerocher/2007/01/10/my_funky_blog_entry /graemerocher/2007/01/10 /graemerocher/2007/01 /graemerocher/2007 /graemerocher
Arbitrary VariablesYou can also pass arbitrary parameters from the URL mapping into the controller by merely setting them in the block passed to the mapping:
This variables will be available within the params object passed to the controller.
"/holiday/win" { id = "Marrakech" year = 2007 }
Dynamically Resolved VariablesThe hard coded arbitrary variables are useful, but sometimes you need to calculate the name of the variable based on runtime factors. This is also possible by assigning a block to the variable name:
In the above case the code within the blocks is resolved when the URL is actually matched and hence can be used in combination with all sorts of logic.
"/holiday/win" { id = { params.id } isEligible = { session.user != null } // must be logged in }
6.4.3 Mapping to ViewsIf you want to resolve a URL to a view, without a controller or action involved, you can do so too. For example if you wanted to map the root URL
/ to a GSP at the location
grails-app/views/index.gsp you could use:
Alternatively if you need a view that is specific to a given controller you could use:
static mappings = { "/"(view:"/index") // map the root URL }
static mappings = { "/help"(controller:"site",view:"help") // to a view for a controller }
6.4.4 Mapping to Response CodesGrails also allows you to map HTTP response codes to controllers, actions or views. All you have to do is use a method name that matches the response code you are interested in:
Or alternatively if you merely want to provide custom error pages:
static mappings = { "403"(controller: "errors", action: "forbidden") "404"(controller: "errors", action: "notFound") "500"(controller: "errors", action: "serverError") }
static mappings = { "403"(view: "/errors/forbidden") "404"(view: "/errors/notFound") "500"(view: "/errors/serverError") }
Declarative Error HandlingIn addition you can configure handlers for individual exceptions:
With this configuration, an
static mappings = { "403"(view: "/errors/forbidden") "404"(view: "/errors/notFound") "500"(controller: "errors", action: "illegalArgument", exception: IllegalArgumentException) "500"(controller: "errors", action: "nullPointer", exception: NullPointerException) "500"(controller: "errors", action: "customException", exception: MyException) "500"(view: "/errors/serverError") }
IllegalArgumentException will be handled by the
illegalArgument action in
ErrorsController, a
NullPointerException will be handled by the
nullPointer action, and a
MyException will be handled by the
customException action. Other exceptions will be handled by the catch-all rule and use the
/errors/serverError view.You can access the exception from your custom error handing view or controller action via the request's
exception attribute like so:
class ErrorController { def handleError = { def exception = request.exception // perform desired processing to handle the exception } }
If your error-handling controller action throws an exception as well, you'll end up with a
6.4.5 Mapping to HTTP methodsURL mappings can also be configured to map based on the HTTP method (GET, POST, PUT or DELETE). This is extremely useful for RESTful APIs and for restricting mappings based on HTTP method.As an example the following mappings provide a RESTful API URL mappings for the
static mappings = { "/product/$id"(controller:"product"){ action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"] } }
6.4.6 Mapping WildcardsGrails' URL mappings mechanism also supports wildcard mappings. For example consider the following mapping:
This mapping will match all paths to images such as
static mappings = { "/images/*.jpg"(controller:"image") }
/image/logo.jpg. Of course you can achieve the same effect with a variable:
However, you can also use double wildcards to match more than one level below:
static mappings = { "/images/$name.jpg"(controller:"image") }
In this cases the mapping will match
static mappings = { "/images/**.jpg"(controller:"image") }
/image/logo.jpg as well as
/image/other/logo.jpg. Even better you can use a double wildcard variable:
In this case it will store the path matched by the wildcard inside a
static mappings = { // will match /image/logo.jpg and /image/other/logo.jpg "/images/$name**.jpg"(controller:"image") }
name parameter obtainable from the params object:
If you are using wildcard URL mappings then you may want to exclude certain URIs from Grails' URL mapping process. To do this you can provide an
def name = params.name println name // prints "logo" or "other/logo"
excludes setting inside the
UrlMappings.groovy class:
