[java] How to add an extra source directory for maven to compile and include in the build jar?

In addition to the src/main/java, I am adding a src/bootstrap directory that I want to include in my build process, in other words, I want maven to compile and include the sources there in my build. How!?

Update the pom.xml as -

<project>   
 ....
    <build>
      <resources>
        <resource>
          <directory>src/main/config</directory>
        </resource>
      </resources>
     ...
    </build>
...
</project>

http://maven.apache.org/guides/mini/guide-using-one-source-directory.html

<build>
<sourceDirectory>../src/main/java</sourceDirectory>

also see

Maven compile with multiple src directories

With recent Maven versions (3) and recent version of the maven compiler plugin (3.7.0), I notice that adding a source folder with the build-helper-maven-plugin is not required if the folder that contains the source code to add in the build is located in the target folder or a subfolder of it.
It seems that the compiler maven plugin compiles any java source code located inside this folder whatever the directory that contains them.
For example having some (generated or no) source code in target/a, target/generated-source/foo will be compiled and added in the outputDirectory : target/classes.

You can add the directories for your build process like:

    ...
   <resources>
     <resource>
       <directory>src/bootstrap</directory>
     </resource>
   </resources>
   ...

The src/main/java is the default path which is not needed to be mentioned in the pom.xml