信长笔记2.0

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JCA分析线程快照 ​

  • JCA是IBM提供的一个线程快照的图形化分析工具

下载链接: https://www.ibm.com/support/pages/ibm-thread-and-monitor-dump-analyzer-java-tmda

  • jstack -l <PID>生成线程快照, -l表示除堆栈外, 显示关于锁的附加信息
  • 生成线程快照的目的主要在于定位线程出现长时间停顿(死锁, 死循环, 慢线程等)或CPU使用率高的原因

线程状态 ​

1745408551436

  • new, 新建
  • runnable, 运行中, 可能持有锁(blocked)
  • blocked, 阻塞并等待监视器锁
  • waiting, 无限期等待
  • timed_waiting, 计时等待
  • terminated, 结束

分析线程死锁案例 ​

死锁示例代码 ​

java
public class DeadLockDemo {
    private static Object resource1 = new Object();//资源 1
    private static Object resource2 = new Object();//资源 2

    public static void main(String[] args) {
        new Thread(() -> {
            synchronized (resource1) {
                System.out.println(Thread.currentThread() + "get resource1");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get resource2");
                synchronized (resource2) {
                    System.out.println(Thread.currentThread() + "get resource2");
                }
            }
        }, "线程 1").start();

        new Thread(() -> {
            synchronized (resource2) {
                System.out.println(Thread.currentThread() + "get resource2");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get resource1");
                synchronized (resource1) {
                    System.out.println(Thread.currentThread() + "get resource1");
                }
            }
        }, "线程 2").start();
    }
}


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Files\JetBrains\IntelliJ IDEA 2022.2.3\lib\idea_rt.jar" com.pingan.lcloud.stack.DeadLockDemo
Connected to the target VM, address: '127.0.0.1:4226', transport: 'socket'
Thread[线程 1,5,main]get resource1
Thread[线程 2,5,main]get resource2
Thread[线程 1,5,main]waiting get resource2
Thread[线程 2,5,main]waiting get resource1

打印快照 ​

java
# win下找到java进程
tasklist | findstr "java"
# 打印快照, 注意win环境下需要再将txt文件转为utf-8编码
jstack -l 1396 > C:\xz\tool\0712.txt

使用JCA打开快照 ​

java
java -jar jca4614.jar

1745408566271

检测到死锁 ​

1745408572003

分析热点线程案例 ​

  1. 先通过top -Hp <PID>找到java进程中占用CPU资源较高的线程id
  2. 使用printf "%x" <id>将id转为16进制
  3. 打印线程快照, 找到该id对应的线程在执行的代码, 即可定位到具体的问题

参考 ​