概述

使用jstack命令，排查線程死鎖問題。

jstack是什麼？

jstack是java虛擬機自帶的一種堆棧跟蹤工具。jstack用於打印出給定的java進程ID或core file或遠程調試服務的Java堆棧信息

jstack有什麼作用

jstack主要用於生成java虛擬機當前時刻的線程快照。線程快照是當前java虛擬機內每一條線程正在執行的方法堆棧的集合，生成線程快照的主要目的是定位線程出現長時間停頓的原因，如線程間死鎖、死循環、請求外部資源導致的長時間等待等。線程出現停頓的時候通過jstack來查看各個線程的調用堆棧，就可以知道沒有響應的線程到底在後臺做什麼事情，或者等待什麼資源。如果現在運行的java程序呈現hung的狀態，jstack是非常有用的。

如何使用？

使用jstack -help查看官方文檔

Usage:
    jstack [-l] <pid>
        (to connect to running process)
    jstack -F [-m] [-l] <pid>
        (to connect to a hung process)
    jstack [-m] [-l] <executable> <core>
        (to connect to a core file)
    jstack [-m] [-l] [server_id@]<remote server IP or hostname>
        (to connect to a remote debug server)

Options:
    -F  to force a thread dump. Use when jstack <pid> does not respond (process is hung)
    -m  to print both java and native frames (mixed mode)
    -l  long listing. Prints additional information about locks
    -h or -help to print this help message

參數解釋

-F：當正常輸出的請求不被響應時（jstack <pid>無響應），強制輸出線程堆棧。

-m：如果調用到本地方法的話，可以顯示C/C++的堆棧 

-l：除堆棧外，會打印出額外的鎖信息，在發生死鎖時可以用jstack -l pid來觀察鎖持有情況

實戰使用

1. 模擬死鎖情形

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

/**
 * <pre>
 * 	程序目的：模擬線程死鎖
 * </pre>
 * created at 2020-06-21 17:54
 * @author lerry
 */
public class DeathLockDemo {
	public static void main(String[] args) {
		deathLock();
	}

	/**
	 * 一個模擬死鎖的方法。
	 */
	public static void deathLock() {
		ReentrantLock lock1 = new ReentrantLock();
		ReentrantLock lock2 = new ReentrantLock();

		Thread t1 = new Thread(() -> {
			try {
				lock1.lock();
				TimeUnit.SECONDS.sleep(1);
				lock2.lock();
			}
			catch (InterruptedException e) {
				e.printStackTrace();
			}
		});
		Thread t2 = new Thread(() -> {
			try {
				lock2.lock();
				TimeUnit.SECONDS.sleep(1);
				lock1.lock();
			}
			catch (InterruptedException e) {
				e.printStackTrace();
			}
		});
		t1.setName("mythread1");
		t2.setName("mythread2");

		System.out.println("啓動兩個線程");
		t1.start();
		t2.start();
	}
}

2. 查找該程序的pid

1）方法1 使用ps命令

ps -ef | grep 'DeathLockDemo'

輸出結果爲：

圖：DeathLockDemo的進程信息
可以查到，該程序的進程編號爲15019
2)方法2 使用jps命令

jps

輸出結果爲：

➜  ~ jps
14736 KotlinCompileDaemon
10720 Launcher
10721 Bootstrap
15462 Jps
3608 
15017 Launcher
15019 DeathLockDemo
13471

同樣可以查看到該程序的進程編號爲15019

3. 使用jstack

jstack -l 15019

輸出結果如下：

2020-06-21 18:25:09
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.251-b08 mixed mode):

"Attach Listener" #16 daemon prio=9 os_prio=31 tid=0x00007faae8835800 nid=0x5c03 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"DestroyJavaVM" #15 prio=5 os_prio=31 tid=0x00007faae684e800 nid=0x1603 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"mythread2" #14 prio=5 os_prio=31 tid=0x00007faae684e000 nid=0xa803 waiting on condition [0x000070001022c000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x000000076aea4848> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
	at com.hua.jvm.DeathLockDemo.lambda$deathLock$1(DeathLockDemo.java:39)
	at com.hua.jvm.DeathLockDemo$$Lambda$2/240650537.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
	- <0x000000076aea4878> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)

"mythread1" #13 prio=5 os_prio=31 tid=0x00007faae684d000 nid=0xa903 waiting on condition [0x0000700010129000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x000000076aea4878> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
	at com.hua.jvm.DeathLockDemo.lambda$deathLock$0(DeathLockDemo.java:29)
	at com.hua.jvm.DeathLockDemo$$Lambda$1/1809787067.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
	- <0x000000076aea4848> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)

