Java 中的死锁示例

java中的死锁是两个或多个线程永远被阻塞的一种编程情况。 Java 死锁情况发生在至少两个线程和两个或更多资源的情况下。这里我写了一个简单的程序，该程序会导致java死锁场景，然后我们将看到如何分析它。

Java 中的死锁

Let’s have a look at a simple program where I will create deadlock in java threads.

package com.journaldev.threads;

public class ThreadDeadlock {

    public static void main(String[] args) throws InterruptedException {
        Object obj1 = new Object();
        Object obj2 = new Object();
        Object obj3 = new Object();
    
        Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
        Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
        Thread t3 = new Thread(new SyncThread(obj3, obj1), "t3");
        
        t1.start();
        Thread.sleep(5000);
        t2.start();
        Thread.sleep(5000);
        t3.start();
        
    }

}

class SyncThread implements Runnable{
    private Object obj1;
    private Object obj2;

    public SyncThread(Object o1, Object o2){
        this.obj1=o1;
        this.obj2=o2;
    }
    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name + " acquiring lock on "+obj1);
        synchronized (obj1) {
         System.out.println(name + " acquired lock on "+obj1);
         work();
         System.out.println(name + " acquiring lock on "+obj2);
         synchronized (obj2) {
            System.out.println(name + " acquired lock on "+obj2);
            work();
        }
         System.out.println(name + " released lock on "+obj2);
        }
        System.out.println(name + " released lock on "+obj1);
        System.out.println(name + " finished execution.");
    }
    private void work() {
        try {
            Thread.sleep(30000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在上面的程序中，SyncThread 实现了 Runnable 接口，它通过使用同步块一一获取每个对象的锁来对两个对象进行操作。在 main 方法中，我为 SyncThread 运行三个线程，并且每个线程之间都有共享资源。线程的运行方式使得它能够获取第一个对象的锁，但是当它尝试获取第二个对象的锁时，它会进入等待状态，因为它已经被另一个线程锁定。这形成了线程之间资源的循环依赖，导致死锁。当我执行上面的程序时，这是生成的输出，但由于 java 线程中的死锁，程序永远不会终止。

t1 acquiring lock on java.lang.Object@6d9dd520
t1 acquired lock on java.lang.Object@6d9dd520
t2 acquiring lock on java.lang.Object@22aed3a5
t2 acquired lock on java.lang.Object@22aed3a5
t3 acquiring lock on java.lang.Object@218c2661
t3 acquired lock on java.lang.Object@218c2661
t1 acquiring lock on java.lang.Object@22aed3a5
t2 acquiring lock on java.lang.Object@218c2661
t3 acquiring lock on java.lang.Object@6d9dd520

在这里，我们可以从输出中清楚地识别出死锁情况，但在实际应用中，很难找到死锁情况并对其进行调试。

如何检测 Java 中的死锁

要检测java中的死锁，我们需要查看java线程转储在上一篇文章中，我解释了如何使用 VisualVM 分析器或使用jstack公用事业。这是上述程序的线程转储。

2012-12-27 19:08:34
Full thread dump Java HotSpot(TM) 64-Bit Server VM (23.5-b02 mixed mode):

"Attach Listener" daemon prio=5 tid=0x00007fb0a2814000 nid=0x4007 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" prio=5 tid=0x00007fb0a2801000 nid=0x1703 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"t3" prio=5 tid=0x00007fb0a204b000 nid=0x4d07 waiting for monitor entry [0x000000015d971000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f658> (a java.lang.Object)
	- locked <0x000000013df2f678> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"t2" prio=5 tid=0x00007fb0a1073000 nid=0x4207 waiting for monitor entry [0x000000015d209000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f678> (a java.lang.Object)
	- locked <0x000000013df2f668> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"t1" prio=5 tid=0x00007fb0a1072000 nid=0x5503 waiting for monitor entry [0x000000015d86e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f668> (a java.lang.Object)
	- locked <0x000000013df2f658> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"Service Thread" daemon prio=5 tid=0x00007fb0a1038000 nid=0x5303 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=5 tid=0x00007fb0a1037000 nid=0x5203 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=5 tid=0x00007fb0a1016000 nid=0x5103 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=5 tid=0x00007fb0a4003000 nid=0x5003 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=5 tid=0x00007fb0a4800000 nid=0x3f03 in Object.wait() [0x000000015d0c0000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
	- locked <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
	at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:177)