In this case Grails won't attempt to match any URIs that start with
class UrlMappings = { static excludes = ["/images/*", "/css/*"] static mappings = { … } }
/images or
6.4.7 Automatic Link Re-WritingAnother great feature of URL mappings is that they automatically customize the behaviour of the link tag so that changing the mappings don't require you to go and change all of your links.This is done through a URL re-writing technique that reverse engineers the links from the URL mappings. So given a mapping such as the blog one from an earlier section:
If you use the link tag as follows:
static mappings = { "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show") }
Grails will automatically re-write the URL in the correct format:
<g:link controller="blog" action="show" params="[blog:'fred', year:2007]">My Blog</g:link> <g:link controller="blog" action="show" params="[blog:'fred', year:2007, month:10]">My Blog - October 2007 Posts</g:link>
<a href="/fred/2007">My Blog</a> <a href="/fred/2007/10">My Blog - October 2007 Posts</a>
6.4.8 Applying ConstraintsURL Mappings also support Grails' unified validation constraints mechanism, which allows you to further "constrain" how a URL is matched. For example, if we revisit the blog sample code from earlier, the mapping currently looks like this:
This allows URLs such as:
static mappings = { "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show") }
However, it would also allow:
This is problematic as it forces you to do some clever parsing in the controller code. Luckily, URL Mappings can be constrained to further validate the URL tokens:
In this case the constraints ensure that the
"/$blog/$year?/$month?/$day?/$id?" { controller = "blog" action = "show" constraints { year(matches:/\d{4}/) month(matches:/\d{2}/) day(matches:/\d{2}/) } }
month and
day parameters match a particular valid pattern thus relieving you of that burden later on.
6.4.9 Named URL MappingsURL Mappings also support named mappings. Simply put, named mappings are mappings which have a name associated with them. The name may be used to refer to a specific mapping when links are being generated.The syntax for defining a named mapping is as follows:
An example:
static mappings = {
name <mapping name>: <url pattern> {
// …
The mapping may be referenced in a link tag in a GSP.
static mappings = { name personList: "/showPeople" { controller = 'person' action = 'list' } name accountDetails: "/details/$acctNumber" { controller = 'product' action = 'accountDetails' } }
That would result in:
<g:link mapping="personList">List People</g:link>
Parameters may be specified using the params attribute.
<a href="/showPeople">List People</a>
That would result in:
<g:link mapping="accountDetails" params="[acctNumber:'8675309']">Show Account</g:link>
Alternatively you may reference a named mapping using the link namespace.
<a href="/details/8675309">Show Account</a>
That would result in:
<link:personList>List People</link:personList>
The link namespace approach allows parameters to be specified as attributes.
<a href="/showPeople">List People</a>
That would result in:
<link:accountDetails acctNumber="8675309">Show Account</link:accountDetails>
To specify attributes that should be applied to the generated href, specify a
<a href="/details/8675309">Show Account</a>
Map value to the
attrs attribute. These attributes will be applied
directly to the href, not passed through to be used as request parameters.
That would result in:
<link:accountDetails attrs="[class: 'fancy']" acctNumber="8675309">Show Account</link:accountDetails>
<a href="/details/8675309" class="fancy">Show Account</a>
6.5 Web Flow
OverviewGrails supports the creation of web flows built on the Spring Web Flow project. A web flow is a conversation that spans multiple requests and retains state for the scope of the flow. A web flow also has a defined start and end state.Web flows don't require an HTTP session, but instead store their state in a serialized form, which is then restored using a flow execution key that Grails passes around as a request parameter. This makes flows far more scalable than other forms of stateful application that use the HttpSession and its inherit memory and clustering concerns.Web flow is essentially an advanced state machine that manages the "flow" of execution from one state to the next. Since the state is managed for you, you don't have to be concerned with ensuring that users enter an action in the middle of some multi step flow, as web flow manages that for you. This makes web flow perfect for use cases such as shopping carts, hotel booking and any application that has multi page work flows.