"Service Thread" #12 daemon prio=9 os_prio=31 tid=0x00007faae580c800 nid=0x5703 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"C1 CompilerThread3" #11 daemon prio=9 os_prio=31 tid=0x00007faae802b800 nid=0x5503 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"C2 CompilerThread2" #10 daemon prio=9 os_prio=31 tid=0x00007faae580b800 nid=0x3f03 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"C2 CompilerThread1" #9 daemon prio=9 os_prio=31 tid=0x00007faae800f800 nid=0x4003 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"C2 CompilerThread0" #8 daemon prio=9 os_prio=31 tid=0x00007faae8828800 nid=0x4103 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"JDWP Command Reader" #7 daemon prio=10 os_prio=31 tid=0x00007faae802b000 nid=0x4203 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"JDWP Event Helper Thread" #6 daemon prio=10 os_prio=31 tid=0x00007faae6010800 nid=0x4303 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"JDWP Transport Listener: dt_socket" #5 daemon prio=10 os_prio=31 tid=0x00007faae8824800 nid=0x4507 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007faae6819800 nid=0x4607 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
	- None

"Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007faae6810800 nid=0x5003 in Object.wait() [0x000070000f608000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000076ab08ee0> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
	- locked <0x000000076ab08ee0> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
	at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)

   Locked ownable synchronizers:
	- None

"Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007faae7815800 nid=0x5103 in Object.wait() [0x000070000f505000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000076ab06c00> (a java.lang.ref.Reference$Lock)
	at java.lang.Object.wait(Object.java:502)
	at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
	- locked <0x000000076ab06c00> (a java.lang.ref.Reference$Lock)
	at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

   Locked ownable synchronizers:
	- None

"VM Thread" os_prio=31 tid=0x00007faae680e000 nid=0x5203 runnable 

"GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007faae8803800 nid=0x1e07 runnable 

"GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007faae9000000 nid=0x1f03 runnable 

"GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007faae6804000 nid=0x2a03 runnable 

"GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007faae780c800 nid=0x2c03 runnable 

"GC task thread#4 (ParallelGC)" os_prio=31 tid=0x00007faae780d000 nid=0x2e03 runnable 

"GC task thread#5 (ParallelGC)" os_prio=31 tid=0x00007faae780d800 nid=0x5403 runnable 

"GC task thread#6 (ParallelGC)" os_prio=31 tid=0x00007faae780e800 nid=0x3003 runnable 

"GC task thread#7 (ParallelGC)" os_prio=31 tid=0x00007faae780f000 nid=0x3203 runnable 

"VM Periodic Task Thread" os_prio=31 tid=0x00007faae7880800 nid=0x5903 waiting on condition 

JNI global references: 2537


Found one Java-level deadlock:
=============================
"mythread2":
  waiting for ownable synchronizer 0x000000076aea4848, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "mythread1"
"mythread1":
  waiting for ownable synchronizer 0x000000076aea4878, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "mythread2"

Java stack information for the threads listed above:
===================================================
"mythread2":
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x000000076aea4848> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
	at com.hua.jvm.DeathLockDemo.lambda$deathLock$1(DeathLockDemo.java:39)
	at com.hua.jvm.DeathLockDemo$$Lambda$2/240650537.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)
"mythread1":
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x000000076aea4878> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
	at com.hua.jvm.DeathLockDemo.lambda$deathLock$0(DeathLockDemo.java:29)
	at com.hua.jvm.DeathLockDemo$$Lambda$1/1809787067.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.

4. 結果分析

圖：截取的部分jstack輸出結果
根據輸出，可以發現：
線程mythread2在等待正在由mythread1持有的鎖0x000000076aea4848。
而線程mythread1同時又在等待正在由mythread2持有的鎖0x000000076aea4878。造成了死鎖問題。

jstack日誌分析工具

可以把jstack輸出到文本中，然後使用分析工具查看

~ jstack -l 15019 > ~/Temp/jstack15019.txt

圖：fastthread.io的在線分析結果

參考

jvm 性能調優工具之 jstack - 簡書

環境說明

java -version

java version "1.8.0_251"
Java(TM) SE Runtime Environment (build 1.8.0_251-b08)
Java HotSpot(TM) 64-Bit Server VM (build 25.251-b08, mixed mode)

OS：macOS High Sierra 10.13.4

使用jstack命令，排查線程死鎖問題

概述

jstack是什麼？

jstack有什麼作用

如何使用？

參數解釋

實戰使用

1. 模擬死鎖情形

2. 查找該程序的pid

3. 使用jstack

4. 結果分析

jstack日誌分析工具

參考

環境說明

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使用jstack命令，排查線程死鎖問題

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