什么是死锁

java中的死锁是一种编程情况，其中两个或多个线程被永久阻塞，Java死锁情况出现至少两个线程和两个或更多资源。

在这里，我们将写了一个简单的程序，它将导致java死锁场景，然后我们将分析它。

怎么实现死锁

下面我们一起看一个简单的死锁事例,在主线程中每隔5秒钟启动一个线程，每个线程的执行时间是3秒钟

  1. package com.lkf.mulithread;
  3. public class ThreadDeadlock {
  4.     public static void main(String[] args) throws InterruptedException {
  5.         Object obj1 = new Object();
  6.         Object obj2 = new Object();
  7.         Object obj3 = new Object();
  9.         Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
  10.         Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
  11.         Thread t3 = new Thread(new SyncThread(obj3, obj1), "t3");
  13.         t1.start();
  14.         Thread.sleep(5000);
  15.         t2.start();
  16.         Thread.sleep(5000);
  17.         t3.start();
  19.     }
  21. }
  23. class SyncThread implements Runnable{
  24.     private Object obj1;
  25.     private Object obj2;
  27.     public SyncThread(Object o1, Object o2){
  28.         this.obj1=o1;
  29.         this.obj2=o2;
  30.     }
  31.     @Override
  32.     public void run() {
  33.         String name = Thread.currentThread().getName();
  34.         System.out.println(name + " acquiring lock on "+obj1);
  35.         synchronized (obj1) {
  36.             System.out.println(name + " acquired lock on "+obj1);
  37.             work();
  38.             System.out.println(name + " acquiring lock on "+obj2);
  39.             synchronized (obj2) {
  40.                 System.out.println(name + " acquired lock on "+obj2);
  41.                 work();
  42.             }
  43.             System.out.println(name + " released lock on "+obj2);
  44.         }
  45.         System.out.println(name + " released lock on "+obj1);
  46.         System.out.println(name + " finished execution.");
  47.     }
  48.     private void work() {
  49.         try {
  50.             Thread.sleep(30000);
  51.         } catch (InterruptedException e) {
  52.             e.printStackTrace();
  53.         }
  54.     }
  55. }

运行结果：

  1. t1 acquiring lock on java.lang.Object@25d849fd
  2. t1 acquired lock on java.lang.Object@25d849fd
  3. t2 acquiring lock on java.lang.Object@30298d1f
  4. t2 acquired lock on java.lang.Object@30298d1f
  5. t3 acquiring lock on java.lang.Object@11d32fc4
  6. t3 acquired lock on java.lang.Object@11d32fc4
  7. t1 acquiring lock on java.lang.Object@30298d1f
  8. t2 acquiring lock on java.lang.Object@11d32fc4
  9. t3 acquiring lock on java.lang.Object@25d849fd