From Grails 1.2 onwards you must install the Webflow plugin to use this feature:
grails install-plugin webflow
Creating a FlowTo create a flow create a regular Grails controller and then add an action that ends with the convention
Flow. For example:
Notice when redirecting or referring to the flow as an action we omit the
class BookController {
def index = {
def shoppingCartFlow = {
Flow suffix. In other words the name of the action of the above flow is
6.5.1 Start and End StatesAs mentioned before a flow has a defined start and end state. A start state is the state which is entered when a user first initiates a conversation (or flow). The start state of A Grails flow is the first method call that takes a block. For example:
Here the
class BookController { … def shoppingCartFlow = { showCart { on("checkout").to "enterPersonalDetails" on("continueShopping").to "displayCatalogue" } … displayCatalogue { redirect(controller:"catalogue", action:"show") } displayInvoice() } }
showCart node is the start state of the flow. Since the showCart state doesn't define an action or redirect it is assumed be a view state that, by convention, refers to the view
grails-app/views/book/shoppingCart/showCart.gsp.Notice that unlike regular controller actions, the views are stored within a directory that matches the name of the flow:
shoppingCart flow also has two possible end states. The first is
displayCatalogue which performs an external redirect to another controller and action, thus exiting the flow. The second is
displayInvoice which is an end state as it has no events at all and will simply render a view called
grails-app/views/book/shoppingCart/displayInvoice.gsp whilst ending the flow at the same time.Once a flow has ended it can only be resumed from the start state, in this case
showCart, and not from any other state.
6.5.2 Action States and View States
View statesA view state is a one that doesn't define an
action or a
redirect. So for example the below is a view state:
It will look for a view called
enterPersonalDetails { on("submit").to "enterShipping" on("return").to "showCart" }
grails-app/views/book/shoppingCart/enterPersonalDetails.gsp by default. Note that the
enterPersonalDetails state defines two events:
submit and
return. The view is responsible for triggering these events. If you want to change the view to be rendered you can do so with the render method:
Now it will look for
enterPersonalDetails { render(view:"enterDetailsView") on("submit").to "enterShipping" on("return").to "showCart" }
grails-app/views/book/shoppingCart/enterDetailsView.gsp. If you want to use a shared view, start with a / in view argument:
Now it will look for
enterPersonalDetails { render(view:"/shared/enterDetailsView") on("submit").to "enterShipping" on("return").to "showCart" }
Action StatesAn action state is a state that executes code but does not render any view. The result of the action is used to dictate flow transition. To create an action state you need to define an action to to be executed. This is done by calling the
action method and passing it a block of code to be executed:
As you can see an action looks very similar to a controller action and in fact you can re-use controller actions if you want. If the action successfully returns with no errors the
listBooks { action { [ bookList:Book.list() ] } on("success").to "showCatalogue" on(Exception).to "handleError" }
success event will be triggered. In this case since we return a map, this is regarded as the "model" and is automatically placed in flow scope.
In addition, in the above example we also use an exception handler to deal with errors on the line:
What this does is make the flow transition to a state called
on(Exception).to "handleError"
handleError in the case of an exception.You can write more complex actions that interact with the flow request context:
Here is a more complex action that gathers all the information accumulated from the flow scope and creates an
processPurchaseOrder { action { def a = flow.address def p = flow.person def pd = flow.paymentDetails def cartItems = flow.cartItems flow.clear() def o = new Order(person:p, shippingAddress:a, paymentDetails:pd) o.invoiceNumber = new Random().nextInt(9999999) cartItems.each { o.addToItems(it) } o.save() [order:o] } on("error").to "confirmPurchase" on(Exception).to "confirmPurchase" on("success").to "displayInvoice" }
Order object. It then returns the order as the model. The important thing to note here is the interaction with the request context and "flow scope".
Transition ActionsAnother form of action is what is known as a transition action. A transition action is executed directly prior to state transition once an event has been triggered. A trivial example of a transition action can be seen below:
Notice how we pass a block of the code to
enterPersonalDetails { on("submit") { log.trace "Going to enter shipping" }.to "enterShipping" on("return").to "showCart" }
submit event that simply logs the transition. Transition states are extremely useful for data binding and validation, which is covered in a later section.
6.5.3 Flow Execution EventsIn order to transition execution of a flow from one state to the next you need some way of trigger an event that indicates what the flow should do next. Events can be triggered from either view states or action states.
