Doclet-demo is a project mainly written in Java, it's free.
A java demo project how to write a javadoc extension a doclet.
A java demo project how to write a javadoc extension as a doclet. Or how to read custom javadoc tags.
The demo doclet code is in the class DocDoclet here: https://github.com/tux2323/doclet-demo/blob/master/src/main/java/com/github/tux2323/doclet/DocDoclet.java
See also the test DocDocletTest how to run a doclet from a java application: https://github.com/tux2323/doclet-demo/blob/master/src/test/java/com/github/tux2323/doclet/DocDocletTest.java