<dependency>
 <groupId>org.javassist</groupId>
 <artifactId>javassist</artifactId>
 <version>3.25.0-GA</version>
</dependency>

package com.rickiyang.learn.javassist;

import javassist.*;

/**
 * @author rickiyang
 * @date 2019-08-06
 * @Desc
 */
public class CreatePerson {

 /**
 * 創(chuàng)建一個(gè)Person 對(duì)象
 *
 * @throws Exception
 */
 public static void createPseson() throws Exception {
 ClassPool pool = ClassPool.getDefault();

 // 1. 創(chuàng)建一個(gè)空類
 CtClass cc = pool.makeClass("com.rickiyang.learn.javassist.Person");

 // 2. 新增一個(gè)字段 private String name;
 // 字段名為name
 CtField param = new CtField(pool.get("java.lang.String"), "name", cc);
 // 訪問(wèn)級(jí)別是 private
 param.setModifiers(Modifier.PRIVATE);
 // 初始值是 "xiaoming"
 cc.addField(param, CtField.Initializer.constant("xiaoming"));

 // 3. 生成 getter、setter 方法
 cc.addMethod(CtNewMethod.setter("setName", param));
 cc.addMethod(CtNewMethod.getter("getName", param));

 // 4. 添加無(wú)參的構(gòu)造函數(shù)
 CtConstructor cons = new CtConstructor(new CtClass[]{}, cc);
 cons.setBody("{name = \"xiaohong\";}");
 cc.addConstructor(cons);

 // 5. 添加有參的構(gòu)造函數(shù)
 cons = new CtConstructor(new CtClass[]{pool.get("java.lang.String")}, cc);
 // $0=this / $1,$2,$3... 代表方法參數(shù)
 cons.setBody("{$0.name = $1;}");
 cc.addConstructor(cons);

 // 6. 創(chuàng)建一個(gè)名為printName方法，無(wú)參數(shù)，無(wú)返回值，輸出name值
 CtMethod ctMethod = new CtMethod(CtClass.voidType, "printName", new CtClass[]{}, cc);
 ctMethod.setModifiers(Modifier.PUBLIC);
 ctMethod.setBody("{System.out.println(name);}");
 cc.addMethod(ctMethod);

 //這里會(huì)將這個(gè)創(chuàng)建的類對(duì)象編譯為.class文件
 cc.writeFile("/Users/yangyue/workspace/springboot-learn/java-agent/src/main/java/");
 }

 public static void main(String[] args) {
 try {
  createPseson();
 } catch (Exception e) {
  e.printStackTrace();
 }
 }
}

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.rickiyang.learn.javassist;

public class Person {
 private String name = "xiaoming";

 public void setName(String var1) {
 this.name = var1;
 }

 public String getName() {
 return this.name;
 }

 public Person() {
 this.name = "xiaohong";
 }

 public Person(String var1) {
 this.name = var1;
 }

 public void printName() {
 System.out.println(this.name);
 }
}

// 這里不寫入文件，直接實(shí)例化
Object person = cc.toClass().newInstance();
// 設(shè)置值
Method setName = person.getClass().getMethod("setName", String.class);
setName.invoke(person, "cunhua");
// 輸出值
Method execute = person.getClass().getMethod("printName");
execute.invoke(person);

ClassPool pool = ClassPool.getDefault();
// 設(shè)置類路徑
pool.appendClassPath("/Users/yangyue/workspace/springboot-learn/java-agent/src/main/java/");
CtClass ctClass = pool.get("com.rickiyang.learn.javassist.Person");
Object person = ctClass.toClass().newInstance();
// ...... 下面和通過(guò)反射的方式一樣去使用

package com.rickiyang.learn.javassist;

/**
 * @author rickiyang
 * @date 2019-08-07
 * @Desc
 */
public interface PersonI {

 void setName(String name);

 String getName();

 void printName();

}

ClassPool pool = ClassPool.getDefault();
pool.appendClassPath("/Users/yangyue/workspace/springboot-learn/java-agent/src/main/java/");

// 獲取接口
CtClass codeClassI = pool.get("com.rickiyang.learn.javassist.PersonI");
// 獲取上面生成的類
CtClass ctClass = pool.get("com.rickiyang.learn.javassist.Person");
// 使代碼生成的類，實(shí)現(xiàn) PersonI 接口
ctClass.setInterfaces(new CtClass[]{codeClassI});

// 以下通過(guò)接口直接調(diào)用 強(qiáng)轉(zhuǎn)
PersonI person = (PersonI)ctClass.toClass().newInstance();
System.out.println(person.getName());
person.setName("xiaolv");
person.printName();

package com.rickiyang.learn.javassist;

/**
 * @author rickiyang
 * @date 2019-08-07
 * @Desc
 */
public class PersonService {

 public void getPerson(){
 System.out.println("get Person");
 }

 public void personFly(){
 System.out.println("oh my god,I can fly");
 }
}

package com.rickiyang.learn.javassist;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.Modifier;

import java.lang.reflect.Method;

/**
 * @author rickiyang
 * @date 2019-08-07
 * @Desc
 */
public class UpdatePerson {

 public static void update() throws Exception {
 ClassPool pool = ClassPool.getDefault();
 CtClass cc = pool.get("com.rickiyang.learn.javassist.PersonService");

 CtMethod personFly = cc.getDeclaredMethod("personFly");
 personFly.insertBefore("System.out.println(\"起飛之前準(zhǔn)備降落傘\");");
 personFly.insertAfter("System.out.println(\"成功落地。。。。\");");


 //新增一個(gè)方法
 CtMethod ctMethod = new CtMethod(CtClass.voidType, "joinFriend", new CtClass[]{}, cc);
 ctMethod.setModifiers(Modifier.PUBLIC);
 ctMethod.setBody("{System.out.println(\"i want to be your friend\");}");
 cc.addMethod(ctMethod);

 Object person = cc.toClass().newInstance();
 // 調(diào)用 personFly 方法
 Method personFlyMethod = person.getClass().getMethod("personFly");
 personFlyMethod.invoke(person);
 //調(diào)用 joinFriend 方法
 Method execute = person.getClass().getMethod("joinFriend");
 execute.invoke(person);
 }

 public static void main(String[] args) {
 try {
  update();
 } catch (Exception e) {
  e.printStackTrace();
 }
 }
}