Triggering Events from a View StateAs discussed previously the start state of the flow in a previous code listing deals with two possible events. A
checkout event and a
continueShopping event:
Since the
def shoppingCartFlow = { showCart { on("checkout").to "enterPersonalDetails" on("continueShopping").to "displayCatalogue" } … }
showCart event is a view state it will render the view
grails-app/book/shoppingCart/showCart.gsp. Within this view you need to have components that trigger flow execution. On a form this can be done use the submitButton tag:
The form must submit back to the
<g:form action="shoppingCart"> <g:submitButton name="continueShopping" value="Continue Shopping"></g:submitButton> <g:submitButton name="checkout" value="Checkout"></g:submitButton> </g:form>
shoppingCart flow. The name attribute of each submitButton tag signals which event will be triggered. If you don't have a form you can also trigger an event with the link tag as follows:
<g:link action="shoppingCart" event="checkout" />
Triggering Events from an ActionTo trigger an event from an
action you need to invoke a method. For example there is the built in
error() and
success() methods. The example below triggers the
error() event on validation failure in a transition action:
In this case because of the error the transition action will make the flow go back to the
enterPersonalDetails { on("submit") { def p = new Person(params) flow.person = p if(!p.validate())return error() }.to "enterShipping" on("return").to "showCart" }
enterPersonalDetails state.With an action state you can also trigger events to redirect flow:
shippingNeeded { action { if(params.shippingRequired) yes() else no() } on("yes").to "enterShipping" on("no").to "enterPayment" }
6.5.4 Flow Scopes
Scope BasicsYou'll notice from previous examples that we used a special object called
flow to store objects within "flow scope". Grails flows have 5 different scopes you can utilize:
request- Stores an object for the scope of the current request
flash- Stores the object for the current and next request only
flow- Stores objects for the scope of the flow, removing them when the flow reaches an end state
conversation- Stores objects for the scope of the conversation including the root flow and nested subflows
session- Stores objects inside the users session
Grails service classes can be automatically scoped to a web flow scope. See the documentation on Services for more information.Also returning a model map from an action will automatically result in the model being placed in flow scope. For example, using a transition action, you can place objects within
flow scope as follows:
Be aware that a new request is always created for each state, so an object placed in request scope in an action state (for example) will not be available in a subsequent view state. Use one of the other scopes to pass objects from one state to another. Also note that Web Flow:
enterPersonalDetails { on("submit") { [person:new Person(params)] }.to "enterShipping" on("return").to "showCart" }
- Moves objects from flash scope to request scope upon transition between states;
- Merges objects from the flow and conversation scopes into the view model before rendering (so you shouldn't include a scope prefix when referencing these objects within a view, e.g. GSP pages).
Flow Scopes and SerializationWhen placing objects in
flow or
conversation scope they must implement
java.io.Serializable otherwise you will get an error. This has an impact on domain classes in that domain classes are typically placed within a scope so that they can be rendered in a view. For example consider the following domain class:
In order to place an instance of the
class Book {
String title
Book class in a flow scope you will need to modify it as follows:
This also impacts associations and closures you declare within a domain class. For example consider this:
class Book implements Serializable { String title }
Here if the
class Book implements Serializable { String title Author author }
Author association is not
Serializable you will also get an error. This also impacts closures used in GORM events such as
onSave and so on. The following domain class will cause an error if an instance is placed in a flow scope:
The reason is that the assigned block on the
class Book implements Serializable { String title def onLoad = { println "I'm loading" } }
onLoad event cannot be serialized. To get around this you should declare all events as
class Book implements Serializable { String title transient onLoad = { println "I'm loading" } }
6.5.5 Data Binding and ValidationIn the section on start and end states, the start state in the first example triggered a transition to the
enterPersonalDetails state. This state renders a view and waits for the user to enter the required information:
The view contains a form with two submit buttons that either trigger the submit event or the return event:
enterPersonalDetails { on("submit").to "enterShipping" on("return").to "showCart" }
However, what about the capturing the information submitted by the form? To to capture the form info we can use a flow transition action:
<g:form action="shoppingCart"> <!-- Other fields --> <g:submitButton name="submit" value="Continue"></g:submitButton> <g:submitButton name="return" value="Back"></g:submitButton> </g:form>
Notice how we perform data binding from request parameters and place the
enterPersonalDetails { on("submit") { flow.person = new Person(params) !flow.person.validate() ? error() : success() }.to "enterShipping" on("return").to "showCart" }
Person instance within
flow scope. Also interesting is that we perform validation and invoke the
error() method if validation fails. This signals to the flow that the transition should halt and return to the
enterPersonalDetails view so valid entries can be entered by the user, otherwise the transition should continue and go to the
enterShipping state.Like regular actions, flow actions also support the notion of Command Objects by defining the first argument of the closure:
enterPersonalDetails { on("submit") { PersonDetailsCommand cmd -> flow.personDetails = cmd !flow.personDetails.validate() ? error() : success() }.to "enterShipping" on("return").to "showCart" }
6.5.6 Subflows and ConversationsGrails' Web Flow integration also supports subflows. A subflow is like a flow within a flow. For example take this search flow:
It references a subflow in the
def searchFlow = { displaySearchForm { on("submit").to "executeSearch" } executeSearch { action { [results:searchService.executeSearch(params.q)] } on("success").to "displayResults" on("error").to "displaySearchForm" } displayResults { on("searchDeeper").to "extendedSearch" on("searchAgain").to "displaySearchForm" } extendedSearch { subflow(controller: "searchExtensions", action: "extendedSearch") // <--- extended search subflow on("moreResults").to "displayMoreResults" on("noResults").to "displayNoMoreResults" } displayMoreResults() displayNoMoreResults() }
extendedSearch state. The controller parameter is optional if the subflow is defined in the same controller as the calling flow.
