搬运一下jstack的简单介绍

# 能干嘛
jstack用于生成java虚拟机当前时刻的线程快照。
# 什么是线程快照
线程快照是当前java虚拟机内每一条线程正在执行的方法堆栈的集合，生成线程快照的主要目的是定位线程出现长时间停顿的原因，如线程间死锁、死循环、请求外部资源导致的长时间等待等。

线程出现停顿的时候通过jstack来查看各个线程的调用堆栈，就可以知道没有响应的线程到底在后台做什么事情，或者等待什么资源。 如果java程序崩溃生成core文件，jstack工具可以用来获得core文件的java stack和native stack的信息，从而可以轻松地知道java程序是如何崩溃和在程序何处发生问题。

然后我们实现一个简单的死锁, 两个线程互相需要对方持有的资源(锁), 会导致死锁

public class DeadLockTest {

    private final Object lockA = new Object();
    private final Object lockB = new Object();

    public void m1() {
        synchronized (lockA) {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (lockB) {
                System.out.println("m1 .... do");
            }
        }
    }


    public void m2() {
        synchronized (lockB) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (lockA) {
                System.out.println("m2 .... do");
            }
        }
    }


    public static void main(String[] args) {
        DeadLockTest deadLockTest = new DeadLockTest();

        Runnable r1 = deadLockTest::m1;

        Runnable r2 = deadLockTest::m2;

        new Thread(r1).start();
        new Thread(r2).start();


    }

}

此时我们用 jstack + PID 查看一下

bogon:~ kayz$ jstack 4529
2018-10-26 12:10:44
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.181-b13 mixed mode):

"Attach Listener" #16 daemon prio=9 os_prio=31 tid=0x00007fa54502e000 nid=0xf07 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" #15 prio=5 os_prio=31 tid=0x00007fa542811800 nid=0x2603 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Thread-1" #14 prio=5 os_prio=31 tid=0x00007fa545051800 nid=0x5a03 waiting for monitor entry [0x000070000a3b1000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.zk.juc.deadlock.DeadLockTest.m2(DeadLockTest.java:30)
    - waiting to lock <0x000000076ae207f0> (a java.lang.Object)
    - locked <0x000000076ae20800> (a java.lang.Object)
    at com.zk.juc.deadlock.DeadLockTest$$Lambda$2/1207140081.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:748)

"Thread-0" #13 prio=5 os_prio=31 tid=0x00007fa545051000 nid=0x5903 waiting for monitor entry [0x000070000a2ae000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.zk.juc.deadlock.DeadLockTest.m1(DeadLockTest.java:16)
    - waiting to lock <0x000000076ae20800> (a java.lang.Object)
    - locked <0x000000076ae207f0> (a java.lang.Object)
    at com.zk.juc.deadlock.DeadLockTest$$Lambda$1/457233904.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:748)

"Service Thread" #12 daemon prio=9 os_prio=31 tid=0x00007fa54382a000 nid=0xa503 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread3" #11 daemon prio=9 os_prio=31 tid=0x00007fa54381d800 nid=0xa703 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread2" #10 daemon prio=9 os_prio=31 tid=0x00007fa541027000 nid=0xa903 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #9 daemon prio=9 os_prio=31 tid=0x00007fa544801000 nid=0x5503 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #8 daemon prio=9 os_prio=31 tid=0x00007fa54181d800 nid=0x3f03 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Command Reader" #7 daemon prio=10 os_prio=31 tid=0x00007fa54400e000 nid=0x3d03 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Event Helper Thread" #6 daemon prio=10 os_prio=31 tid=0x00007fa544007800 nid=0x4103 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Transport Listener: dt_socket" #5 daemon prio=10 os_prio=31 tid=0x00007fa541827800 nid=0x4307 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fa543829000 nid=0x4403 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fa544036800 nid=0x3403 in Object.wait() [0x000070000978d000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x000000076ab08ed0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
    - locked <0x000000076ab08ed0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)

"Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fa541007000 nid=0x4903 in Object.wait() [0x000070000968a000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x000000076ab06bf8> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:502)
    at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    - locked <0x000000076ab06bf8> (a java.lang.ref.Reference$Lock)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=31 tid=0x00007fa544034000 nid=0x4b03 runnable 

"GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fa544800000 nid=0x1e07 runnable 

"GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fa543000800 nid=0x2a03 runnable 

"GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fa54380a800 nid=0x5403 runnable 

"GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fa54380b000 nid=0x2c03 runnable 

"GC task thread#4 (ParallelGC)" os_prio=31 tid=0x00007fa541006800 nid=0x2e03 runnable 

"GC task thread#5 (ParallelGC)" os_prio=31 tid=0x00007fa542801000 nid=0x5203 runnable 

"GC task thread#6 (ParallelGC)" os_prio=31 tid=0x00007fa545000000 nid=0x5103 runnable 

"GC task thread#7 (ParallelGC)" os_prio=31 tid=0x00007fa54380b800 nid=0x4f03 runnable 

"GC task thread#8 (ParallelGC)" os_prio=31 tid=0x00007fa54380c000 nid=0x4e03 runnable 

"GC task thread#9 (ParallelGC)" os_prio=31 tid=0x00007fa545001000 nid=0x4d03 runnable 

"VM Periodic Task Thread" os_prio=31 tid=0x00007fa541826800 nid=0x5703 waiting on condition 

JNI global references: 2357


Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x00007fa541017ea8 (object 0x000000076ae207f0, a java.lang.Object),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x00007fa541013e08 (object 0x000000076ae20800, a java.lang.Object),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
    at com.zk.juc.deadlock.DeadLockTest.m2(DeadLockTest.java:30)
    - waiting to lock <0x000000076ae207f0> (a java.lang.Object)
    - locked <0x000000076ae20800> (a java.lang.Object)
    at com.zk.juc.deadlock.DeadLockTest$$Lambda$2/1207140081.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:748)
"Thread-0":
    at com.zk.juc.deadlock.DeadLockTest.m1(DeadLockTest.java:16)
    - waiting to lock <0x000000076ae20800> (a java.lang.Object)
    - locked <0x000000076ae207f0> (a java.lang.Object)
    at com.zk.juc.deadlock.DeadLockTest$$Lambda$1/457233904.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.

Found 1 deadlock.