title: Error creating assembly archive in multi module maven project
body: <p>I have created a multi-module maven project with 2 modules(child1, child2).<br> I am trying to create a package - APP.zip which should contain app.properties, child1.war and child2.war by using assembly plugin.<br>But I am getting the below error when I run <strong>mvn assembly:single</strong> command</p>
  
  <pre><code>[ERROR] Failed to execute goal org.apache.maven.plugins:maven-assembly-plugin:2.6:single (default-cli) on project app: Failed to create assembly: Error creating assembly archive My_Package: You must set at least one file. -&gt; [Help 1]
  </code></pre>
  
  <p>Project folder structure</p>
  
  <pre><code>app
    |
     pom.xml
     assembly.xml
     app.properties 
     child 1
         | 
         pom.xml src target
                     |
                       child1.war
     child 2
         |
         pom.xml src target
                     |
                       child2.war   
  </code></pre>
  
  <p>Parent pom (app/pom.xml)</p>
  
  <pre><code>    &lt;project&gt;
          &lt;groupId&gt;com.karthik&lt;/groupId&gt;
          &lt;artifactId&gt;parent&lt;/artifactId&gt;
          &lt;packaging&gt;pom&lt;/packaging&gt;
          &lt;version&gt;1.0-SNAPSHOT&lt;/version&gt;
          &lt;name&gt;parent&lt;/name&gt;
  
          &lt;modules&gt;
             &lt;module&gt;child 1&lt;/module&gt;
             &lt;module&gt;child 2&lt;/module&gt;
          &lt;/modules&gt;
          &lt;build&gt;
             &lt;pluginManagement&gt;
                 &lt;plugins&gt;
                    &lt;plugin&gt;
                  &lt;groupId&gt;org.apache.maven.plugins&lt;/groupId&gt;
                  &lt;artifactId&gt;maven-assembly-plugin&lt;/artifactId&gt;
                  &lt;configuration&gt;
                      &lt;finalName&gt;APP&lt;/finalName&gt;
                      &lt;appendAssemblyId&gt;false&lt;/appendAssemblyId&gt;
                      &lt;descriptors&gt;
                      &lt;descriptor&gt;assembly.xml&lt;/descriptor&gt;
                      &lt;/descriptors&gt;
                  &lt;/configuration&gt;
                  &lt;executions&gt;
                       &lt;execution&gt;
                      &lt;id&gt;make-bundles&lt;/id&gt;
                      &lt;goals&gt;
                          &lt;goal&gt;single&lt;/goal&gt;
                      &lt;/goals&gt;
                      &lt;phase&gt;package&lt;/phase&gt;
                        &lt;/execution&gt;
                  &lt;/executions&gt;
              &lt;/plugin&gt;
          &lt;/plugins&gt;
    &lt;/pluginManagement&gt;
  &lt;/build&gt;
  &lt;/project&gt;
  </code></pre>
  
  <p>assembly.xml</p>
  
  <pre><code>&lt;assembly
      xmlns="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.2"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.2 http://maven.apache.org/xsd/assembly-1.1.2.xsd"&gt;
      &lt;formats&gt;
          &lt;format&gt;zip&lt;/format&gt;
      &lt;/formats&gt;
      &lt;id&gt;My_Package&lt;/id&gt;
      &lt;includeBaseDirectory&gt;false&lt;/includeBaseDirectory&gt;
      &lt;moduleSets&gt;
          &lt;moduleSet&gt;
              &lt;useAllReactorProjects&gt;true&lt;/useAllReactorProjects&gt;
              &lt;includes&gt;
                  &lt;include&gt;com.karthik:child1&lt;/include&gt;
                  &lt;include&gt;com.karthik:child2&lt;/include&gt;
              &lt;/includes&gt;
          &lt;/moduleSet&gt;
      &lt;/moduleSets&gt;
  &lt;/assembly&gt;
  </code></pre>
  
  <p>I believe I have missed something in the assembly descriptor that is causing this error.<br>
  1) Can you please help to resolve this issue &amp; help creating a zip(APP.zip) containing app.properties, child1.war and child2.war?<br>
  2) When do we need to use <code>&lt;fileSet&gt;</code> &amp; <code>&lt;moduleSet&gt;</code> tags?</p>