Prior to 1.3.5, the previous subflow call would look likeThe subflow is another flow entirely:
subflow(extendedSearchFlow), with the requirement that the name of the subflow state was the same as the called subflow (minus
Flow). This way of calling a subflow is deprecated and only supported for backward compatibility.
Notice how it places the
def extendedSearchFlow = { startExtendedSearch { on("findMore").to "searchMore" on("searchAgain").to "noResults" } searchMore { action { def results = searchService.deepSearch(ctx.conversation.query) if(!results)return error() conversation.extendedResults = results } on("success").to "moreResults" on("error").to "noResults" } moreResults() noResults() }
extendedResults in conversation scope. This scope differs to flow scope as it allows you to share state that spans the whole conversation not just the flow. Also notice that the end state (either
moreResults or
noResults of the subflow triggers the events in the main flow:
extendedSearch { subflow(controller: "searchExtensions", action: "extendedSearch") // <--- extended search subflow on("moreResults").to "displayMoreResults" on("noResults").to "displayNoMoreResults" }
6.6 FiltersAlthough Grails controllers support fine grained interceptors, these are only really useful when applied to a few controllers and become difficult to manage with larger applications. Filters on the other hand can be applied across a whole group of controllers, a URI space or a to a specific action. Filters are far easier to plug-in and maintain completely separately to your main controller logic and are useful for all sorts of cross cutting concerns such as security, logging, and so on.
6.6.1 Applying FiltersTo create a filter create a class that ends with the convention
Filters in the
grails-app/conf directory. Within this class define a code block called
filters that contains the filter definitions:
Each filter you define within the
class ExampleFilters { def filters = { // your filters here } }
filters block has a name and a scope. The name is the method name and the scope is defined using named arguments. For example if you need to define a filter that applies to all controllers and all actions you can use wildcards:
The scope of the filter can be one of the following things:
sampleFilter(controller:'*', action:'*') { // interceptor definitions }
- A controller and/or action name pairing with optional wildcards
- A URI, with Ant path matching syntax
controller- controller matching pattern, by default * is replaced with .* and a regex is compiled
action- action matching pattern, by default * is replaced with .* and a regex is compiled
regex(true/false) - use regex syntax (don't replace '*' with '.*')
uri- a uri to match, expressed with as Ant style path (e.g. /book/**)
find(true/false) - rule matches with partial match (see java.util.regex.Matcher.find())
invert(true/false) - invert the rule (NOT rule)
- All controllers and actions
all(controller:'*', action:'*') {}
- Only for the
justBook(controller:'book', action:'*') {}
- All controllers except the
notBook(controller:'book', invert:true) {}
- All actions containing 'save' in the action name
saveInActionName(action:'save', find:true) {}
- Applied to a URI space
someURIs(uri:'/book/**') {}
- Applied to all URIs
In addition, the order in which you define the filters within the
allURIs(uri:'/**') {}
filters code block dictates the order in which they are executed. To control the order of execution between
Filters classes, you can use the
dependsOn property discussed in filter dependencies section.
6.6.2 Filter TypesWithin the body of the filter you can then define one or several of the following interceptor types for the filter:
before- Executed before the action. Can return false to indicate all future filters and the action should not execute
after- Executed after an action. Takes a first argument as the view model
afterView- Executed after view rendering. Takes an Exception as an argument. Note: this closure is called before the layout is applied.
Here the
class SecurityFilters { def filters = { loginCheck(controller:'*', action:'*') { before = { if(!session.user && !actionName.equals('login')) { redirect(action:'login') return false } } } } }
loginCheck filter uses a
before interceptor to execute a block of code that checks if a user is in the session and if not redirects to the login action. Note how returning false ensure that the action itself is not executed.