可以看到每个线程中都能成功获取第一个对象的锁，最终却阻塞在了获取第二个对象，造成了线程之间的互相等待，形成了死锁

怎么排查死锁

我们通过jstack拿到该应用程序死锁的线程转储文件如下（获取线程转储文件方法）：


  2. 2018-03-31 15:33:34
  3. Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.161-b12 mixed mode):
  5. "RMI TCP Connection(2)-192.168.1.101" #22 daemon prio=9 os_prio=31 tid=0x00007fe2c81cc000 nid=0xa503 runnable [0x000070000ba7c000]
  6.    java.lang.Thread.State: RUNNABLE
  7.         at java.net.SocketInputStream.socketRead0(Native Method)
  8.         at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
  9.         at java.net.SocketInputStream.read(SocketInputStream.java:171)
  10.         at java.net.SocketInputStream.read(SocketInputStream.java:141)
  11.         at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
  12.         at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
  13.         - locked <0x0000000797612b30> (a java.io.BufferedInputStream)
  14.         at java.io.FilterInputStream.read(FilterInputStream.java:83)
  15.         at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:550)
  16.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
  17.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
  18.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$6/556824457.run(Unknown Source)
  19.         at java.security.AccessController.doPrivileged(Native Method)
  20.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
  21.         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  22.         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  23.         at java.lang.Thread.run(Thread.java:748)
  25.    Locked ownable synchronizers:
  26.         - <0x0000000797612c78> (a java.util.concurrent.ThreadPoolExecutor$Worker)
  28. "JMX server connection timeout 21" #21 daemon prio=9 os_prio=31 tid=0x00007fe2c8968800 nid=0xa703 in Object.wait() [0x000070000b97a000]
  29.    java.lang.Thread.State: TIMED_WAITING (on object monitor)
  30.         at java.lang.Object.wait(Native Method)
  31.         at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:168)
  32.         - locked <0x0000000797600720> (a [I)
  33.         at java.lang.Thread.run(Thread.java:748)
  35.    Locked ownable synchronizers:
  36.         - None
  38. "RMI Scheduler(0)" #20 daemon prio=9 os_prio=31 tid=0x00007fe2c81bc800 nid=0xa903 waiting on condition [0x000070000b877000]
  39.    java.lang.Thread.State: TIMED_WAITING (parking)
  40.         at sun.misc.Unsafe.park(Native Method)
  41.         - parking to wait for  <0x0000000797618010> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
  42.         at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
  43.         at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)
  44.         at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)
  45.         at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
  46.         at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
  47.         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
  48.         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  49.         at java.lang.Thread.run(Thread.java:748)
  51.    Locked ownable synchronizers:
  52.         - None
  54. "RMI TCP Connection(1)-192.168.1.101" #19 daemon prio=9 os_prio=31 tid=0x00007fe2c884e000 nid=0x5903 runnable [0x000070000b773000]
  55.    java.lang.Thread.State: RUNNABLE
  56.         at java.net.SocketInputStream.socketRead0(Native Method)
  57.         at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
  58.         at java.net.SocketInputStream.read(SocketInputStream.java:171)
  59.         at java.net.SocketInputStream.read(SocketInputStream.java:141)
  60.         at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
  61.         at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
  62.         - locked <0x0000000797600eb8> (a java.io.BufferedInputStream)
  63.         at java.io.FilterInputStream.read(FilterInputStream.java:83)
  64.         at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:550)
  65.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
  66.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
  67.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$6/556824457.run(Unknown Source)
  68.         at java.security.AccessController.doPrivileged(Native Method)
  69.         at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
  70.         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  71.         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  72.         at java.lang.Thread.run(Thread.java:748)
  74.    Locked ownable synchronizers:
  75.         - <0x0000000797600ff8> (a java.util.concurrent.ThreadPoolExecutor$Worker)
  77. "RMI TCP Accept-0" #18 daemon prio=9 os_prio=31 tid=0x00007fe2c8855000 nid=0x5703 runnable [0x000070000b671000]
  78.    java.lang.Thread.State: RUNNABLE
  79.         at java.net.PlainSocketImpl.socketAccept(Native Method)
  80.         at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
  81.         at java.net.ServerSocket.implAccept(ServerSocket.java:545)
  82.         at java.net.ServerSocket.accept(ServerSocket.java:513)
  83.         at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:52)
  84.         at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:400)
  85.         at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:372)
  86.         at java.lang.Thread.run(Thread.java:748)
  88.    Locked ownable synchronizers:
  89.         - None
  91. "DestroyJavaVM" #16 prio=5 os_prio=31 tid=0x00007fe2c90cf800 nid=0x2803 waiting on condition [0x0000000000000000]
  92.    java.lang.Thread.State: RUNNABLE
  94.    Locked ownable synchronizers:
  95.         - None
  97. "t3" #14 prio=5 os_prio=31 tid=0x00007fe2c7865000 nid=0x5503 waiting for monitor entry [0x000070000b56e000]
  98.    java.lang.Thread.State: BLOCKED (on object monitor)
  99.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  100.         - waiting to lock <0x0000000797618188> (a java.lang.Object)
  101.         - locked <0x0000000797618198> (a java.lang.Object)
  102.         at java.lang.Thread.run(Thread.java:748)
  104.    Locked ownable synchronizers:
  105.         - None
  107. "Attach Listener" #15 daemon prio=9 os_prio=31 tid=0x00007fe2c884d800 nid=0xb07 waiting on condition [0x0000000000000000]
