Test的代码比较简单

public class Test {
/**
 * @param args
 */
public static void main(String[] args) {
    test01();
}

static void test01() {
    System.out.println("Hello In Test01!");
}

}

对Test.class进行处理，添加test03的生成及调用

mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodAdapter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
public class Test_08 {
    /**
     * @param args
     * @throws IOException 
     */
    public static void main(String[] args) throws IOException {
        FileInputStream fis = new FileInputStream("D:\\Test.class");
        ClassReader reader = new ClassReader(fis);
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        reader.accept(new MyClassVisivator(cw), 0);
        writeToFile(cw.toByteArray(), "D:\\Test.class");
    }

    static void writeToFile(byte[] bytes, String fileName) {
        try {
            (new File(fileName)).createNewFile();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(fileName);
            fos.write(bytes);
            fos.flush();
        } catch (Exception e) {
        }finally{
            try {
                if (fos != null) {
                    fos.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    static class MyClassVisivator extends ClassAdapter {
        ClassVisitor mCv;
        public MyClassVisivator(ClassVisitor cv) {
            super(cv);
            mCv = cv;
        }

        @Override
        public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
            MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
            if ("main".equals(name)) {
                MyMethodVisitor mmv = new MyMethodVisitor(mv);
                return mmv;
            }
            return mv;
        }

        @Override
        public void visitEnd() {
            MethodVisitor mv = mCv.visitMethod(Opcodes.ACC_STATIC, "test03", "()V", null, null);
            if (mv != null) {
                mv.visitCode();
                mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
                mv.visitLdcInsn("Hello In Test03!");
                mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V");
                mv.visitInsn(Opcodes.RETURN);
                mv.visitMaxs(0, 0);
                mv.visitEnd();
            }
        }
    }

    static class MyMethodVisitor extends MethodAdapter {
        MethodVisitor mMv;
        public MyMethodVisitor(MethodVisitor mv) {
            super(mv);
            mMv = mv;
        }

        @Override
        public void visitInsn(int opcode) {
            if (opcode == Opcodes.RETURN) {
                System.out.println("Debug");
                mv.visitMethodInsn(Opcodes.INVOKESTATIC, "Test", "test03", "()V");
            }
            super.visitInsn(opcode);
        }

        @Override
        public void visitEnd() {
            super.visitEnd();
        }
    }
}

相关jar包

Java ASM 简单例子