Merged configuration between parent & child?

[Andres Almiray]

Hello everyone,

I wonder if it's possible to define a plugin's configuration on both parent
& child in such a way that the resulting configuration is merged, first
the  parent's then the child's. My use case is defining a set of annotation
processors in the maven-compiler-plugin at the parent, also adding another
set in the child's maven-compiler-plugin.

The following example is NOT explicitly related to Lombok nor AutoValue,
they are used just as reference. Suggestions to skip Lombok and/or
AutoValue are not needed, thanks.

Given a parent pom such as

<project xmlns="http://maven.apache.org/POM/4.0.0"; xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance";
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd";>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.acme</groupId>
    <artifactId>parent</artifactId>
    <version>0.0.0</version>
    <packaging>pom</packaging>

    <properties>
        <auto-value.version>1.7.4</auto-value.version>
        <lombok.version>1.18.14</lombok.version>
    </properties>

    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                  <groupId>org.apache.maven.plugins</groupId>
                  <artifactId>maven-compiler-plugin</artifactId>
                  <version>3.8.1</version>
                  <configuration>
                      <annotationProcessorPaths>
                          <path>
                              <groupId>org.projectlombok</groupId>
                              <artifactId>lombok</artifactId>
                              <version>1.18.14</version>
                          </path>
                      </annotationProcessorPaths>
                  </configuration>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

And a child pom as

<project xmlns="http://maven.apache.org/POM/4.0.0"; xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance";
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd";>
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>com.acme</groupId>
        <artifactId>parent</artifactId>
        <version>0.0.0</version>
    </parent>

    <artifactId>child</artifactId>

    <build>
        <plugins>
            <plugin>
              <groupId>org.apache.maven.plugins</groupId>
              <artifactId>maven-compiler-plugin</artifactId>
              <version>3.8.1</version>
              <inherited>true</inherited>
              <configuration>
                  <annotationProcessorPaths>
                      <path>
                          <groupId>com.google.auto.value</groupId>
                          <artifactId>auto-value</artifactId>
                          <version>${auto-value.version}</version>
                      </path>
                  </annotationProcessorPaths>
              </configuration>
            </plugin>
        </plugins>
    </build>
</project>

The desired result when evaluating the child's pom is to have both Lombok
and AutoValue (in that order) in the compiler's configuration. As it
currently stands with Maven 3.6.3 only the child configuration is
available, as shown by running `mvn help:effective-pom`

      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.1</version>
        <executions>
          <execution>
            <id>default-compile</id>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
            <configuration>
              <annotationProcessorPaths>
                <path>
                  <groupId>com.google.auto.value</groupId>
                  <artifactId>auto-value</artifactId>
                  <version>1.7.4</version>
                </path>
              </annotationProcessorPaths>
            </configuration>
          </execution>
          <execution>
            <id>default-testCompile</id>
            <phase>test-compile</phase>
            <goals>
              <goal>testCompile</goal>
            </goals>
            <configuration>
              <annotationProcessorPaths>
                <path>
                  <groupId>com.google.auto.value</groupId>
                  <artifactId>auto-value</artifactId>
                  <version>1.7.4</version>
                </path>
              </annotationProcessorPaths>
            </configuration>
          </execution>
        </executions>
        <inherited>true</inherited>
        <configuration>
          <annotationProcessorPaths>
            <path>
              <groupId>com.google.auto.value</groupId>
              <artifactId>auto-value</artifactId>
              <version>1.7.4</version>
            </path>
          </annotationProcessorPaths>
        </configuration>
      </plugin>

Cheers,
Andres