Javaassist

package com.an.bytecode.javaassist;

import javassist.*;
import java.io.File;
import java.io.FileOutputStream;

/**
 * @description:
 * @author: anpeiyong
 * @date: Created in 2020/4/7 11:25
 * @since:
 */
public class JavaAssistTest {

    public static void main(String[] args) throws Exception {
        //CtClass对象容器
        ClassPool classPool=ClassPool.getDefault();
        //CtClass对象容器中创建一个public的JATest类
        CtClass jATestClazz=classPool.makeClass("com.an.bytecode.javaassist.JATest");

        //***属性操作
        //JATest类中添加private int id
        CtField ctIdField=new CtField(classPool.getCtClass("int"),"id",jATestClazz);
        ctIdField.setModifiers(Modifier.PRIVATE);
        jATestClazz.addField(ctIdField);

        //JATest类中添加private String username
        CtField ctUserNameField=new CtField(classPool.getCtClass("java.lang.String"),"username",jATestClazz);
        ctUserNameField.setModifiers(Modifier.PRIVATE);
        jATestClazz.addField(ctUserNameField);

        //添加getter,setter方法
        jATestClazz.addMethod(CtNewMethod.getter("getId",ctIdField));
        jATestClazz.addMethod(CtNewMethod.getter("setId",ctIdField));
        jATestClazz.addMethod(CtNewMethod.getter("getUsername",ctUserNameField));
        jATestClazz.addMethod(CtNewMethod.getter("setUsername",ctUserNameField));

        //添加构造函数
        CtConstructor ctConstructor=new CtConstructor(new CtClass[]{},jATestClazz);
        //添加构造函数方法体
        StringBuffer sb = new StringBuffer();
        sb.append("{\n").append("this.id = 27;\n").append("this.username=\"carl\";\n}");
        ctConstructor.setBody(sb.toString());
        jATestClazz.addConstructor(ctConstructor);

        // 添加自定义方法
        CtMethod method = new CtMethod(CtClass.voidType, "say", new CtClass[]{}, jATestClazz);
        method.setModifiers(Modifier.PUBLIC);
        StringBuffer printSb = new StringBuffer();
        printSb.append("{\nSystem.out.println(\"begin!\");\n")
                .append("System.out.println(id);\n")
                .append("System.out.println(username);\n")
                .append("System.out.println(\"end!\");\n")
                .append("}");
        method.setBody(printSb.toString());
        jATestClazz.addMethod(method);

        //生成一个Class对象
        Class<?> clazz=jATestClazz.toClass();
        Object object=clazz.newInstance();

        //反射执行方法
        clazz.getMethod("say",new Class[]{}).invoke(object,new Object[]{});

        //将生成的class写入文件中
        FileOutputStream fileOutputStream=new FileOutputStream(new File("JATest.class"));
        fileOutputStream.write(jATestClazz.toBytecode());
        fileOutputStream.close();

    }
}