WAR (file format)

In software engineering, a WAR file (Web Application Resource or Web application ARchive) is a file used to distribute a collection of JAR-files, JavaServer Pages, Java Servlets, Java classes, XML files, tag libraries, static web pages (HTML and related files) and other resources that together constitute a web application.
Content and structure.
A WAR file may be digitally signed in the same way as a JAR file in order to allow others to determine where the source code came from.
There are special files and directories within a WAR file:
Advantages of WAR files.
Assuming production environments do not promote a fix without sufficient testing prior to deployment, a WAR file has a distinct advantage when properties files are used to identify environment specific variables. For example, an LDAP server in a testing environment may be something like codice_1. The LDAP server in a production environment is codice_2. An external properties file would define the link with some thing like:
 LINKED_PAGE=ldaps://testauth.example.com:636
The source code reads the property file to determine the target LDAP server. In this way, developers can be certain that the WAR file tested and verified is exactly the same as that which is being promoted to production.
Disadvantages of WAR files.
Some consider web deployment using WAR files to be disadvantageous when minor changes to source code are required for dynamic environments. Each change to source code must be repackaged and deployed in development. This does not require stopping the web server if configured for runtime deployment. 
Example.
The following sample "web.xml" file demonstrates the declaration and association of a servlet:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app
 PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
 "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
<web-app>
 <servlet>
 <servlet-name>HelloServlet</servlet-name>
 <servlet-class>mypackage.HelloServlet</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>HelloServlet</servlet-name>
 <url-pattern>/HelloServlet</url-pattern>
 </servlet-mapping>
 <resource-ref>
 <description>
 Resource reference to a factory for javax.mail.Session
 instances that may be used for sending electronic mail messages,
 preconfigured to connect to the appropriate SMTP server.
 </description>
 <res-ref-name>mail/Session</res-ref-name>
 <res-type>javax.mail.Session</res-type>
 <res-auth>Container</res-auth>
 </resource-ref>
</web-app>
The codice_3 directory is on the ClassLoader's classpath. (The classpath consists of a list of locations from which codice_4 files can be loaded and executed by the JVM.) The codice_3 directory contains the classes associated with the web application itself. 
Any JAR files placed in the codice_6 directory will also be placed on the ClassLoader's classpath.