通过Arthas修改并热发布代码

主要使用jad、mc、retransform三个命令：

jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.javamc /tmp/UserController.java -d /tmpretransform /tmp/com/example/demo/arthas/user/UserController.class

通过`jad`反编译看源码情况

[arthas@16897]$ jad demo.MathGameClassLoader:                                                                    
+-sun.misc.Launcher$AppClassLoader@5c647e05                                     +-sun.misc.Launcher$ExtClassLoader@4554617c                                   Location:                                                                       
/Users/admin/arthas-demo.jar                                                    /** Decompiled with CFR.*/package demo;import java.util.ArrayList;import java.util.List;import java.util.Random;import java.util.concurrent.TimeUnit;public class MathGame {private static Random random = new Random();public int illegalArgumentCount = 0;public static void main(String[] args) throws InterruptedException {MathGame game = new MathGame();while (true) {
/*16*/             game.run();
/*17*/             TimeUnit.SECONDS.sleep(1L);}}public void run() throws InterruptedException {try {
/*23*/             int number = random.nextInt() / 10000;
/*24*/             List<Integer> primeFactors = this.primeFactors(number);
/*25*/             MathGame.print(number, primeFactors);}catch (Exception e) {
/*28*/             System.out.println(String.format("illegalArgumentCount:%3d, ", this.illegalArgumentCount) + e.getMessage());}}public static void print(int number, List<Integer> primeFactors) {StringBuffer sb = new StringBuffer(number + "=");
/*34*/         for (int factor : primeFactors) {
/*35*/             sb.append(factor).append('*');}
/*37*/         if (sb.charAt(sb.length() - 1) == '*') {
/*38*/             sb.deleteCharAt(sb.length() - 1);}
/*40*/         System.out.println(sb);}public List<Integer> primeFactors(int number) {
/*44*/         if (number < 2) {
/*45*/             ++this.illegalArgumentCount;throw new IllegalArgumentException("number is: " + number + ", need >= 2");}ArrayList<Integer> result = new ArrayList<Integer>();
/*50*/         int i = 2;
/*51*/         while (i <= number) {
/*52*/             if (number % i == 0) {
/*53*/                 result.add(i);
/*54*/                 number /= i;
/*55*/                 i = 2;continue;}
/*57*/             ++i;}
/*61*/         return result;}}Affect(row-cnt:1) cost in 491 ms.
[arthas@16897]$

下载源代码

jad --source-only demo.MathGame > /Users/admin/MathGame.java

修改源代码

vim /Users/admin/MathGame.java

我在中间加了一行代码。
在这里插入图片描述

从新编译源代码

mc /Users/admin/MathGame.java -d /Users/admin/

[arthas@17448]$ mc /Users/admin/MathGame.java -d /Users/admin/
Memory compiler output:
/Users/admin/demo/MathGame.class
Affect(row-cnt:1) cost in 355 ms.

重载class文件

retransform /Users/admin/demo/MathGame.class

[arthas@17448]$ retransform /Users/admin/demo/MathGame.class
retransform success, size: 1, classes:
demo.MathGame

代码已经实时生效了
在这里插入图片描述

retransform 的限制

不允许新增加 field/method
正在跑的函数，没有退出不能生效，比如下面新增加的System.out.println，只有run()函数里的会生效

public class MathGame {public static void main(String[] args) throws InterruptedException {MathGame game = new MathGame();while (true) {game.run();TimeUnit.SECONDS.sleep(1);// 这个不生效，因为代码一直跑在 while里System.out.println("in loop");}}public void run() throws InterruptedException {// 这个生效，因为run()函数每次都可以完整结束System.out.println("call run()");try {int number = random.nextInt();List<Integer> primeFactors = primeFactors(number);print(number, primeFactors);} catch (Exception e) {System.out.println(String.format("illegalArgumentCount:%3d, ", illegalArgumentCount) + e.getMessage());}}
}