一、概述

两个或两个以上的进程在执行任务的过程中，因争抢资源而造成的一种互相等待的现象，若无外力干涉将无法推进下去。

二、死锁演示

  1. public class HoldLockThread implements Runnable {
  3.     private String lock1;
  4.     private String lock2;
  6.     public HoldLockThread(String lock1, String lock2) {
  7.         this.lock1 = lock1;
  8.         this.lock2 = lock2;
  9.     }
  11.     // A线程进来获得锁A，（等2s）然后B线程进来获得锁B，然后A想去获得锁B（此时B还持有锁B）
  12.     @Override
  13.     public void run() {
  14.         synchronized (lock1) {
  15.             System.out.println(Thread.currentThread().getName() + "\t自己持有lock1：" + lock1 + "，尝试获得lock2：" + lock2);
  16.             try {
  17.                 // 等待两秒，让第二B线程启动并获取锁B
  18.                 TimeUnit.SECONDS.sleep(2);
  19.             } catch (InterruptedException e) {
  20.                 e.printStackTrace();
  21.             }
  22.             synchronized (lock2) {
  23.                 System.out.println(Thread.currentThread().getName() + "\t自己持有lock2：" + lock2 + "，尝试获得lock1：" + lock1);
  24.             }
  25.         }
  26.     }
  27. }
  1. public class DeadLockDemo {
  2.     public static void main(String[] args) {
  3.         String lockA = "lockA";
  4.         String lockB = "lockB";
  5.         new Thread(new HoldLockThread(lockA,lockB),"ThreadAAA").start();
  6.         new Thread(new HoldLockThread(lockB,lockA),"ThreadBBB").start();
  7.     }
  8. }

1、上述代码就会出现死锁，A线程进来获得锁A，（等2s）然后B线程进来获得锁B，然后A想去获得锁B（此时B还持有锁B）。
2、首先通过jps命令定位进程号，可以看到DeadLockDemo正在运行，进程号是54760

  1. superking@wangchaodeMacBook-Pro examination % jps -l
  2. 54835 jdk.jcmd/sun.tools.jps.Jps
  3. 54708 org.jetbrains.idea.maven.server.RemoteMavenServer36
  4. 54758 org.jetbrains.jps.cmdline.Launcher
  5. 54760 com.supkingx.base.f_thread.deadlock.DeadLockDemo

