asm demo

package com.gxf.asm;

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

import java.io.FileOutputStream;

/**
 * @Author: <guanxiangfei@meituan.com>
 * @Description:
 * @Date: Created in : 2018/8/19 下午6:15
 **/
public class Helloworld extends ClassLoader implements Opcodes {

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


        //定义一个叫做Example的类
        ClassWriter cw = new ClassWriter(0);
        cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

        //生成默认的构造方法
        MethodVisitor mw = cw.visitMethod(ACC_PUBLIC,
                "<init>",
                "()V",
                null,
                null);

        //生成构造方法的字节码指令
        mw.visitVarInsn(ALOAD, 0);
        mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
        mw.visitInsn(RETURN);
        mw.visitMaxs(1, 1);
        mw.visitEnd();

        //生成main方法
        mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC,
                "main",
                "([Ljava/lang/String;)V",
                null,
                null);

        //生成main方法中的字节码指令
        mw.visitFieldInsn(GETSTATIC,
                "java/lang/System",
                "out",
                "Ljava/io/PrintStream;");

        mw.visitLdcInsn("Hello world!");
        mw.visitMethodInsn(INVOKEVIRTUAL,
                "java/io/PrintStream",
                "println",
                "(Ljava/lang/String;)V");
        mw.visitInsn(RETURN);
        mw.visitMaxs(2, 2);

        //字节码生成完成
        mw.visitEnd();

        // 获取生成的class文件对应的二进制流
        byte[] code = cw.toByteArray();


        //将二进制流写到本地磁盘上
        FileOutputStream fos = new FileOutputStream("Example.class");
        fos.write(code);
        fos.close();

        //直接将二进制流加载到内存中
        Helloworld loader = new Helloworld();
        Class<?> exampleClass = loader.defineClass("Example", code, 0, code.length);

        //通过反射调用main方法
        exampleClass.getMethods()[0].invoke(null, new Object[] { null });


    }
}