使用ASM操作Java字节码，实现AOP原理

本文通过一个的例子来实现：使用ASM动态生成Java字节码文件（.class） 或者 加载字节码后动态修改字节码，添加我们需要执行的代码，来模拟实现Spring AOP。

年底了，也没心情抠字了，把写demo包含的几个类代码直接贴出来吧，代码拷贝下来后可以直接使用，不会有什么其他错误。
使用 asm-5.0.3.jar
demo工程的package为com.shanhy.demo.asm.hello

5个Java文件：
AopClassAdapter.java 用来处理哪些方法需要进行修改
AopInteceptor.java 要修改的方法中，准备添加的我们自己的代码逻辑
AopMethodVisitor.java 修改字节码
HelloGeneratorClass.java 通过asm动态生成java类，测试的main方法也在这里
MyClassLoader.java 自定义ClassLoader

package com.shanhy.demo.asm.hello;

import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class AopClassAdapter extends ClassVisitor implements Opcodes {

    public AopClassAdapter(int api, ClassVisitor cv) {
        super(api, cv);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
        MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
        // 对test开头的方法进行特殊处理
        if (name.startsWith("test")) {
            mv = new AopMethodVisitor(this.api, mv);
        }
        return mv;
    }

}

package com.shanhy.demo.asm.hello;

public class AopInteceptor {

    public static void before(){
        System.out.println(".......before().......");
    }

    public static void after(){
        System.out.println(".......after().......");
    }


}

package com.shanhy.demo.asm.hello;

import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class AopMethodVisitor extends MethodVisitor implements Opcodes {

    public AopMethodVisitor(int api, MethodVisitor mv) {
        super(api, mv);
    }

    @Override
    public void visitCode() {
        super.visitCode();
        this.visitMethodInsn(INVOKESTATIC, "com/shanhy/demo/asm/hello/AopInteceptor", "before", "()V", false);
    }

    @Override
    public void visitInsn(int opcode) {
        if (opcode >= IRETURN && opcode <= RETURN)// 在返回之前安插after 代码。
            this.visitMethodInsn(INVOKESTATIC, "com/shanhy/demo/asm/hello/AopInteceptor", "after", "()V", false);
        super.visitInsn(opcode);
    }

}

package com.shanhy.demo.asm.hello;

import java.io.File;
import java.io.FileOutputStream;
import java.lang.reflect.Method;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

/** * 通过ASM生成类的字节码 * * @author Administrator * */
public class HelloGeneratorClass implements Opcodes {

    /** * 使用构造Hello类class字节码<br/> * package中的Hello.java只是代码原型，本例中只供对比用，没有实际使用到这个类。<br/> * ASM代码生成工具使用 bytecode * * @return * @throws Exception * @author SHANHY * @create 2016年2月3日 */
    public static byte[] generatorHelloClass() throws Exception {

        ClassWriter cw = new ClassWriter(0);
        FieldVisitor fv;
        MethodVisitor mv;

        cw.visit(V1_7, ACC_PUBLIC + ACC_SUPER, "com/shanhy/demo/asm/hello/Hello", null, "java/lang/Object", null);

