• 1 概述
• 2 类加载
• 3 字节码
• 4 简单使用

https://mp.weixin.qq.com/s/wDwU7ndRwHdxqIIR43YNuQ

ASM: https://asm.ow2.io/
Guid: https://www.baeldung.com/java-asm
极客时间：https://github.com/AndroidAdvanceWithGeektime/Chapter-ASM

1 概述

• 背景：高效修改字节码，编码 source code 中的功能需要拓展，修改 、增强

• ASM ：字节码修改工具，可以修改class文件，实现在 非运行期 修改字节码

• ASM 和平台无关提供修改字节码的能力

• 场景

  • 代码插桩
  • 性能优化
  • 热修复
  • AOP编程

字节码 处理

2 类加载

方法区： 保存 Class 相关信息，同类存一份，method等相关信息

自定义类加载器

3 字节码

操作数入栈
操作符入栈

（最底层控制线程栈执行方法的寄存器 ebp esp）

一个个方法都构成一个个 栈帧 运行在线程

线程本身也是以栈来执行方法

线程栈最下面就是main 或者 run

然后调用的方法会在其上方入栈执行

入栈方法占用栈空间，执行结束从线程栈pop

一段代码被多个线程执行，那么其中的寄存器，或者内存中的存储变量就会被多个线程同时操作，出现并发问题

---

对于修改字节码，可以使用工具直接生成修改的代码
IDE Plugin ： ASM ByteCode Outline

---

java code

public class MainActivity {

    private int id;

    public MainActivity(int id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return super.toString();
    }

    public int sum(int l, int r){
        int result = l + r;
        return result;
    }
}

---

bytecode

// class version 49.0 (49)
// access flags 0x21
public class MainActivity {

  // compiled from: MainActivity.java

  // access flags 0x2
  private I id

  // access flags 0x1
  public <init>(I)V
   L0
    LINENUMBER 5 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L1
    LINENUMBER 6 L1
    ALOAD 0
    ILOAD 1
    PUTFIELD MainActivity.id : I
   L2
    LINENUMBER 7 L2
    RETURN
   L3
    LOCALVARIABLE this LMainActivity; L0 L3 0
    LOCALVARIABLE id I L0 L3 1
    MAXSTACK = 2
    MAXLOCALS = 2

  // access flags 0x1
  public toString()Ljava/lang/String;
   L0
    LINENUMBER 11 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.toString ()Ljava/lang/String;
    ARETURN
   L1
    LOCALVARIABLE this LMainActivity; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x1
  public sum(II)I
   L0
    LINENUMBER 15 L0
    ILOAD 1
    ILOAD 2
    IADD
    ISTORE 3
   L1
    LINENUMBER 16 L1
    ILOAD 3
    IRETURN
   L2
    LOCALVARIABLE this LMainActivity; L0 L2 0
    LOCALVARIABLE l I L0 L2 1
    LOCALVARIABLE r I L0 L2 2
    LOCALVARIABLE result I L1 L2 3
    MAXSTACK = 2
    MAXLOCALS = 4
}

---

ASMifed

import java.util.*;

import org.objectweb.asm.*;

public class MainActivityDump implements Opcodes {

    public static byte[] dump() throws Exception {

        ClassWriter cw = new ClassWriter(0);
        FieldVisitor fv;
        MethodVisitor mv;
        AnnotationVisitor av0;

        cw.visit(V1_5, ACC_PUBLIC + ACC_SUPER, "MainActivity", null, "java/lang/Object", null);

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

        {
            fv = cw.visitField(ACC_PRIVATE, "id", "I", null, null);
            fv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "<init>", "(I)V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(5, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLineNumber(6, l1);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitVarInsn(ILOAD, 1);
            mv.visitFieldInsn(PUTFIELD, "MainActivity", "id", "I");
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLineNumber(7, l2);
            mv.visitInsn(RETURN);
            Label l3 = new Label();
            mv.visitLabel(l3);
            mv.visitLocalVariable("this", "LMainActivity;", null, l0, l3, 0);
            mv.visitLocalVariable("id", "I", null, l0, l3, 1);
            mv.visitMaxs(2, 2);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "toString", "()Ljava/lang/String;", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(11, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "toString", "()Ljava/lang/String;", false);
            mv.visitInsn(ARETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "LMainActivity;", null, l0, l1, 0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "sum", "(II)I", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(15, l0);
            mv.visitVarInsn(ILOAD, 1);
            mv.visitVarInsn(ILOAD, 2);
            mv.visitInsn(IADD);
            mv.visitVarInsn(ISTORE, 3);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLineNumber(16, l1);
            mv.visitVarInsn(ILOAD, 3);
            mv.visitInsn(IRETURN);
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLocalVariable("this", "LMainActivity;", null, l0, l2, 0);
            mv.visitLocalVariable("l", "I", null, l0, l2, 1);
            mv.visitLocalVariable("r", "I", null, l0, l2, 2);
            mv.visitLocalVariable("result", "I", null, l1, l2, 3);
            mv.visitMaxs(2, 4);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }
}

4 简单使用

通过一个例子简单学习ASM的使用

目的：在指定方法执行之前 插入一行代码

修改前

public class MainActivity {
    @Override
    public String toString() {
        return super.toString();
    }
}

修改后

public class MainActivity {
    public String toString() {
        System.out.println("插入：Hello World!");
        return super.toString();
    }
}

实现：

/**
 * @author gongshijie
 *
 */
public class Main {

    public static void main(String[] args) {

        try {
            // 修改字节码
            ClassReader classReader = new ClassReader(MainActivity.class.getName());
            ClassWriter classWriter = new ClassWriter(classReader, ClassWriter.COMPUTE_MAXS);
            ClassVisitor classVisitor = new LogVisitor(Opcodes.ASM5, classWriter);
            classReader.accept(classVisitor, ClassReader.EXPAND_FRAMES);
            byte[] bytecode = classWriter.toByteArray();

            // 修改后的结果
            FileOutputStream fos = new FileOutputStream(new File("MainActivity.class"));
            fos.write(bytecode);
            fos.flush();
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

/**
 * @author gongshijie
 */
public class MainActivity {
    @Override
    public String toString() {
        return super.toString();
    }
}

/**
 * @author gongshijie
 * 遍历方法 定位 目标方法
 */
public class LogVisitor extends ClassVisitor {
    public LogVisitor(int i) {
        super(i);
    }

    public LogVisitor(int i, ClassVisitor classVisitor) {
        super(i, classVisitor);
    }

    @Override
    public MethodVisitor visitMethod(int i, String s, String s1, String s2, String[] strings) {
        MethodVisitor methodVisitor = cv.visitMethod(i, s, s1, s2, strings);
        if (s.equals("toString")) {
            // 若是我们需要增强的方法 toString() 就进入我们的 visitor 操作字节码
            methodVisitor = new LogAdapter(Opcodes.ASM5, methodVisitor, i, s, s1);
        }
        return methodVisitor;
    }
}

/**
 * @author gongshijie
 * 对 目标方法 进行字节码修改
 */
public class LogAdapter extends AdviceAdapter {
    protected LogAdapter(int i, MethodVisitor methodVisitor, int i1, String s, String s1) {
        super(i, methodVisitor, i1, s, s1);
    }

    @Override
    protected void onMethodEnter() {
        mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        mv.visitLdcInsn("\u63d2\u5165\uff1aHello World!");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
    }
}

结果

总之， ASM提供修改字节码的能力，怎么使用可以结合场景
不会带来性能变化，在 编译期 完成字节码操作