Today I’ve had a headache with the maven assembly plugin, and the way it inherits from a parent. The story goes as follows:

I have a uber parent pom, which defines the assembly plugin in the pluginManagement section, and it looks like this:

<artifactId>maven-assembly-plugin</artifactId>
<version>2.2-beta-1</version>
<configuration>
<descriptors>
<descriptor>src/assembly/kit.xml</descriptor>
</descriptors>
<finalName>${pom.artifactId}-${pom.version}</finalName>
<outputDirectory>build/maven/${pom.artifactId}/target</outputDirectory>
<workDirectory>build/maven/${pom.artifactId}/target/assembly/work</workDirectory>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
</execution>
</executions>

Then I have a parent pom for a collection of projects, and this parent pom has the following definition in it:

<artifactId>maven-assembly-plugin</artifactId>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>attached</goal>
</goals>
</execution>
</executions>
<configuration>
<descriptors>
<descriptor>src/main/assembly/kit.xml</descriptor>
</descriptors>
</configuration>

Now, if I simply put <artifactId>maven-assembly-plugin</artifactId> in one of the module’s pom files, it inherits most of everything from the project parent, which makes sense to me. The problem arises when we want to do something nifty with the assembly plugin, namely, create a jar with dependencies, and THEN include that in the package as described by an assembly descriptor which is different to the parent.

Here’s what I had in my project pom:

<artifactId>maven-assembly-plugin</artifactId>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>attached</goal>
</goals>
</execution>
<execution>
<id>copy-fields-conf-file</id>
<phase>package</phase>
<goals>
<goal>attached</goal>
</goals>
</execution>
</executions>
<configuration>
<descriptors>
<descriptor>src/main/assembly/kit.xml</descriptor>
</descriptors>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifestEntries>
<build-number>${build-number}</build-number>
</manifestEntries>
</archive>
</configuration>

The problem was that the initial make-assembly execution was inheriting its configuration from the parent, which tells it there’s an assembly descriptor in src/main/assembly/kit.xml. That file DOES exist there, but inside it, it has:

<file>
<source>build/maven/permissioning-auth-module/target/permissioning-auth-module-${project.version}-jar-with-dependencies.jar</source>
<outputDirectory>/</outputDirectory>
</file>

Now, this is a problem because this jar hasn’t been created yet, that’s what we’re trying to create! So, I commented this section out from the parent, only to find that it inherits it from the uber parent, and here’s why:

In the assembly plugin definition you have execution sections and configurations. You can tie a configuration in to an execution phase, by simply adding it inside the execution scope. If you don’t, and you leave it outside, then it becomes a general definition which is inherited by any child projects whenever they call the assembly plugin and don’t override it explicitly. This is usually fine, but not when you don’t want to declare an assembly descriptor. Because the asembly descriptor is defined in the parent(s), it always goes looking for it even if you don’t want it to in your execution (which we don’t). There’s a workaround: create a blank assembly descriptor and point your execution at that, but that’s not very elegant. The trick is to always tie your configurations in to an execution phase, so the parent pom(s) end up looking something more like this:

<artifactId>maven-assembly-plugin</artifactId>
<version>2.2-beta-1</version>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
<configuration>
<descriptors>
<descriptor>src/assembly/kit.xml</descriptor>
</descriptors>
<finalName>${pom.artifactId}-${pom.version}</finalName>
<outputDirectory>build/maven/${pom.artifactId}/target</outputDirectory>
<workDirectory>build/maven/${pom.artifactId}/target/assembly/work</workDirectory>
</configuration>
</execution>
</executions>

And the module’s pom can now look like this:

<artifactId>maven-assembly-plugin</artifactId>
<executions>
<execution>
<id>pack-assembly</id>
<phase>prepare-package</phase>
<goals>
<goal>single</goal>
</goals>
<configuration>
<descriptors>
</descriptors>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifestEntries>
<build-number>${build-number}</build-number>
</manifestEntries>
</archive>
</configuration>
</execution>
<execution>
<id>copy-fields-conf-file</id>
<phase>package</phase>
<goals>
<goal>attached</goal>
</goals>
<configuration>
<descriptors>
<descriptor>src/main/assembly/kit.xml</descriptor>
</descriptors>
</configuration>
</execution>
</executions>

Headache over 🙂