3、通过jstack分析问题。

  1. superking@wangchaodeMacBook-Pro examination % jstack 54760
  2. 2021-10-17 10:45:26
  3. Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.251-b08 mixed mode):
  5. "Attach Listener" #14 daemon prio=9 os_prio=31 tid=0x00007fb823810000 nid=0xa703 waiting on condition [0x0000000000000000]
  6.    java.lang.Thread.State: RUNNABLE
  8. "DestroyJavaVM" #13 prio=5 os_prio=31 tid=0x00007fb824080000 nid=0xe03 waiting on condition [0x0000000000000000]
  9.    java.lang.Thread.State: RUNNABLE
  11. "ThreadBBB" #12 prio=5 os_prio=31 tid=0x00007fb82980a000 nid=0xa803 waiting for monitor entry [0x0000700004e87000]
  12.    java.lang.Thread.State: BLOCKED (on object monitor)
  13.         at com.supkingx.base.f_thread.deadlock.HoldLockThread.run(HoldLockThread.java:32)
  14.         - waiting to lock <0x000000076ac231d0> (a java.lang.String)
  15.         - locked <0x000000076ac23208> (a java.lang.String)
  16.         at java.lang.Thread.run(Thread.java:748)
  17. "ThreadAAA" #11 prio=5 os_prio=31 tid=0x00007fb82407f000 nid=0x5a03 waiting for monitor entry [0x0000700004d84000]
  18.    java.lang.Thread.State: BLOCKED (on object monitor)
  19.         at com.supkingx.base.f_thread.deadlock.HoldLockThread.run(HoldLockThread.java:32)
  20.         - waiting to lock <0x000000076ac23208> (a java.lang.String)
  21.         - locked <0x000000076ac231d0> (a java.lang.String)
  22.         at java.lang.Thread.run(Thread.java:748)
  24. "Service Thread" #10 daemon prio=9 os_prio=31 tid=0x00007fb825819800 nid=0xa903 runnable [0x0000000000000000]
  25.    java.lang.Thread.State: RUNNABLE
  27. "C1 CompilerThread3" #9 daemon prio=9 os_prio=31 tid=0x00007fb82402e800 nid=0x5503 waiting on condition [0x0000000000000000]
  28.    java.lang.Thread.State: RUNNABLE
  30. "C2 CompilerThread2" #8 daemon prio=9 os_prio=31 tid=0x00007fb82402d800 nid=0x3d03 waiting on condition [0x0000000000000000]
  31.    java.lang.Thread.State: RUNNABLE
  33. "C2 CompilerThread1" #7 daemon prio=9 os_prio=31 tid=0x00007fb82505b800 nid=0x3e03 waiting on condition [0x0000000000000000]
  34.    java.lang.Thread.State: RUNNABLE
  36. "C2 CompilerThread0" #6 daemon prio=9 os_prio=31 tid=0x00007fb82505b000 nid=0x4003 waiting on condition [0x0000000000000000]
  37.    java.lang.Thread.State: RUNNABLE
  39. "Monitor Ctrl-Break" #5 daemon prio=5 os_prio=31 tid=0x00007fb82606c800 nid=0x3a03 runnable [0x000070000466f000]
  40.    java.lang.Thread.State: RUNNABLE
  41.         at java.net.SocketInputStream.socketRead0(Native Method)
  42.         at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
  43.         at java.net.SocketInputStream.read(SocketInputStream.java:171)
  44.         at java.net.SocketInputStream.read(SocketInputStream.java:141)
  45.         at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
  46.         at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
  47.         at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
  48.         - locked <0x000000076ac83cc0> (a java.io.InputStreamReader)
  49.         at java.io.InputStreamReader.read(InputStreamReader.java:184)
  50.         at java.io.BufferedReader.fill(BufferedReader.java:161)
  51.         at java.io.BufferedReader.readLine(BufferedReader.java:324)
  52.         - locked <0x000000076ac83cc0> (a java.io.InputStreamReader)
  53.         at java.io.BufferedReader.readLine(BufferedReader.java:389)
  54.         at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:47)
  56. "Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fb82500d000 nid=0x3903 runnable [0x0000000000000000]
  57.    java.lang.Thread.State: RUNNABLE
  59. "Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fb82402d000 nid=0x4d03 in Object.wait() [0x0000700004363000]
  60.    java.lang.Thread.State: WAITING (on object monitor)
  61.         at java.lang.Object.wait(Native Method)
  62.         - waiting on <0x000000076ab08ee0> (a java.lang.ref.ReferenceQueue$Lock)
  63.         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
  64.         - locked <0x000000076ab08ee0> (a java.lang.ref.ReferenceQueue$Lock)
  65.         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
  66.         at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)
  68. "Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fb825811000 nid=0x3403 in Object.wait() [0x0000700004260000]
  69.    java.lang.Thread.State: WAITING (on object monitor)
  70.         at java.lang.Object.wait(Native Method)
  71.         - waiting on <0x000000076ab06c00> (a java.lang.ref.Reference$Lock)
  72.         at java.lang.Object.wait(Object.java:502)
  73.         at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
  74.         - locked <0x000000076ab06c00> (a java.lang.ref.Reference$Lock)
  75.         at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
  77. "VM Thread" os_prio=31 tid=0x00007fb826812800 nid=0x3303 runnable 
  79. "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fb824013000 nid=0x2207 runnable 
  81. "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fb825008800 nid=0x2103 runnable 
  83. "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fb825009000 nid=0x1f03 runnable 
  85. "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fb825808800 nid=0x2a03 runnable 
  87. "GC task thread#4 (ParallelGC)" os_prio=31 tid=0x00007fb825809000 nid=0x2c03 runnable 
  89. "GC task thread#5 (ParallelGC)" os_prio=31 tid=0x00007fb825809800 nid=0x5403 runnable 
  91. "GC task thread#6 (ParallelGC)" os_prio=31 tid=0x00007fb82500a000 nid=0x2f03 runnable 
  93. "GC task thread#7 (ParallelGC)" os_prio=31 tid=0x00007fb82500a800 nid=0x3003 runnable 
  95. "GC task thread#8 (ParallelGC)" os_prio=31 tid=0x00007fb82580a000 nid=0x3103 runnable 
  97. "GC task thread#9 (ParallelGC)" os_prio=31 tid=0x00007fb824014000 nid=0x5003 runnable 
  99. "VM Periodic Task Thread" os_prio=31 tid=0x00007fb82a008800 nid=0x5803 waiting on condition 
  101. JNI global references: 15
  104. Found one Java-level deadlock:
  105. =============================
  106. "ThreadBBB":
  107.   waiting to lock monitor 0x00007fb82602b5e8 (object 0x000000076ac231d0, a java.lang.String),
  108.   which is held by "ThreadAAA"
  109. "ThreadAAA":
  110.   waiting to lock monitor 0x00007fb8260290c8 (object 0x000000076ac23208, a java.lang.String),
  111.   which is held by "ThreadBBB"
  113. Java stack information for the threads listed above:
  114. ===================================================
  115. "ThreadBBB":
  116.         at com.supkingx.base.f_thread.deadlock.HoldLockThread.run(HoldLockThread.java:32)
  117.         - waiting to lock <0x000000076ac231d0> (a java.lang.String)
  118.         - locked <0x000000076ac23208> (a java.lang.String)
  119.         at java.lang.Thread.run(Thread.java:748)
  120. "ThreadAAA":
  121.         at com.supkingx.base.f_thread.deadlock.HoldLockThread.run(HoldLockThread.java:32)
  122.         - waiting to lock <0x000000076ac23208> (a java.lang.String)
  123.         - locked <0x000000076ac231d0> (a java.lang.String)
  124.         at java.lang.Thread.run(Thread.java:748)
  126. Found 1 deadlock.

通过上述输出结果可以看到最后一行 Found 1 deadlock.。
根据输出结果如下，出现了互相等待，即产生了死锁
image.png