maven-javadoc-plugin does not generate javadocs

I have a project with the following structure:

parent
 |-api
 |-plugin (depends on api)

The project is build using the clean install package tasks from maven on the parent module. The JARs generate correctly but the javadocs don't. How can I fix this?

This is what my parent module's pom looks like:

<?xml version="1.0" encoding="UTF-8"?>
<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>PluginLogger</groupId>
    <artifactId>parent</artifactId>
    <version>1.1</version>
    <packaging>pom</packaging>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
    </properties>

    <modules>
        <module>plugin</module>
        <module>api</module>
    </modules>

    // repositories and dependencies go here...

    <build>
        <defaultGoal>clean install package</defaultGoal>

        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-javadoc-plugin</artifactId>
                    <version>3.6.3</version>
                    <configuration>
                        <outputDirectory>${project.build.directory}/javadoc</outputDirectory>
                        <sourceFileIncludes>
                            <include>**/me/suprB/pluginlogger/api/**/*.java</include>
                        </sourceFileIncludes>
                    </configuration>
                    <executions>
                        <execution>
                            <id>attach-javadocs</id>
                            <phase>package</phase>
                            <goals>
                                <goal>aggregate</goal>
                            </goals>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

I just want to generate the Javadocs for the "api" module, not the "plugin" one.

Solution

I created a small POC with a similar structure you provided. Based on my findings I would like to suggest the following:

Please do not use the trailing ** in the <include> tag
use reportOutputDirectory and destDir configurations instead of outputDirectory
In my opinion you should not use pluginManagement here, since the plugin is used only by the parent pom.

Sample Parent POM

This is the actual pom.xml for my PoC's parent module:

<?xml version="1.0" encoding="UTF-8"?>
<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.github.tcsizmadia</groupId>
    <artifactId>maven-javadoc-sandbox</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>
    <modules>
        <module>api</module>
        <module>plugin</module>
    </modules>

    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-javadoc-plugin</artifactId>
                <version>3.6.3</version>
                <executions>
                    <execution>
                        <id>attach-javadocs</id>
                        <phase>package</phase>
                        <goals>
                            <goal>aggregate</goal>
                        </goals>
                        <configuration>
                            <sourceFileIncludes>
                                <sourceFileInclude>com/github/tcsizmadia/api/**</sourceFileInclude>
                            </sourceFileIncludes>
                            <reportOutputDirectory>${project.build.directory}</reportOutputDirectory>
                            <destDir>javadoc</destDir>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Directory tree after issuing mvn clean install on parent module:

├── javadoc
│   ├── com
│   │   └── github
│   │       └── tcsizmadia
│   │           └── api
│   │               └── class-use
│   ├── legal
│   ├── resources
│   └── script-dir
│       └── images
└── javadoc-bundle-options

As you can see, it does not contain any documentation from plugin module - just from the api, as you requested.