6.6.3 Variables and ScopesFilters support all the common properties available to controllers and tag libraries, plus the application context:
- request - The HttpServletRequest object
- response - The HttpServletResponse object
- session - The HttpSession object
- servletContext - The ServletContext object
- flash - The flash object
- params - The request parameters object
- actionName - The action name that is being dispatched to
- controllerName - The controller name that is being dispatched to
- grailsApplication - The Grails application currently running
- applicationContext - The ApplicationContext object
6.6.4 Filter DependenciesIn a
Filters class, you can specify any other
Filters classes that should first be executed using the
dependsOn property. The
dependsOn property is used when a
Filters class depends on the behavior of another
Filters class (e.g. setting up the environment, modifying the request/session, etc.) and is defined as an array of
Filters classes.Take the following example
Filters classes:
MyFilters specifically
class MyFilters { def dependsOn = [MyOtherFilters] def filters = { checkAwesome(uri:"/*") { before = { if (request.isAwesome) { // do something awesome } } } checkAwesome2(uri:"/*") { before = { if (request.isAwesome) { // do something else awesome } } } } }class MyOtherFilters { def filters = { makeAwesome(uri:"/*") { before = { request.isAwesome = true; } } doNothing(uri:"/*") { before = { // do nothing } } } }
dependsOn MyOtherFilters. This will cause all the filters in MyOtherFilters to be executed before those in MyFilters, given their scope matches the current request. For a request of "/test", which will match the scope of every filter in the example, the execution order would be as follows:
- MyOtherFilters - makeAwesome
- MyOtherFilters - doNothing
- MyFilters - checkAwesome
- MyFilters - checkAwesome2
Filters classes are enabled and the execution order of filters within each
Filters class are preserved.If any cyclical dependencies are detected, the filters with cyclical dependencies will be added to the end of the filter chain and processing will continue. Information about any cyclical dependencies that are detected will be written to the logs. Ensure that your root logging level is set to at least WARN or configure an appender for the Grails Filters Plugin (org.codehaus.groovy.grails.plugins.web.filters.FiltersGrailsPlugin) when debugging filter dependency issues.
6.7 AjaxAjax stands for Asynchronous Javascript and XML and is the driving force behind the shift to richer web applications. These types of applications in general are better suited to agile, dynamic frameworks written in languages like Ruby and Groovy Grails provides support for building Ajax applications through its Ajax tag library for a full list of these see the Tag Library Reference.
6.7.1 Ajax using PrototypeBy default Grails ships with the Prototype library, but through the Plug-in system provides support for other frameworks such as Dojo Yahoo UI and the Google Web ToolkitThis section covers Grails' support for Prototype. To get started you need to add this line to the
<head> tag of your page:
<g:javascript library="prototype" />
This works because of Grails' support for adaptive tag libraries. Thanks to Grails' plugin system there is support for a number of different Ajax libraries including (but not limited to):
<g:javascript library="scriptaculous" />
- prototype
- dojo
- yui
- mootools
18.104.22.168 Remoting LinkingRemote content can be loaded in a number of ways, the most commons way is through the remoteLink tag. This tag allows the creation of HTML anchor tags that perform an asynchronous request and optionally set the response in an element. The simplest way to create a remote link is as follows:
The above link sends an asynchronous request to the
<g:remoteLink action="delete" id="1">Delete Book</g:remoteLink>
delete action of the current controller with an id of
188.8.131.52 Updating ContentThis is great, but usually you would want to provide some kind of feedback to the user as to what has happened:
GSP code:
def delete = {
def b = Book.get( params.id )
render "Book ${b.id} was deleted"
The above example will call the action and set the contents of the
<div id="message"></div> <g:remoteLink action="delete" id="1" update="message">Delete Book</g:remoteLink>
div to the response in this case
"Book 1 was deleted". This is done by the
update attribute on the tag, which can also take a map to indicate what should be updated on failure:
Here the
<div id="message"></div> <div id="error"></div> <g:remoteLink action="delete" id="1" update="[success:'message',failure:'error']">Delete Book</g:remoteLink>
error div will be updated if the request failed.
