前些天看aop就看到了cglib，看cglib又看到了asm，模仿着做了个示例。利用asm修改字节码，能实现编译不通过执行通的过的效果，挺有意思。

一个简单的待修改类：

package com.asm.zjc;
public class C {
	public void m() throws InterruptedException{
		Thread.sleep(300);
	}
}

package com.asm.zjc;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

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

public class Generator {
	public static void main(String[] arg){
		try{
			ClassReader cr = new ClassReader("com/asm/zjc/C");
			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 = http://www.mamicode.com/cw.toByteArray();>
其中用到了类适配器：
package com.asm.zjc;
import org.objectweb.asm.*;

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){
			mv=new AddTimeMethodAdapter(mv);
		}
		return mv;
	}
	
	@Override
	public void visitEnd(){
		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, "timer", "J");
			mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");
			mv.visitInsn(Opcodes.LSUB);
			mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");
		}
		
		@Override
		public void visitInsn(int opcode){
			if((opcode>=Opcodes.IRETURN && opcode<=Opcodes.RETURN) || opcode==Opcodes.ATHROW){
				mv.visitFieldInsn(Opcodes.GETSTATIC, owner, "timer", "J");
				mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");
				mv.visitInsn(Opcodes.LADD);
				mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");
			}
			mv.visitInsn(opcode);
		}
		
		@Override
		public void visitMaxs(int maxStack, int maxLocal){
			mv.visitMaxs(maxStack+4, maxLocal);
		}
	}
}

最后写一个测试程序验证：
package com.asm.zjc;

public class Test {
	@SuppressWarnings("static-access")
	public static void main(String[] args)throws InterruptedException{
		C c1=new C();
		c1.m();
		System.out.println(C.timer);
	}
}

注意第8行是不能通过编译的，但可以执行成功。

其实我们用下面的命令行反编译class文件，也可以了解字节码的结构：
D:\software\eclipse\workspace\study_asm\bin>javap -c -verbose com.asm.zjc.C >aaa.java