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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:

<assembly xmlns="http://maven.apache.org/ASSEMBLY/2.2.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.2.0 http://maven.apache.org/xsd/assembly-2.2.0.xsd">
  <id>appserverA</id>
  <formats>
    <format>zip</format>
  </formats>
  <dependencySets>
    <dependencySet>
      <outputDirectory>/lib</outputDirectory>
      <includes>
        <include>application:logging</include>
        <include>application:core</include>
        <include>application:utils</include>
        <include>application:appserverA</include>
      </includes>
    </dependencySet>
  </dependencySets>
</assembly>

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

<assembly xmlns="http://maven.apache.org/ASSEMBLY/2.2.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.2.0 http://maven.apache.org/xsd/assembly-2.2.0.xsd">
  <id>appserverB</id>
  <formats>
    <format>zip</format>
  </formats>
  <dependencySets>
    <dependencySet>
      <outputDirectory>/lib</outputDirectory>
      <includes>
        <include>application:logging</include>
        <include>application:core</include>
        <include>application:utils</include>
        <include>application:appserverB</include>
      </includes>
    </dependencySet>
  </dependencySets>
</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:

<component>
  <dependencySets>
    <dependencySet>
      <outputDirectory>/lib</outputDirectory>
      <includes>
        <include>application:logging</include>
        <include>application:core</include>
        <include>application:utils</include>
      </includes>
    </dependencySet>
  </dependencySets>
</component>

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

<assembly xmlns="http://maven.apache.org/ASSEMBLY/2.2.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.2.0 http://maven.apache.org/xsd/assembly-2.2.0.xsd">
  <id>appserverA</id>
  <formats>
    <format>zip</format>
  </formats>
  <componentDescriptors>
    <componentDescriptor>src/assembly/component.xml</componentDescriptor>
  </componentDescriptors>
  <dependencySets>
    <dependencySet>
      <outputDirectory>/lib</outputDirectory>
      <includes>
        <include>application:appserverA</include>
      </includes>
    </dependencySet>
  </dependencySets>
</assembly>

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

<assembly xmlns="http://maven.apache.org/ASSEMBLY/2.2.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.2.0 http://maven.apache.org/xsd/assembly-2.2.0.xsd">
  <id>appserverB</id>
  <formats>
    <format>zip</format>
  </formats>
  <componentDescriptors>
    <componentDescriptor>src/assembly/component.xml</componentDescriptor>
  </componentDescriptors>
  <dependencySets>
    <dependencySet>
      <outputDirectory>/lib</outputDirectory>
      <includes>
        <include>application:appserverB</include>
      </includes>
    </dependencySet>
  </dependencySets>
</assembly>

The POM

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

<project>
  [...]
  <build>
    [...]
    <plugins>
      [...]
      <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>3.6.0</version>
        <configuration>
          <descriptors>
            <descriptor>src/assembly/appserverA-assembly.xml</descriptor>
            <descriptor>src/assembly/appserverB-assembly.xml</descriptor>
          </descriptors>
        </configuration>
      </plugin>
   [...]
</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:

  mvn assembly:single