Java 字节码

// 修改字节码文件

public class MyClassVisitor extends ClassVisitor implements Opcodes {

    private static String methodName;

    public MyClassVisitor(ClassVisitor cv) {

        super(ASM5, cv);

    }

    @Override

    public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {

        cv.visit(version, access, name, signature, superName, interfaces);

    }

    @Override

    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {

        MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);

        // 忽略构造方法

        if (!name.equals("<init>") && mv != null) {

            methodName = name;

            mv = new MyMethodVisitor(mv);

        }

        return mv;

    }

    class MyMethodVisitor extends MethodVisitor implements Opcodes {

        public MyMethodVisitor(MethodVisitor mv) {

            super(Opcodes.ASM5, mv);

        }

        public void visitCode() {

            super.visitCode();

            // 方法进入时打印信息

            // 拿到 java.lang.System 类的.out static 字段，并放入栈顶

            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");

            // 将待输出信息放入栈顶

            mv.visitLdcInsn("method " + methodName + " is starting");

            // 调用 println 方法

            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

        }

        public void visitInsn(int opcode) {

            // 判断 return 或抛出异常

            if ((opcode >= Opcodes.IRETURN && opcode <= Opcodes.RETURN) || opcode == Opcodes.ATHROW) {

                mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");

                mv.visitLdcInsn("method " + methodName + " is ending");

                mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

            }

            mv.visitInsn(opcode);

        }

    }

}

// 生成字节码文件

public class Generator {

    public static void main(String[] args) throws Exception {

        modifyGreeting();

        createAsmGreeting();

    }

    public static void modifyGreeting() throws IOException {

        // 读取

        ClassReader classReader = new ClassReader("com/test/Greeting");

        ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS);

        // 处理

        ClassVisitor classVisitor = new MyClassVisitor(classWriter);

        classReader.accept(classVisitor, ClassReader.SKIP_DEBUG);

        byte[] data = classWriter.toByteArray();

        // 输出

        File f = new File("target/classes/com/test/Greeting.class");

        FileOutputStream fout = new FileOutputStream(f);

        fout.write(data);

        fout.close();

        System.out.println("Modify Greeting Class success!!!!!");

    }

    public static void createAsmGreeting() throws Exception {

        ClassWriter cw = new ClassWriter(0);

        MethodVisitor mv;

        // 主版本号设为 49，JDK5 之后都可以运行

        cw.visit(49, Opcodes.ACC_PUBLIC + Opcodes.ACC_SUPER, "com/test/AsmGreeting", null, "java/lang/Object", null);

        cw.visitSource("AsmGreeting.java", null);

        // 无参构造方法

        mv = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);

        mv.visitVarInsn(Opcodes.ALOAD, 0); // 将 this 放入栈顶

        // 调用 super ()

        mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);

        mv.visitInsn(Opcodes.RETURN);

        mv.visitMaxs(1, 1);

        mv.visitEnd();

        //hello 方法，无参无返回值

        mv = cw.visitMethod(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC, "hello", "()V", null, null);

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

        mv.visitLdcInsn("Hello, this class is genrated by ASM");

        mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

        mv.visitInsn(Opcodes.RETURN);

        mv.visitMaxs(2, 1);

        mv.visitEnd();

        cw.visitEnd();

        File f = new File("target/classes/com/test/AsmGreeting.class");

        FileOutputStream fout = new FileOutputStream(f);

        fout.write(cw.toByteArray());

        fout.close();

        System.out.println("Create AsmGreeting success!!!!!");

    }

}