Custom maven plugin fails when mojo extends mojo from different module (no AbstractMojo) - maven

Question

I have mutli project with 2 maven plugin modules: base and child (<packaging>maven-plugin</packaging>). Child depend on base.
Base plugin has single class:
public abstract class BaseMojo extends AbstractMojo {}
with POM:
<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>plugin-set</groupId>
<artifactId>plugin-set</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>plugin-base</artifactId>
<packaging>maven-plugin</packaging>
<dependencies>
<dependency>
<groupId>org.apache.maven.plugin-tools</groupId>
<artifactId>maven-plugin-annotations</artifactId>
<version>3.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-core</artifactId>
<version>3.3.3</version>
<scope>provided</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-plugin-plugin</artifactId>
<version>3.4</version>
<configuration>
<goalPrefix>my</goalPrefix>
<skipErrorNoDescriptorsFound>true</skipErrorNoDescriptorsFound>
</configuration>
<executions>
<execution>
<id>mojo-descriptor</id>
<goals>
<goal>descriptor</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
Child plugin has single class:
public abstract class ChildMojo extends BaseMojo {}
with POM:
<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>plugin-set</groupId>
<artifactId>plugin-set</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>plugin-child</artifactId>
<packaging>maven-plugin</packaging>
<dependencies>
<dependency>
<groupId>org.apache.maven.plugin-tools</groupId>
<artifactId>maven-plugin-annotations</artifactId>
<version>3.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-core</artifactId>
<version>3.3.3</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>plugin-set</groupId>
<artifactId>plugin-base</artifactId>
<version>0.0.1-SNAPSHOT</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-plugin-plugin</artifactId>
<version>3.4</version>
<configuration>
<goalPrefix>my-child</goalPrefix>
<skipErrorNoDescriptorsFound>true</skipErrorNoDescriptorsFound>
</configuration>
<executions>
<execution>
<id>mojo-descriptor</id>
<goals>
<goal>descriptor</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
Base compiles correctly, child pass compilation but fails maven-plugin-plugin:3.4:descriptor:
Failed to execute goal org.apache.maven.plugins:maven-plugin-plugin:3.4:descriptor (default-descriptor) on project plugin-child: The API of the mojo scanner is not compatible with this plugin version. Please check the plugin dependencies configured in the POM and ensure the versions match. org/apache/maven/plugin/AbstractMojo: org.apache.maven.plugin.AbstractMojo -> [Help 1]
How to build child module?
Both base and child has almost identical pom.xml, child has 1 more dependency: to base
Source: https://github.com/michaldo/mojo-extend-mojo

Answer 1

The direct solution to your compilation issue is to remove <scope>provided</scope> from the maven-core dependency in plugin-child.
However, although it will compile after that, there are a lot of changes to be made to your project for it to work.
You should have a clear view of what a Mojo is and how it is implemented.
First of all, a Mojo should comply to a simple contract, which is implementing the Mojo interface.
Although the requirements on Mojos are minimal by design, there are still a very few requirements that Mojo developers must keep in mind. Basically, these Mojo requirements are embodied by the org.apache.maven.plugin.Mojo interface, which the Mojo must implement (or else extend its abstract base class counterpart org.apache.maven.plugin.AbstractMojo).
You can have a hierarchy of abstract classes but the top-most abstract one should inherit from AbstractMojo (or implement Mojo). Having a common abstract parent class is not uncommon. It is done by a lot of standard Maven plugins, like the maven-install-plugin.
What is important to note that the abstract classes are simply there to factor common code from multiple Mojo in a plugin. They do not represent a goal. As such, the plugin-base module, which contains your abstract superclass:
should not be packaged as a maven-plugin.
should not declare an execution of the maven-plugin-plugin plugin.
The concrete Mojo is inside plugin-child so it is this module that should be packaged as a maven-plugin. Since this will be the concrete implementation, it must not be abstract and it can be annotated with #Mojo to ease its declaration:
#Mojo(name = "child")
public class ChildMojo extends BaseMojo {
#Override
public void execute() throws MojoExecutionException, MojoFailureException {
}
}
This declares the goal "child" for this Mojo.
Sample code
plugin-base
<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>plugin-set</groupId>
<artifactId>plugin-set</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>plugin-base</artifactId>
<dependencies>
<dependency>
<groupId>org.apache.maven.plugin-tools</groupId>
<artifactId>maven-plugin-annotations</artifactId>
<version>3.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-core</artifactId>
<version>3.3.3</version>
</dependency>
</dependencies>
</project>
with the only class being
import org.apache.maven.plugin.AbstractMojo;
public abstract class BaseMojo extends AbstractMojo { }
plugin-child
<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>plugin-set</groupId>
<artifactId>plugin-set</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>plugin-child</artifactId>
<packaging>maven-plugin</packaging>
<dependencies>
<dependency>
<groupId>org.apache.maven.plugin-tools</groupId>
<artifactId>maven-plugin-annotations</artifactId>
<version>3.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-core</artifactId>
<version>3.3.3</version>
</dependency>
<dependency>
<groupId>plugin-set</groupId>
<artifactId>plugin-base</artifactId>
<version>${project.version}</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-plugin-plugin</artifactId>
<version>3.4</version>
<configuration>
<goalPrefix>my-child</goalPrefix>
</configuration>
<executions>
<execution>
<id>mojo-descriptor</id>
<goals>
<goal>descriptor</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
with the following Java code:
import org.apache.maven.plugin.MojoExecutionException;
import org.apache.maven.plugin.MojoFailureException;
import org.apache.maven.plugins.annotations.Mojo;
#Mojo(name = "child")
public class ChildMojo extends BaseMojo {
#Override
public void execute() throws MojoExecutionException, MojoFailureException {
}
}
Finally, you can test that this works by depending on this plugin:
<plugin>
<groupId>plugin-set</groupId>
<artifactId>plugin-child</artifactId>
<version>0.0.1-SNAPSHOT</version>
<executions>
<execution>
<id>foo</id>
<goals>
<goal>child</goal>
</goals>
<phase>install</phase>
</execution>
</executions>
</plugin>
If you build the test project depending on plugin-child then you will correctly see Maven invoking it, on the install phase here.