jstack [ options ] pid
jstack [ options ] executable core
jstack [ options ] [ server-id@ ] remote-hostname-or-IP
OPTIONS
       -F
              Force a stack dump when jstack [-l] pid does not respond.
       -l
              Long listing. Prints additional information about locks such as a list of owned java.util.concurrent ownable synchronizers. See the
              AbstractOwnableSynchronizer class description at
              http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/AbstractOwnableSynchronizer.html
       -m
              Prints a mixed mode stack trace that has both Java and native C/C++ frames.

-F 當(dāng)正常的請求不被響應(yīng)時(shí)，強(qiáng)制輸出堆棧信息。
-l 額外打印鎖的信息，當(dāng)發(fā)生死鎖是可以查看鎖的信息
-m 如果調(diào)用本地方法棧的信息，可以打印 C/C++的堆棧

以一個(gè)發(fā)生死鎖的例子來看一下使用 Jstack 查看到的信息

public class Jstack {
    private static Object obj1 = new Object();
    private static Object obj2 = new Object();
    public static void main(String[] args) {
        new Thread(() -> {
            synchronized (obj1) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (obj2) {
                }
            }
        }).start();
        new Thread(() -> {
            synchronized (obj2) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (obj1) {
                }
            }
        }).start();
    }
}

上面代碼中，第一個(gè)線程拿到 obj1 的鎖，等待 obj2 的鎖，第二個(gè)線程拿到 obj2 的鎖，等待 obj1 的鎖，這樣就會發(fā)生死鎖。

先通過jps命令獲取到先拿到當(dāng)前的進(jìn)程 pid，然后通過 jstack 獲取線程的信息?？梢钥吹接袃蓚€(gè)線程都處于阻塞狀態(tài)。

"Thread-1" #12 prio=5 os_prio=0 tid=0x00007fdff871c800 nid=0x3cc2 waiting for monitor entry [0x00007fdfce0fc000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.example.demo.jstack.Jstack.lambda$main$1(Jstack.java:36)
	- waiting to lock <0x000000076e925a90> (a java.lang.Object)
	- locked <0x000000076e925aa0> (a java.lang.Object)
	at com.example.demo.jstack.Jstack$$Lambda$2/2052001577.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)

"Thread-0" #11 prio=5 os_prio=0 tid=0x00007fdff871a800 nid=0x3cc1 waiting for monitor entry [0x00007fdfce1fc000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.example.demo.jstack.Jstack.lambda$main$0(Jstack.java:25)
	- waiting to lock <0x000000076e925aa0> (a java.lang.Object)
	- locked <0x000000076e925a90> (a java.lang.Object)
	at com.example.demo.jstack.Jstack$$Lambda$1/1174361318.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)

第一行顯示可線程名、線程優(yōu)先級、線程 id、線程狀態(tài)描述等信息

第二行顯示的是當(dāng)前線程的狀態(tài)

? Java 中線程的狀態(tài)分為 NEW、RUNNABLE、BLOCKED、WATING、TIMED_WATING、TERMINATED，但是在快照中 NEW 狀態(tài)是不會出現(xiàn)的。

再下面的就是當(dāng)前線程的調(diào)用棧的信息。調(diào)用棧中包含了鎖的信息。

? locked 表示使用 synchronized 申請對象鎖成功,監(jiān)視器的擁有者

? waiting to lock 表示使用 synchronized 申請對象鎖未成功,進(jìn)入等待區(qū)。

? waiting on 表示用 synchronized 申請對象鎖成功后,調(diào)用了 wait 方法,進(jìn)入對象的等待區(qū)等待。

? parking to wait for park 是基本的線程阻塞原語,不通過監(jiān)視器在對象上阻塞。隨 concurrent 包會出現(xiàn)的新的機(jī)制,與 synchronized 體系不同。

在最后也顯示出了代碼中出現(xiàn)死鎖的信息

Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x00007fdfac006638 (object 0x000000076e925a90, a java.lang.Object),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x00007fdfac003da8 (object 0x000000076e925aa0, a java.lang.Object),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
	at com.example.demo.jstack.Jstack.lambda$main$1(Jstack.java:36)
	- waiting to lock <0x000000076e925a90> (a java.lang.Object)
	- locked <0x000000076e925aa0> (a java.lang.Object)
	at com.example.demo.jstack.Jstack$$Lambda$2/2052001577.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)
"Thread-0":
	at com.example.demo.jstack.Jstack.lambda$main$0(Jstack.java:25)
	- waiting to lock <0x000000076e925aa0> (a java.lang.Object)
	- locked <0x000000076e925a90> (a java.lang.Object)
	at com.example.demo.jstack.Jstack$$Lambda$1/1174361318.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)
Found 1 deadlock.

好了，熟悉了 Jstack，我們用一段死循環(huán)的代碼，通過 Jstack 來定位到使 CPU 占用 100%的代碼行

public class JstackDemo {
    public static Executor executor = Executors.newFixedThreadPool(3);
    private static Object lock = new Object();
    public static void main(String[] args) {
        Task task1 = new Task();
        Task task2 = new Task();
        executor.execute(task1);
        executor.execute(task2);
    }

    public static class Task implements Runnable {
        @Override
        public void run() {
            synchronized (lock) {
                run0();
            }
        }

        private void run0() {
            int i = 0;
            while (true) {
                i++;
            }
        }
    }
}

1、首先通過top查看到使 CPU 占用到 100%的進(jìn)程 id