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

        {
            fv = cw.visitField(ACC_PUBLIC + ACC_FINAL + ACC_STATIC, "FLAG", "Ljava/lang/String;", null,
                    "\u6211\u662f\u5e38\u91cf");
            fv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
            mv.visitInsn(RETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "display", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitInsn(ICONST_0);
            mv.visitVarInsn(ISTORE, 1);
            Label l1 = new Label();
            mv.visitLabel(l1);
            Label l2 = new Label();
            mv.visitJumpInsn(GOTO, l2);
            Label l3 = new Label();
            mv.visitLabel(l3);
            mv.visitFrame(Opcodes.F_APPEND, 1, new Object[] { Opcodes.INTEGER }, 0, null);
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitLdcInsn(">>>>>>>>>>\u6211\u662f\u5e38\u91cf");
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
            Label l4 = new Label();
            mv.visitLabel(l4);
            mv.visitIincInsn(1, 1);
            mv.visitLabel(l2);
            mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
            mv.visitVarInsn(ILOAD, 1);
            mv.visitIntInsn(BIPUSH, 8);
            mv.visitJumpInsn(IF_ICMPLT, l3);
            Label l5 = new Label();
            mv.visitLabel(l5);
            mv.visitInsn(RETURN);
            Label l6 = new Label();
            mv.visitLabel(l6);
            mv.visitMaxs(2, 2);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "testList", "()Ljava/util/List;", "()Ljava/util/List<Ljava/lang/String;>;",
                    null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitTypeInsn(NEW, "java/util/ArrayList");
            mv.visitInsn(DUP);
            mv.visitMethodInsn(INVOKESPECIAL, "java/util/ArrayList", "<init>", "()V", false);
            mv.visitVarInsn(ASTORE, 1);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitVarInsn(ALOAD, 1);
            mv.visitLdcInsn("Tome");
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
            mv.visitInsn(POP);
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitVarInsn(ALOAD, 1);
            mv.visitLdcInsn("Jack");
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
            mv.visitInsn(POP);
            Label l3 = new Label();
            mv.visitLabel(l3);
            mv.visitVarInsn(ALOAD, 1);
            mv.visitLdcInsn("Lily");
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
            mv.visitInsn(POP);
            Label l4 = new Label();
            mv.visitLabel(l4);
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
            mv.visitInsn(DUP);
            mv.visitLdcInsn(">>>>>>>>>>testList > list.size = ");
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "(Ljava/lang/String;)V", false);
            mv.visitVarInsn(ALOAD, 1);
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "size", "()I", true);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(I)Ljava/lang/StringBuilder;",
                    false);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
            Label l5 = new Label();
            mv.visitLabel(l5);
            mv.visitVarInsn(ALOAD, 1);
            mv.visitInsn(ARETURN);
            Label l6 = new Label();
            mv.visitLabel(l6);
            mv.visitMaxs(4, 2);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC + ACC_VARARGS, "testMapList", "(Z[Ljava/util/Map;)Ljava/util/List;",
                    "(Z[Ljava/util/Map<Ljava/lang/String;Ljava/lang/String;>;)Ljava/util/List<Ljava/util/Map<Ljava/lang/String;Ljava/lang/String;>;>;",
                    null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitTypeInsn(NEW, "java/util/ArrayList");
            mv.visitInsn(DUP);
            mv.visitMethodInsn(INVOKESPECIAL, "java/util/ArrayList", "<init>", "()V", false);
            mv.visitVarInsn(ASTORE, 3);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitVarInsn(ILOAD, 1);
            Label l2 = new Label();
            mv.visitJumpInsn(IFEQ, l2);
            Label l3 = new Label();
            mv.visitLabel(l3);
            mv.visitVarInsn(ALOAD, 2);
            mv.visitInsn(DUP);
            mv.visitVarInsn(ASTORE, 7);
            mv.visitInsn(ARRAYLENGTH);
            mv.visitVarInsn(ISTORE, 6);
            mv.visitInsn(ICONST_0);
            mv.visitVarInsn(ISTORE, 5);
            Label l4 = new Label();
            mv.visitJumpInsn(GOTO, l4);
            Label l5 = new Label();
            mv.visitLabel(l5);
            mv.visitFrame(Opcodes.F_FULL, 8,
                    new Object[] { "com/shanhy/demo/asm/hello/Hello", Opcodes.INTEGER, "[Ljava/util/Map;",
                            "java/util/List", Opcodes.TOP, Opcodes.INTEGER, Opcodes.INTEGER, "[Ljava/util/Map;" },
                    0, new Object[] {});
            mv.visitVarInsn(ALOAD, 7);
            mv.visitVarInsn(ILOAD, 5);
            mv.visitInsn(AALOAD);
            mv.visitVarInsn(ASTORE, 4);
            Label l6 = new Label();
            mv.visitLabel(l6);
            mv.visitVarInsn(ALOAD, 3);
            mv.visitVarInsn(ALOAD, 4);
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
            mv.visitInsn(POP);
            Label l7 = new Label();
            mv.visitLabel(l7);
            mv.visitIincInsn(5, 1);
            mv.visitLabel(l4);
            mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
            mv.visitVarInsn(ILOAD, 5);
            mv.visitVarInsn(ILOAD, 6);
            mv.visitJumpInsn(IF_ICMPLT, l5);
            mv.visitLabel(l2);
            mv.visitFrame(Opcodes.F_FULL, 4, new Object[] { "com/shanhy/demo/asm/hello/Hello", Opcodes.INTEGER,
                    "[Ljava/util/Map;", "java/util/List" }, 0, new Object[] {});
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
            mv.visitInsn(DUP);
            mv.visitLdcInsn(">>>>>>>>>>testMapList > list.size = ");
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "(Ljava/lang/String;)V", false);
            mv.visitVarInsn(ALOAD, 3);
            mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "size", "()I", true);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(I)Ljava/lang/StringBuilder;",
                    false);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
            Label l8 = new Label();
            mv.visitLabel(l8);
            mv.visitVarInsn(ALOAD, 3);
            mv.visitInsn(ARETURN);
            Label l9 = new Label();
            mv.visitLabel(l9);
            mv.visitMaxs(4, 8);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }

