Using Component Descriptors

Introduction

Suppose you have a project which will be distributed in two forms: one for use with appserver A and another for appserver B. And as customization for these two servers, you need to exclude some dependencies which are not used by the appserver you will be distributing.

NOTE: Although putting <excludes> inside <dependencySets> may provide the result we want, it is not recommended because if a new appserver becomes available, you will have to maintain the excludes of the other distributions to exclude dependencies meant for the new appserver.

This example demonstrate the use of <componentDescriptors>, more information can be found here.

The Assembly Descriptors

First, let's write the assembly descriptor for appserver A distribution. It should like this:

  1. <assembly xmlns="http://maven.apache.org/ASSEMBLY/2.0.0"
  2.     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  3.     xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.0.0 http://maven.apache.org/xsd/assembly-2.0.0.xsd">
  4.   <id>appserverA</id>
  5.   <formats>
  6.     <format>zip</format>
  7.   </formats>
  8.   <dependencySets>
  9.     <dependencySet>
  10.       <outputDirectory>/lib</outputDirectory>
  11.       <includes>
  12.         <include>application:logging</include>
  13.         <include>application:core</include>
  14.         <include>application:utils</include>
  15.         <include>application:appserverA</include>
  16.       </includes>
  17.     </dependencySet>
  18.   </dependencySets>
  19. </assembly>

The assembly descriptor for appserver B distribution would then be similar:

  1. <assembly xmlns="http://maven.apache.org/ASSEMBLY/2.0.0"
  2.     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  3.     xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.0.0 http://maven.apache.org/xsd/assembly-2.0.0.xsd">
  4.   <id>appserverB</id>
  5.   <formats>
  6.     <format>zip</format>
  7.   </formats>
  8.   <dependencySets>
  9.     <dependencySet>
  10.       <outputDirectory>/lib</outputDirectory>
  11.       <includes>
  12.         <include>application:logging</include>
  13.         <include>application:core</include>
  14.         <include>application:utils</include>
  15.         <include>application:appserverB</include>
  16.       </includes>
  17.     </dependencySet>
  18.   </dependencySets>
  19. </assembly>

From the two descriptors shown, we can say that there are three artifacts common for both, thus we separate them into a common component descriptor and save it as src/assembly/component.xml. Its contents would be:

  1. <component>
  2.   <dependencySets>
  3.     <dependencySet>
  4.       <outputDirectory>/lib</outputDirectory>
  5.       <includes>
  6.         <include>application:logging</include>
  7.         <include>application:core</include>
  8.         <include>application:utils</include>
  9.       </includes>
  10.     </dependencySet>
  11.   </dependencySets>
  12. </component>

Then the final assembly descriptor for the appserver A would be:

  1. <assembly xmlns="http://maven.apache.org/ASSEMBLY/2.0.0"
  2.     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  3.     xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.0.0 http://maven.apache.org/xsd/assembly-2.0.0.xsd">
  4.   <id>appserverA</id>
  5.   <formats>
  6.     <format>zip</format>
  7.   </formats>
  8.   <componentDescriptors>
  9.     <componentDescriptor>src/assembly/component.xml</componentDescriptor>
  10.   </componentDescriptors>
  11.   <dependencySets>
  12.     <dependencySet>
  13.       <outputDirectory>/lib</outputDirectory>
  14.       <includes>
  15.         <include>application:appserverA</include>
  16.       </includes>
  17.     </dependencySet>
  18.   </dependencySets>
  19. </assembly>

And the corresponding assembly descriptor for the appserver B then would be:

  1. <assembly xmlns="http://maven.apache.org/ASSEMBLY/2.0.0"
  2.     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  3.     xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.0.0 http://maven.apache.org/xsd/assembly-2.0.0.xsd">
  4.   <id>appserverB</id>
  5.   <formats>
  6.     <format>zip</format>
  7.   </formats>
  8.   <componentDescriptors>
  9.     <componentDescriptor>src/assembly/component.xml</componentDescriptor>
  10.   </componentDescriptors>
  11.   <dependencySets>
  12.     <dependencySet>
  13.       <outputDirectory>/lib</outputDirectory>
  14.       <includes>
  15.         <include>application:appserverB</include>
  16.       </includes>
  17.     </dependencySet>
  18.   </dependencySets>
  19. </assembly>

The POM

Now we should update the POM configuration of the project for the Assembly Plugin, which should look like:

  1. <project>
  2.   [...]
  3.   <build>
  4.     [...]
  5.     <plugins>
  6.       [...]
  7.       <plugin>
  8.         <artifactId>maven-assembly-plugin</artifactId>
  9.         <version>3.0.0</version>
  10.         <configuration>
  11.           <descriptors>
  12.             <descriptor>src/assembly/appserverA-assembly.xml</descriptor>
  13.             <descriptor>src/assembly/appserverB-assembly.xml</descriptor>
  14.           </descriptors>
  15.         </configuration>
  16.       </plugin>
  17.    [...]
  18. </project>

Creating The Distributions

Since we didn't configure the assembly plugin to always generate the configured assemblies during the project's normal build lifecycle, we create the distributions by:

  1.   mvn assembly:single