220.127.116.11 Remote Form SubmissionAn HTML form can also be submitted asynchronously in one of two ways. Firstly using the formRemote tag which expects similar attributes to those for the remoteLink tag:
Or alternatively you can use the submitToRemote tag to create a submit button. This allows some buttons to submit remotely and some not depending on the action:
<g:formRemote url="[controller:'book',action:'delete']" update="[success:'message',failure:'error']"> <input type="hidden" name="id" value="1" /> <input type="submit" value="Delete Book!" /> </g:formRemote >
<form action="delete"> <input type="hidden" name="id" value="1" /> <g:submitToRemote action="delete" update="[success:'message',failure:'error']" /> </form>
188.8.131.52 Ajax EventsSpecific javascript can be called if certain events occur, all the events start with the "on" prefix and allow you to give feedback to the user where appropriate, or take other action:
The above code will execute the "showProgress()" function which may show a progress bar or whatever is appropriate. Other events include:
<g:remoteLink action="show" id="1" update="success" onLoading="showProgress()" onComplete="hideProgress()">Show Book 1</g:remoteLink>
onSuccess- The javascript function to call if successful
onFailure- The javascript function to call if the call failed
on_ERROR_CODE- The javascript function to call to handle specified error codes (eg on404="alert('not found!')")
onUninitialized- The javascript function to call the a ajax engine failed to initialise
onLoading- The javascript function to call when the remote function is loading the response
onLoaded- The javascript function to call when the remote function is completed loading the response
onComplete- The javascript function to call when the remote function is complete, including any updates
XmlHttpRequest object you can use the implicit event parameter
e to obtain it:
<g:javascript> function fireMe(e) { alert("XmlHttpRequest = " + e) } } </g:javascript> <g:remoteLink action="example" update="success" onSuccess="fireMe(e)">Ajax Link</g:remoteLink>
6.7.2 Ajax with DojoGrails features an external plug-in to add Dojo support to Grails. To install the plug-in type the following command from the root of your project in a terminal window:
This will download the current supported version of Dojo and install it into your Grails project. With that done you can add the following reference to the top of your page:
grails install-plugin dojo
Now all of Grails tags such as remoteLink, formRemote and submitToRemote work with Dojo remoting.
<g:javascript library="dojo" />
6.7.3 Ajax with GWTGrails also features support for the Google Web Toolkit through a plug-in comprehensive documentation for can be found on the Grails wiki.
6.7.4 Ajax on the ServerAlthough Ajax features the X for XML there are a number of different ways to implement Ajax which are typically broken down into:
- Content Centric Ajax - Where you merely use the HTML result of a remote call to update the page
- Data Centric Ajax - Where you actually send an XML or JSON response from the server and programmatically update the page
- Script Centric Ajax - Where the server sends down a stream of Javascript to be evaluated on the fly
Content Centric AjaxJust to re-cap, content centric Ajax involves sending some HTML back from the server and is typically done by rendering a template with the render method:
Calling this on the client involves using the remoteLink tag:
def showBook = {
def b = Book.get(params.id) render(template:"bookTemplate", model:[book:b])
<g:remoteLink action="showBook" id="${book.id}" update="book${book.id}">Update Book</g:remoteLink><div id="book${book.id}"> <!--existing book mark-up --> </div>
Data Centric Ajax with JSONData Centric Ajax typically involves evaluating the response on the client and updating programmatically. For a JSON response with Grails you would typically use Grails' JSON marshaling capability:
And then on the client parse the incoming JSON request using an Ajax event handler:
import grails.converters.*def showBook = {
def b = Book.get(params.id) render b as JSON
<g:javascript> function updateBook(e) { var book = eval("("+e.responseText+")") // evaluate the JSON $("book" + book.id + "_title").innerHTML = book.title } <g:javascript> <g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">Update Book</g:remoteLink> <g:set var="bookId">book${book.id}</g:set> <div id="${bookId}"> <div id="${bookId}_title">The Stand</div> </div>
Data Centric Ajax with XMLOn the server side using XML is equally trivial:
However, since DOM is involved the client gets more complicated:
import grails.converters.*def showBook = {
def b = Book.get(params.id) render b as XML
<g:javascript> function updateBook(e) { var xml = e.responseXML var id = xml.getElementsByTagName("book").getAttribute("id") $("book" + id + "_title") = xml.getElementsByTagName("title")[0].textContent } <g:javascript> <g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">Update Book</g:remoteLink> <g:set var="bookId">book${book.id}</g:set> <div id="${bookId}"> <div id="${bookId}_title">The Stand</div> </div>
Script Centric Ajax with JavaScriptScript centric Ajax involves actually sending Javascript back that gets evaluated on the client. An example of this can be seen below:
The important thing to remember is to set the
def showBook = { def b = Book.get(params.id) response.contentType = "text/javascript" String title = b.title.encodeAsJavascript() render "$('book${b.id}_title')='${title}'" }
contentType to
text/javascript. If you are using Prototype on the client the returned Javascript will automatically be evaluated due to this
contentType setting.Obviously in this case it is critical that you have an agreed client-side API as you don't want changes on the client breaking the server. This is one of the reasons Rails has something like RJS. Although Grails does not currently have a feature such as RJS there is a Dynamic JavaScript Plug-in that offers similar capabilities.