    /** * 生成保存class文件 * * @param args * @author SHANHY * @create 2016年2月3日 */
    public static void main3(String[] args) {
        try {
            byte[] data = generatorHelloClass();
            File file = new File("H:/Hello.class");
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(data);
            fos.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /** * AOP测试 * * @param args * @author SHANHY * @create 2016年2月3日 */
    public static void main(String[] args) {
        try {
            byte[] data = generatorHelloClass();

            ClassWriter cw = new ClassWriter(0);
            ClassReader cr = new ClassReader(data);
            cr.accept(new AopClassAdapter(ASM5,  cw), ClassReader.SKIP_DEBUG);

            data = cw.toByteArray();

            MyClassLoader myClassLoader = new MyClassLoader();
            Class<?> helloClass = myClassLoader.defineClass("com.shanhy.demo.asm.hello.Hello", data);
            Object obj = helloClass.newInstance();
            Method method = helloClass.getMethod("display", null);
            method.invoke(obj, null);

            method = helloClass.getMethod("testList", null);
            Object result = method.invoke(obj, null);
            System.out.println(result);

            Class<?> stuClass = Thread.currentThread().getContextClassLoader().loadClass("com.shanhy.demo.asm.example.StudentServiceImpl");
            obj = stuClass.newInstance();
            method = stuClass.getMethod("print", String.class);
            System.out.println(method.invoke(obj, "单单"));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

package com.shanhy.demo.asm.hello;

import org.objectweb.asm.Opcodes;

/** * 自定义ClassLoader * * @author 单红宇(365384722) * @myblog http://blog.csdn.net/catoop/ * @create 2016年2月2日 */
public class MyClassLoader extends ClassLoader implements Opcodes {


    public MyClassLoader() {
        super();
    }

    public MyClassLoader(ClassLoader parent) {
        super(parent);
    }

    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        return super.loadClass(name);
    }

    public Class<?> defineClass(String name, byte[] b){
        return super.defineClass(name, b, 0, b.length);
    }

}

最后这个是我们动态生成的Hello类的源码原型。

package com.shanhy.demo.asm.hello;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Hello {

    // 声明 一个常量
    public static final String FLAG = "我是常量";

    // 普通方法
    public void display(){
        for (int i = 0; i < 8; i++) {
            System.out.println(">>>>>>>>>>" + FLAG);
        }
    }

    // 带有List返回值
    public List<String> testList(){
        List<String> list = new ArrayList<>();
        list.add("Tome");
        list.add("Jack");
        list.add("Lily");
        System.out.println(">>>>>>>>>>testList > list.size = " + list.size());
        return list;
    }

    // 泛型返回值，包含List和Map
    // 两个参数，参数为Map动参
    public List<Map<String, String>> testMapList(boolean val, Map<String, String>... map){
        List<Map<String, String>> list = new ArrayList<>();
        if(val){
            for (Map<String, String> m : map) {
                list.add(m);
            }
        }
        System.out.println(">>>>>>>>>>testMapList > list.size = " + list.size());
        return list;
    }

}

仅供参考，关于ASM的API看看官方文档即可。bytecode这个Eclipse插件大家可以用一下，用了你就知道是干什么的了。
实际项目中一般不需要我们去用字节码来生成类，只有在特殊需求的时候才会使用。
话说“不常用的东西解决不常见的问题”嘛。