"Reference Handler" daemon prio=5 tid=0x00007fb0a4002000 nid=0x3e03 in Object.wait() [0x000000015cfbd000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000013de75320> (a java.lang.ref.Reference$Lock)
	at java.lang.Object.wait(Object.java:503)
	at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
	- locked <0x000000013de75320> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=5 tid=0x00007fb0a2049800 nid=0x3d03 runnable 

"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007fb0a300d800 nid=0x3503 runnable 

"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007fb0a2001800 nid=0x3603 runnable 

"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007fb0a2003800 nid=0x3703 runnable 

"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007fb0a2004000 nid=0x3803 runnable 

"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007fb0a2005000 nid=0x3903 runnable 

"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007fb0a2005800 nid=0x3a03 runnable 

"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007fb0a2006000 nid=0x3b03 runnable 

"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007fb0a2006800 nid=0x3c03 runnable 

"VM Periodic Task Thread" prio=5 tid=0x00007fb0a1015000 nid=0x5403 waiting on condition 

JNI global references: 114


Found one Java-level deadlock:
=============================
"t3":
  waiting to lock monitor 0x00007fb0a1074b08 (object 0x000000013df2f658, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x00007fb0a1010f08 (object 0x000000013df2f668, a java.lang.Object),
  which is held by "t2"
"t2":
  waiting to lock monitor 0x00007fb0a1012360 (object 0x000000013df2f678, a java.lang.Object),
  which is held by "t3"

Java stack information for the threads listed above:
===================================================
"t3":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f658> (a java.lang.Object)
	- locked <0x000000013df2f678> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)
"t1":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f668> (a java.lang.Object)
	- locked <0x000000013df2f658> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)
"t2":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f678> (a java.lang.Object)
	- locked <0x000000013df2f668> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

Found 1 deadlock.

线程转储输出清楚地显示了死锁情况以及导致死锁情况所涉及的线程和资源。为了分析死锁，我们需要寻找状态为BLOCKED然后是资源等待锁定。每个资源都有一个唯一的 ID，使用它我们可以找到哪个线程已经持有该对象的锁。例如，线程“t3”正在等待锁定 0x000000013df2f658，但它已经被线程“t1”锁定。一旦我们分析了死锁情况并找出了导致死锁的线程，我们就需要更改代码以避免死锁情况。

java中如何避免死锁

这些是我们可以避免大多数僵局情况的一些准则。

• 避免嵌套锁：这是死锁的最常见原因，如果您已经持有一个资源，请避免锁定另一个资源。如果您只使用一个对象锁，则几乎不可能出现死锁情况。例如，这里是 run() 方法的另一种实现，没有嵌套锁，程序成功运行，没有出现死锁情况。

      public void run() {
          String name = Thread.currentThread().getName();
          System.out.println(name + " acquiring lock on " + obj1);
          synchronized (obj1) {
              System.out.println(name + " acquired lock on " + obj1);
              work();
          }
          System.out.println(name + " released lock on " + obj1);
          System.out.println(name + " acquiring lock on " + obj2);
          synchronized (obj2) {
              System.out.println(name + " acquired lock on " + obj2);
              work();
          }
          System.out.println(name + " released lock on " + obj2);
  
          System.out.println(name + " finished execution.");
      }
  

• 仅锁定需要的内容：您应该仅对必须处理的资源获取锁定，例如在上面的程序中，我锁定了完整的对象资源，但如果我们只对其其中一个字段感兴趣，那么我们应该仅锁定该特定字段而不是完整的对象。

• 避免无限期等待：如果两个线程无限期地等待对方完成，则可能会出现死锁螺纹连接。如果您的线程必须等待另一个线程完成，那么最好使用连接并设置您想要等待线程完成的最长时间。

这就是java线程死锁的全部内容。