Package jdk.javadoc.doclet

   public boolean run(DocletEnvironment environment)

import com.sun.source.doctree.DocCommentTree;
import com.sun.source.util.DocTrees;
import jdk.javadoc.doclet.Doclet;
import jdk.javadoc.doclet.DocletEnvironment;
import jdk.javadoc.doclet.Reporter;

import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.ElementFilter;
import javax.tools.Diagnostic.Kind;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
import java.util.Locale;
import java.util.Set;

public class Example implements Doclet {
    private Reporter reporter;
    private PrintWriter stdout;
    private String overviewFile;

    @Override
    public void init(Locale locale, Reporter reporter) {
        reporter.print(Kind.NOTE, "Doclet using locale: " + locale);
        this.reporter = reporter;
        stdout = reporter.getStandardWriter();
    }

    @Override
    public String getName() {
        return "Example";
    }

    @Override
    public Set<? extends Option> getSupportedOptions() {
        Option[] options = {
            new Option() {
                private final List<String> someOption = List.of(
                        "--overview-file",
                        "-overviewfile",
                        "-o"
                );

                @Override
                public int getArgumentCount() {
                    return 1;
                }

                @Override
                public String getDescription() {
                    return "an option with aliases";
                }

                @Override
                public Option.Kind getKind() {
                    return Option.Kind.STANDARD;
                }

                @Override
                public List<String> getNames() {
                    return someOption;
                }

                @Override
                public String getParameters() {
                    return "file";
                }

                @Override
                public boolean process(String opt, List<String> arguments) {
                    overviewFile = arguments.get(0);
                    return true;
                }
            }
        };

        return Set.of(options);
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        // support the latest release
        return SourceVersion.latest();
    }

    @Override
    public boolean run(DocletEnvironment docEnv) {
        reporter.print(Kind.NOTE, "overviewFile: " + overviewFile);

        // get the DocTrees utility class to access document comments
        DocTrees docTrees = docEnv.getDocTrees();

        // location of an element in the same directory as overview.html
        try {
            Element e = ElementFilter.typesIn(docEnv.getSpecifiedElements()).iterator().next();
            DocCommentTree docCommentTree
                    = docTrees.getDocCommentTree(e, overviewFile);
            if (docCommentTree != null) {
                stdout.println("Overview html: " + docCommentTree.getFullBody());
            }
        } catch (IOException missing) {
            reporter.print(Kind.ERROR, "No overview.html found.");
        }

        for (TypeElement t : ElementFilter.typesIn(docEnv.getIncludedElements())) {
            stdout.println(t.getKind() + ":" + t);
            for (Element e : t.getEnclosedElements()) {
                printElement(docTrees, e);
            }
        }
        return true;
    }

    private void printElement(DocTrees trees, Element e) {
        DocCommentTree docCommentTree = trees.getDocCommentTree(e);
        if (docCommentTree != null) {
            stdout.println("Element (" + e.getKind() + ": "
                    + e + ") has the following comments:");
            stdout.println("Entire body: " + docCommentTree.getFullBody());
            stdout.println("Block tags: " + docCommentTree.getBlockTags());
        }
    }
}

javadoc -docletpath doclet-classes \
  -doclet Example \
  --overview-file overview.html \
  --source-path source-location \
  source-location/Example.java