  108.    java.lang.Thread.State: RUNNABLE
  110.    Locked ownable synchronizers:
  111.         - None
  113. "t2" #13 prio=5 os_prio=31 tid=0x00007fe2c884b800 nid=0x3e03 waiting for monitor entry [0x000070000b368000]
  114.    java.lang.Thread.State: BLOCKED (on object monitor)
  115.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  116.         - waiting to lock <0x0000000797618198> (a java.lang.Object)
  117.         - locked <0x00000007976181e8> (a java.lang.Object)
  118.         at java.lang.Thread.run(Thread.java:748)
  120.    Locked ownable synchronizers:
  121.         - None
  123. "t1" #12 prio=5 os_prio=31 tid=0x00007fe2c7885000 nid=0x4103 waiting for monitor entry [0x000070000b265000]
  124.    java.lang.Thread.State: BLOCKED (on object monitor)
  125.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  126.         - waiting to lock <0x00000007976181e8> (a java.lang.Object)
  127.         - locked <0x0000000797618188> (a java.lang.Object)
  128.         at java.lang.Thread.run(Thread.java:748)
  130.    Locked ownable synchronizers:
  131.         - None
  133. "Service Thread" #11 daemon prio=9 os_prio=31 tid=0x00007fe2c884c800 nid=0x4503 runnable [0x0000000000000000]
  134.    java.lang.Thread.State: RUNNABLE
  136.    Locked ownable synchronizers:
  137.         - None
  139. "C1 CompilerThread2" #10 daemon prio=9 os_prio=31 tid=0x00007fe2c906c000 nid=0x4603 waiting on condition [0x0000000000000000]
  140.    java.lang.Thread.State: RUNNABLE
  142.    Locked ownable synchronizers:
  143.         - None
  145. "C2 CompilerThread1" #9 daemon prio=9 os_prio=31 tid=0x00007fe2c884a800 nid=0x3c03 waiting on condition [0x0000000000000000]
  146.    java.lang.Thread.State: RUNNABLE
  148.    Locked ownable synchronizers:
  149.         - None
  151. "C2 CompilerThread0" #8 daemon prio=9 os_prio=31 tid=0x00007fe2c806c800 nid=0x3a03 waiting on condition [0x0000000000000000]
  152.    java.lang.Thread.State: RUNNABLE
  154.    Locked ownable synchronizers:
  155.         - None
  157. "JDWP Command Reader" #7 daemon prio=10 os_prio=31 tid=0x00007fe2c7811000 nid=0x4903 runnable [0x0000000000000000]
  158.    java.lang.Thread.State: RUNNABLE
  160.    Locked ownable synchronizers:
  161.         - None
  163. "JDWP Event Helper Thread" #6 daemon prio=10 os_prio=31 tid=0x00007fe2c7810000 nid=0x3803 runnable [0x0000000000000000]
  164.    java.lang.Thread.State: RUNNABLE
  166.    Locked ownable synchronizers:
  167.         - None
  169. "JDWP Transport Listener: dt_socket" #5 daemon prio=10 os_prio=31 tid=0x00007fe2c780f800 nid=0x4a07 runnable [0x0000000000000000]
  170.    java.lang.Thread.State: RUNNABLE
  172.    Locked ownable synchronizers:
  173.         - None
  175. "Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fe2c801d000 nid=0x3507 runnable [0x0000000000000000]
  176.    java.lang.Thread.State: RUNNABLE
  178.    Locked ownable synchronizers:
  179.         - None
  181. "Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fe2c7820800 nid=0x3003 in Object.wait() [0x000070000a847000]
  182.    java.lang.Thread.State: WAITING (on object monitor)
  183.         at java.lang.Object.wait(Native Method)
  184.         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
  185.         - locked <0x0000000797618850> (a java.lang.ref.ReferenceQueue$Lock)
  186.         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
  187.         at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
  189.    Locked ownable synchronizers:
  190.         - None
  192. "Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fe2c8017000 nid=0x2e03 in Object.wait() [0x000070000a744000]
  193.    java.lang.Thread.State: WAITING (on object monitor)
  194.         at java.lang.Object.wait(Native Method)
  195.         at java.lang.Object.wait(Object.java:502)
  196.         at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
  197.         - locked <0x0000000797618890> (a java.lang.ref.Reference$Lock)
  198.         at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
  200.    Locked ownable synchronizers:
  201.         - None
  203. "VM Thread" os_prio=31 tid=0x00007fe2c8014800 nid=0x5303 runnable 
  205. "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fe2c8011800 nid=0x1b07 runnable 
  207. "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fe2c8012000 nid=0x1d03 runnable 
  209. "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fe2c8012800 nid=0x2a03 runnable 
  211. "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fe2c9010000 nid=0x5403 runnable 
  213. "VM Periodic Task Thread" os_prio=31 tid=0x00007fe2c781a000 nid=0x4303 waiting on condition 
  215. JNI global references: 5038
  217. Found one Java-level deadlock:
  218. =============================
  219. "t3":
  220.   waiting to lock monitor 0x00007fe2c91c7198 (object 0x0000000797618188, a java.lang.Object),
  221.   which is held by "t1"
  222. "t1":
  223.   waiting to lock monitor 0x00007fe2c9046568 (object 0x00000007976181e8, a java.lang.Object),
  224.   which is held by "t2"
  225. "t2":
  226.   waiting to lock monitor 0x00007fe2c91c5b98 (object 0x0000000797618198, a java.lang.Object),
  227.   which is held by "t3"
  229. Java stack information for the threads listed above:
  230. ===================================================
  231. "t3":
  232.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  233.         - waiting to lock <0x0000000797618188> (a java.lang.Object)
  234.         - locked <0x0000000797618198> (a java.lang.Object)
  235.         at java.lang.Thread.run(Thread.java:748)
  236. "t1":
  237.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  238.         - waiting to lock <0x00000007976181e8> (a java.lang.Object)
  239.         - locked <0x0000000797618188> (a java.lang.Object)
  240.         at java.lang.Thread.run(Thread.java:748)
  241. "t2":
  242.         at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40)
  243.         - waiting to lock <0x0000000797618198> (a java.lang.Object)
  244.         - locked <0x00000007976181e8> (a java.lang.Object)
  245.         at java.lang.Thread.run(Thread.java:748)
  247. Found 1 deadlock.

