首先实现一个字节码替换的玩具

package com.yyds;

public class TransClass {

public int getNumber() {

return 1;

}

}

将retrun改为2之后实例化运行，并改类名

package com.yyds;

public class MyAgentTest {

public static void main(String[] args) {

System.out.println(new TransClass().getNumber());

}

}

之后写一个Transformer类

package com.yyds;

import javassist.CannotCompileException;

import javassist.ClassPool;

import javassist.NotFoundException;

import java.io.ByteArrayOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import java.lang.instrument.ClassFileTransformer;

import java.lang.instrument.IllegalClassFormatException;

import java.security.ProtectionDomain;

public class Transformer implements ClassFileTransformer {

public static final String classNumberReturns2 = "E:\\testJar\\target\\classes\\com\\yyds\\TransClass.class.2";

public static byte[] getBytesFromFile(String fileName) throws Exception{

FileInputStream fileInputStream = new FileInputStream(new File("E:\\testJar\\target\\classes\\com\\yyds\\TransClass.class.2"));

int a;

byte[] bytes = new byte[1024];

ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

while ((a=fileInputStream.read(bytes))!= -1){

outputStream.write(bytes,0,a);

}

return outputStream.toByteArray();

}

@Override

public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) {

if (!className.equals("com/yyds/TransClass")) {

return null;

}

try {

return getBytesFromFile(classNumberReturns2);

} catch (Exception e) {

e.printStackTrace();

}

return null;

}

}

以及MyAgent，按上一篇文章生成jar后用-javaagent运行最后

package com.yyds;

import java.lang.instrument.Instrumentation;

public class MyAgent {

public static void premain(String args, Instrumentation inst) throws Exception {

inst.addTransformer(new Transformer());

}

}

最后结果不是1是2

redefineClasses也是差不多

public class MyAgent {

public static void premain(String agentArgs， Instrumentation inst) throws ClassNotFoundException， UnmodifiableClassException {

ClassDefinition def = new ClassDefinition(TransClass.class， Transformer.getBytesFromFile(Transformer.classNumberReturns2));

inst.redefineClasses(new ClassDefinition[] { def });

System.out.println("success");

}

}