SampleJavacPlugin.java

import com.sun.source.tree.ClassTree;
import com.sun.source.util.*;
import com.sun.tools.javac.api.BasicJavacTask;
import com.sun.tools.javac.code.Flags;
import com.sun.tools.javac.tree.JCTree;
import com.sun.tools.javac.tree.TreeMaker;
import com.sun.tools.javac.util.Context;
import com.sun.tools.javac.util.List;
import com.sun.tools.javac.util.Names;

public class SampleJavacPlugin implements Plugin {

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

    @Override
    public void init(JavacTask task, String... args) {

        Context context = ((BasicJavacTask) task).getContext();

        task.addTaskListener(new TaskListener() {
            @Override
            public void started(TaskEvent e) {
            }

            @Override
            public void finished(TaskEvent e) {
                if (e.getKind() != TaskEvent.Kind.PARSE) {
                    return;
                }

                e.getCompilationUnit().accept(new TreeScanner<Void, Void>() {

                    @Override
                    public Void visitClass(ClassTree node, Void aVoid) {

                        TreeMaker maker = TreeMaker.instance(context);
                        Names symbolsTable = Names.instance(context);

                        JCTree.JCClassDecl myClass = maker
                                .at(((JCTree) node).pos)
                                .ClassDef(maker.Modifiers(Flags.PUBLIC | Flags.STATIC | Flags.FINAL),
                                        symbolsTable.fromString("MyClass"),
                                        List.nil(),
                                        null, // Here
                                        List.nil(),
                                        List.nil()
                                );

                        ((JCTree.JCClassDecl) node).defs = ((JCTree.JCClassDecl) node).defs.append(myClass);

                        return super.visitClass(node, aVoid);
                    }

                }, null);
            }
        });
    }
}