Java线程状态dump

Java线程状态dump是重要的Java应用程序分析工具。它可以帮助我们找到导致诸如高内存/CPU占用、死锁等性能问题的根源。

得益于Java平台丰富的工具，产生Java线程状态dump的方法也很多。比如VisualVM，但这里着重介绍命令行的工具，因为它们是Java自带的，掌握这些工具在生产环境诊断问题更加得心应手。

命令行产生dump主要有两种方法：

1. 向Java进程发SIGQUIT信号kill -3 <java_process_id>
2. jstack <java_process_id>
3. jcmd <java_process_id | java_class_name> Thread.print

真正应该使用的是后一种，因为前一种会往目标Java进程所在终端上直接输出dump内容，这样无法重定向dump内容到文件。而且如果目标Java进程如果事后台运行的，那么就很难找到dump内容。jcmd不仅可以用PID识别目标Java进程，而且可以用Java进程的启动类名识别目标进程。所以，从脚本编程的角度，用jcmd是最佳选择。

 

以下是个用jstack分析死锁的例子。产生死锁的程序如下：

 

import static java.lang.System.out;

public class Deadlock {
    static class Resource {
        private String name;
        private Resource buddy;

        public Resource(String name) {
            this.name = name;
        }

        public synchronized void acquire(long delay) {
            out.printf(
                "Thread %s acquired %s, trying to get %s ...\n",
                Thread.currentThread().getName(),
                this.name,
                this.buddy.getName()
            );
            try {
                Thread.sleep(delay);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
            this.buddy.hold();
        }

        public synchronized void hold() {
            out.printf(
                "Thread %s hold %s\n",
                Thread.currentThread().getName(),
                this.name
            );
        }

        public void setBuddy(Resource buddy) {
            this.buddy = buddy;
        }

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

    static class CompetitorThread extends Thread {
        private Resource resource;
        private long delay;

        public CompetitorThread(Resource resource, long delay) {
            this.resource = resource;
            this.delay = delay;
        }

        @Override
        public void run() {
            resource.acquire(delay);
            out.printf(
                "Thread %s completes execution!\n",
                Thread.currentThread().getName()
            );
        }
    }

    public static void main(String[] args) {

        Resource resourceA = new Resource("Resource A");
        Resource resourceB = new Resource("Resource B");
        resourceA.setBuddy(resourceB);
        resourceB.setBuddy(resourceA);

        Thread thread1 = new CompetitorThread(resourceA, 0);
        Thread thread2 = new CompetitorThread(resourceB, 1000);
        thread1.start();
        /* uncomment the following code to avoid deadlock */
//        try {
//            Thread.sleep(1000);
//        }
//        catch (InterruptedException e) {
//            e.printStackTrace();
//        }
        thread2.start();
    }
}

 你也可以在这里下载这个程序：https://github.com/schnell18/java-honing/blob/master/src/main/java/org/home/hone/thread/Deadlock.java

 

然后编译并运行这个程序：

 

$ javac Deadlock.java
$ java Deadlock
Thread Thread-0 acquired Resource A, trying to get Resource B ...
Thread Thread-1 acquired Resource B, trying to get Resource A ...

打开另一个终端执行：

$ ps -ef |grep Deadlock
fgz       6076  5664  0 22:11 pts/0    00:00:00 java Deadlock
fgz       6144  5879  0 22:38 pts/1    00:00:00 grep Deadlock

 可见陷入死锁的Java进程的PID时6076。然后就可以用jstack产生线程dump了：

 

$ jstack 6076 > deadlock.dump

 deadlock.dump的摘要如下：

2015-02-27 22:42:23
Full thread dump OpenJDK 64-Bit Server VM (23.25-b01 mixed mode):
...
Java stack information for the threads listed above:
===================================================
"Thread-1":
	at Deadlock$Resource.hold(Deadlock.java:29)
	- waiting to lock <0x00000000f224cb28> (a Deadlock$Resource)
	at Deadlock$Resource.acquire(Deadlock.java:25)
	- locked <0x00000000f224cb88> (a Deadlock$Resource)
	at Deadlock$CompetitorThread.run(Deadlock.java:56)
"Thread-0":
	at Deadlock$Resource.hold(Deadlock.java:29)
	- waiting to lock <0x00000000f224cb88> (a Deadlock$Resource)
	at Deadlock$Resource.acquire(Deadlock.java:25)
	- locked <0x00000000f224cb28> (a Deadlock$Resource)
	at Deadlock$CompetitorThread.run(Deadlock.java:56)

Found 1 deadlock.

 这里清除的说明了死锁的存在，并说明了Thread-0和Thread-1都阻塞在hold()这个方法调用上。具体的代码行号是Deadlock.java 29行。这些具体的信息为问题的解决打下了坚实的基础。