Generate Javadoc for multimodule project - maven

Question

I have read everything I can find on solving this and my attempts still fail. The best I can do is to get the Javadoc of exactly one module to show up--the last one built. (For now, I'm not trying to bundle Javadoc into any JARs. I'm also not trying to do anything "site".) I just want to put Javadoc for easy access into a subdirectory under the project root.
Here's what's in my parent pom.xml:
<build>
<plugins>
.
.
.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>3.2.0</version>
<configuration>
<noqualifier>all</noqualifier>
<reportOutputDirectory>${user.dir}/documents</reportOutputDirectory>
<destDir>javadoc</destDir>
</configuration>
<executions>
<execution>
<id>attach-javadocs</id>
<phase>package</phase>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
What I'm putting into subordinate pom.xml files is identical to the above except for
<goals>
<goal>javadoc</goal>
</goals>
I have played with replacing the <execution> in the parent and sometimes subordinate pom.xml files with:
<execution>
<id>aggregate</id>
<goals>
<goal>aggregate</goal>
</goals>
</execution>
but it makes no difference.

Answer 1

I think the following configuration is the reason your reports get overwritten:
<configuration>
<reportOutputDirectory>${user.dir}/documents</reportOutputDirectory>
</configuration>
All module builds will be written to the same directory, hence overwriting the previous build.
The solution is to use the default output directory and configure the output directory for the aggregated javadoc instead. This way the reactor build will create javadoc output files in each module's target directory. These can then be used by the aggregate goal to be combined.
This can be done by configuring your parent POM as follows:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>3.2.0</version>
<configuration>
<!-- Default configuration for all reports -->
<noqualifier>all</noqualifier>
<destDir>javadoc</destDir>
</configuration>
<executions>
<execution>
<id>aggregate</id>
<goals>
<goal>aggregate</goal>
</goals>
<configuration>
<!-- Specific configuration for the aggregate report -->
<reportOutputDirectory>${user.dir}/documents</reportOutputDirectory>
<destDir>javadoc</destDir>
</configuration>
</execution>
...
</executions>
</plugin>
...
</plugins>
</build>
(there is no need for any additional configuration in the module POM files)
The aggregated javadoc can now be created by running
mvn compile javadoc:javadoc javadoc:aggregate
(note that the compile or package goal is required for reactor to resolve inter-module dependencies)