为了分析死锁，我们需要注意那些被阻塞的线程，以及等待锁定的资源，每个资源有一个惟一的ID，我们可以找到哪个线程已经持有对象上的锁，比如：线程t3正在等待获取对象0x00007fe2c91c7198的锁，但是该对象已经被线程t1锁定。

怎么避免死锁

上边，我们分析了死锁的情况，发现了导致死锁的线程，我们需要修改代码以避免死锁的情况，那么怎么避免死锁呢

1、避免嵌套锁

这是死锁最常见的原因，如果您已经持有一个资源，请避免锁定另一个资源。如果只使用一个对象锁，则几乎不可能出现死锁情况，比如以下代码对上边的循环嵌套部分进行修改，则避免了死锁的情况：

  1. public void run() {
  2.         String name = Thread.currentThread().getName();
  3.         System.out.println(name + " acquiring lock on " + obj1);
  4.         synchronized (obj1) {
  5.             System.out.println(name + " acquired lock on " + obj1);
  6.             work();
  7.         }
  8.         System.out.println(name + " released lock on " + obj1);
  9.         System.out.println(name + " acquiring lock on " + obj2);
  10.         synchronized (obj2) {
  11.             System.out.println(name + " acquired lock on " + obj2);
  12.             work();
  13.         }
  14.         System.out.println(name + " released lock on " + obj2);
  16.         System.out.println(name + " finished execution.");
  17.     }

2、只锁需要的部分

只获对需要的资源加锁，例如在上面的程序中，我们锁定了完整的对象资源，但是如果我们只需要其中一个字段，那么我们应该只锁定那个特定的字段而不是完整的对象

3、避免无限期等待

如果两个线程使用 thread join 无限期互相等待也会造成死锁，我们可以设定等待的最大时间来避免这种情况。

翻译自：https://www.journaldev.com/1058/deadlock-in-java-example

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