6.8 Content NegotiationGrails has built in support for Content negotiation using either the HTTP
Accept header, an explicit format request parameter or the extension of a mapped URI.
Configuring Mime TypesBefore you can start dealing with content negotiation you need to tell Grails what content types you wish to support. By default Grails comes configured with a number of different content types within
grails-app/conf/Config.groovy using the
grails.mime.types setting:
The above bit of configuration allows Grails to detect to format of a request containing either the 'text/xml' or 'application/xml' media types as simply 'xml'. You can add your own types by simply adding new entries into the map.
grails.mime.types = [ xml: ['text/xml', 'application/xml'], text: 'text-plain', js: 'text/javascript', rss: 'application/rss+xml', atom: 'application/atom+xml', css: 'text/css', csv: 'text/csv', all: '*/*', json: 'text/json', html: ['text/html','application/xhtml+xml'] ]
Content Negotiation using the Accept headerEvery incoming HTTP request has a special Accept header that defines what media types (or mime types) a client can "accept". In older browsers this is typically:
Which simply means anything. However, on newer browser something all together more useful is sent such as (an example of a Firefox
Accept header):
Grails parses this incoming format and adds a
property to the request object that outlines the preferred request format. For the above example the following assertion would pass:
Why? The
assert 'html' == request.format
text/html media type has the highest "quality" rating of 0.9, therefore is the highest priority. If you have an older browser as mentioned previously the result is slightly different:
In this case 'all' possible formats are accepted by the client. To deal with different kinds of requests from Controllers you can use the withFormat method that acts as kind of a switch statement:
assert 'all' == request.format
What happens here is that if the preferred format is
import grails.converters.*class BookController { def books def list = { this.books = Book.list() withFormat { html bookList:books js { render "alert('hello')" } xml { render books as XML } } } }
html then Grails will execute the
html() call only. What this is does is make Grails look for a view called either
grails-app/views/books/list.html.gsp or
grails-app/views/books/list.gsp. If the format is
xml then the closure will be invoked and an XML response rendered.How do we handle the "all" format? Simply order the content-types within your
withFormat block so that whichever one you want executed comes first. So in the above example, "all" will trigger the
html handler.
When using withFormat make sure it is the last call in your controller action as the return value of the
withFormat method is used by the action to dictate what happens next.
Content Negotiation with the format Request ParameterIf fiddling with request headers if not your favorite activity you can override the format used by specifying a
format request parameter:
You can also define this parameter in the URL Mappings definition:
"/book/list"(controller:"book", action:"list") { format = "xml" }
Content Negotiation with URI ExtensionsGrails also supports content negotiation via URI extensions. For example given the following URI:
Grails will shave off the extension and map it to
/book/list instead whilst simultaneously setting the content format to
xml based on this extension. This behaviour is enabled by default, so if you wish to turn it off, you must set the
grails.mime.file.extensions property in
grails-app/conf/Config.groovy to
grails.mime.file.extensions = false
Testing Content NegotiationTo test content negotiation in an integration test (see the section on Testing) you can either manipulate the incoming request headers:
Or you can set the format parameter to achieve a similar effect:
void testJavascriptOutput() { def controller = new TestController() controller.request.addHeader "Accept", "text/javascript, text/html, application/xml, text/xml, */*" controller.testAction() assertEquals "alert('hello')", controller.response.contentAsString }
void testJavascriptOutput() { def controller = new TestController() controller.params.format = 'js' controller.testAction() assertEquals "alert('hello')", controller.response.contentAsString }