Append or merge default maven plugin configuration

Question

Is it possible to not override but merge or append to default plugin configuration in Apache Maven just like it's possible with parent POM configuration elements?

Answer 1

I'm note sure if i understand your questions correctly:

If you like for example to change the configuration of an already defined plugin you should be aware that you need to use the correct execution id which can be looked at during a default build which is printed out in the log output (something like this):

[INFO] --- maven-clean-plugin:3.0.0:clean (default-clean) @ parent ---
[INFO]
[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-maven) @ parent ---
[INFO]

The value in braces gives the hint: default-clean can now be used to add information to the configuration or also to change behaviour:

<plugins>
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-clean-plugin</artifactId>
    <executions>
      <execution>
        <id>default-clean</id>
        <configuration>
         <.. combine.children="append">
         </...>
        </configuration>

See more explanations following.

You can do this if you need. Lets say you have defined the following in a parent pom file:

<plugin>
 <groupId>..</groupId>
 <artifactId>..</artifactId>
 <configuration>
   <values>
     <value>First</value>
   </values>
 </configuration>
</plugin>

In an inheriting pom file you can now write the following:

<plugin>
 <groupId>..</groupId>
 <artifactId>..</artifactId>
 <configuration>
   <values combine.children="append">
     <value>Second</value>
   </values>
 </configuration>
</plugin>

Or if you do something different:

<plugin>
 <groupId>..</groupId>
 <artifactId>..</artifactId>
 <configuration>
   <values combine.children="override">
     <value>Second</value>
   </values>
 </configuration>
</plugin>

or you can give explicitly what is already the default:

<plugin>
 <groupId>..</groupId>
 <artifactId>..</artifactId>
 <configuration>
   <values combine.children="merge">
     <value>Second</value>
   </values>
 </configuration>
</plugin>

This is documented in the pom reference.