关于java字节码框架ASM的学习

一、什么是ASM
  ASM是一个java字节码操纵框架,它能被用来动态生成类或者增强既有类的功能。ASM 可以直接产生二进制 class 文件,也可以在类被加载入 Java 虚拟机之前动态改变类行为。Java class 被存储在严格格式定义的 .class文件里,这些类文件拥有足够的元数据来解析类中的所有元素:类名称、方法、属性以及 Java 字节码(指令)。ASM从类文件中读入信息后,能够改变类行为,分析类信息,甚至能够根据用户要求生成新类。

  使用ASM框架需要导入asm的jar包,下载链接:asm-3.2.jar
  二、如何使用ASM
  ASM框架中的核心类有以下几个:
  ① ClassReader:该类用来解析编译过的class字节码文件。
  ② ClassWriter:该类用来重新构建编译后的类,比如说修改类名、属性以及方法,甚至可以生成新的类的字节码文件。
  ③ ClassAdapter:该类也实现了ClassVisitor接口,它将对它的方法调用委托给另一个ClassVisitor对象。
  示例1.通过asm生成类的字节码
[java]
package com.asm3;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;

/**

  • 通过asm生成类的字节码
  • @author Administrator
    /
    public class GeneratorClass {

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

    //生成一个类只需要ClassWriter组件即可
    ClassWriter cw = new ClassWriter(0);
    //通过visit方法确定类的头部信息
    cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT+Opcodes.ACC_INTERFACE,
            "com/asm3/Comparable", null, "java/lang/Object", new String[]{"com/asm3/Mesurable"});
    //定义类的属性
    cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
            "LESS", "I", null, new Integer(-1)).visitEnd();
    cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
            "EQUAL", "I", null, new Integer(0)).visitEnd();
    cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
            "GREATER", "I", null, new Integer(1)).visitEnd();
    //定义类的方法
    cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo",
            "(Ljava/lang/Object;)I", null, null).visitEnd();
    cw.visitEnd(); //使cw类已经完成
    //将cw转换成字节数组写到文件里面去
    byte[] data = cw.toByteArray();
    File file = new File("D://Comparable.class");
    FileOutputStream fout = new FileOutputStream(file);
    fout.write(data);
    fout.close();
    

    }
    }
    [/java]

 生成一个类的字节码文件只需要用到ClassWriter类即可,生成Comparable.class后用javap指令对其进行反编译:javap -c Comparable.class >test.txt ,编译后的结果如下
[java]
public interface com.asm3.Comparable extends com.asm3.Mesurable {
public static final int LESS;

public static final int EQUAL;

public static final int GREATER;

public abstract int compareTo(java.lang.Object);
}
[/java]

  示例2.修改类的字节码文件
[java]
package com.asm5;

public class C {
public void m() throws InterruptedException{
Thread.sleep(100);
}
}
[/java]
将C.java类的内容改为如下:
[java]
package com.asm5;

public class C {
public static long timer;
public void m() throws InterruptedException{
timer -= System.currentTimeMillis();
Thread.sleep(100);
timer += System.currentTimeMillis();
}
}
[/java]
 为了弄清楚ASM是如何实现的,我们先编译这两个类,然后比对它们的TraceClassVisitor的输出,我们可以发现如下的不同(粗体表示)

GETSTATIC C.timer : J
INVOKESTATIC java/lang/System.currentTimilis()J
LSUB
PUTSTATIC C.timer : J

LDC 100
INVOKESTATIC java/lang/Thread.sleep(J)V
GETSTATIC C.timer : J
INVOKESTATIC java/lang/System.currentTimilis()J
LADD
PUTSTATIC C.timer : J

RETURN
MAXSTACK=4
MAXLOCALS=1

  通过比对上面的指令,我们可以发现必须在m()方法的最前面增加四条指令,在RETURN指令前也增加四条指令,同时这四条必须位于xRETURN和ATHROW之前,因为这些指令都会结束方法的执行。

具体代码如下:

AddTimeClassAdapter.java
[java]
package com.asm5;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.MethodAdapter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class AddTimeClassAdapter extends ClassAdapter {
private String owner;
private boolean isInterface;
public AddTimeClassAdapter(ClassVisitor cv) {
super(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);
owner = name;
isInterface = (access & Opcodes.ACC_INTERFACE) != 0;
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc,
String signature, String[] exceptions) {
MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);
if(!name.equals("<init>") && !isInterface && mv!=null){
//为方法添加计时功能
mv = new AddTimeMethodAdapter(mv);
}
return mv;
}
@Override
public void visitEnd() {
//添加字段
if(!isInterface){
FieldVisitor fv = cv.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_STATIC, "timer", "J", null, null);
if(fv!=null){
fv.visitEnd();
}
}
cv.visitEnd();
}

class AddTimeMethodAdapter extends MethodAdapter{
    public AddTimeMethodAdapter(MethodVisitor mv) {
        super(mv);
    }
    @Override
    public void visitCode() {
        mv.visitCode();
        mv.visitFieldInsn(Opcodes.GETSTATIC, owner, &quot;timer&quot;, &quot;J&quot;);
        mv.visitMethodInsn(Opcodes.INVOKESTATIC, &quot;java/lang/System&quot;, &quot;currentTimeMillis&quot;, &quot;()J&quot;);
        mv.visitInsn(Opcodes.LSUB);
        mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, &quot;timer&quot;, &quot;J&quot;);
    }
    @Override
    public void visitInsn(int opcode) {
        if((opcode&gt;=Opcodes.IRETURN &amp;&amp; opcode&lt;=Opcodes.RETURN) || opcode==Opcodes.ATHROW){
            mv.visitFieldInsn(Opcodes.GETSTATIC, owner, &quot;timer&quot;, &quot;J&quot;);
            mv.visitMethodInsn(Opcodes.INVOKESTATIC, &quot;java/lang/System&quot;, &quot;currentTimeMillis&quot;, &quot;()J&quot;);
            mv.visitInsn(Opcodes.LADD);
            mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, &quot;timer&quot;, &quot;J&quot;);
        }
        mv.visitInsn(opcode);
    }
    @Override
    public void visitMaxs(int maxStack, int maxLocal) {
        mv.visitMaxs(maxStack+4, maxLocal);
    }
}

}
[/java]
Generator.java
[java]
package com.asm5;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;

public class Generator {

public static void main(String[] args){
    try {
        ClassReader cr = new ClassReader(&quot;com/asm5/C&quot;);
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        ClassAdapter classAdapter = new AddTimeClassAdapter(cw);
        //使给定的访问者访问Java类的ClassReader
        cr.accept(classAdapter, ClassReader.SKIP_DEBUG);
        byte[] data = cw.toByteArray();
        File file = new File(System.getProperty(&quot;user.dir&quot;) + &quot;\\WebRoot\\WEB-INF\\classes\\com\\asm5\\C.class&quot;);
        FileOutputStream fout = new FileOutputStream(file);
        fout.write(data);
        fout.close();
        System.out.println(&quot;success!&quot;);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

}
[/java]
下面是一个测试类:
[java]
package com.asm5;

public class Test {
public static void main(String[] args) throws InterruptedException, NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
C c = new C();
c.m();
Class cc = c.getClass();
System.out.println(cc.getField("timer").get(c));
}
}
[/java]
输出结果为:100