Dependency management does not work for multi-module project

Question

I have a Maven project with multiple modules and I'm trying to set it up so that module dependencies are automatically built to the correct lifecycle phase needed for building depending modules to the requested lifecycle phase.

In the example, the module plugin builds a Maven plugin, which is used to generate source code and is used by the module main. If I just try to use mvn -am -pl main compile, the module plugin is compiled but the process-classes lifecycle phase is not executed (which is necessary for a plugin to be usable). Compiling the module main then fails then with the following error:

[ERROR] Failed to parse plugin descriptor for example:plugin:1.0.0-SNAPSHOT (/Users/ims/Dropbox/IMS/Projects/PARITy_R4/codegen-test-simple/plugin/target/classes): No plugin descriptor found at META-INF/maven/plugin.xml -> [Help 1]

Is Maven, or a plugin for it, capable of resolving the dependencies of modules in a multi-module project and build them to stage necessary by other modules? And if so, how do I need to set up the project for this to work?

These are the POMs of my project:

pom.xml:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<project xmlns="http://maven.apache.org/POM/4.0.0">
    <modelVersion>4.0.0</modelVersion>
    <groupId>example</groupId>
    <artifactId>project</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>pom</packaging>
    <modules>
        <module>plugin</module>
        <module>main</module>
    </modules>
</project>

plugin/pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0">
    <modelVersion>4.0.0</modelVersion>
    <groupId>example</groupId>
    <artifactId>plugin</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>maven-plugin</packaging>
    <parent>
        <groupId>example</groupId>
        <artifactId>project</artifactId>
        <version>1.0.0-SNAPSHOT</version>
    </parent>
    <dependencies>
        <dependency>
            <groupId>org.apache.maven</groupId>
            <artifactId>maven-plugin-api</artifactId>
            <version>2.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.maven.plugin-tools</groupId>
            <artifactId>maven-plugin-annotations</artifactId>
            <version>3.2</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-plugin-plugin</artifactId>
                <version>3.2</version>
                <configuration>
                    <goalPrefix>configurator</goalPrefix>
                </configuration>
                <executions>
                    <execution>
                        <id>default-descriptor</id>
                        <goals>
                            <goal>descriptor</goal>
                        </goals>
                        <phase>process-classes</phase>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

main/pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0">
    <modelVersion>4.0.0</modelVersion>
    <groupId>example</groupId>
    <artifactId>main</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <parent>
        <groupId>example</groupId>
        <artifactId>project</artifactId>
        <version>1.0.0-SNAPSHOT</version>
    </parent>
    <build>
        <plugins>
            <plugin>
                <groupId>example</groupId>
                <artifactId>plugin</artifactId>
                <version>1.0.0-SNAPSHOT</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>codegen</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Answer 1

If you look at the reference documentation for the Maven lifecycle, you'll see that compile is before process-classes.

If you want this step to happen, you need to use mvn -am -pl main process-classes instead.

But I suggest that you always use mvn ... install - it also runs the tests and makes sure that the plugin which main uses is actually the one you think it should: Without install, the build will use an old/outdated version from the local repository (Maven will not magically determine "oh, there is a plugin in my reactor, I'll use that instead of the version from the local repo").