Java死锁范例以及如何分析死锁

左手的ㄟ右手 2022-07-13 16:46 277阅读 0赞

死锁是两个甚至多个线程被永久阻塞时的一种运行局面，这种局面的生成伴随着至少两个线程和两个或者多个资源。在这里我已写好一个简单的程序，它将会引起死锁方案然后我们就会明白如何分析它。

# Java死锁范例 #

ThreadDeadlock.java

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();
    		}
    	}
    	
    }

在上面的程序中同步线程正完成Runnable的接口，它工作的是两个对象，这两个对象向对方寻求死锁而且都在使用同步阻塞。

在主函数中，我使用了三个为同步线程运行的线程，而且在其中每个线程中都有一个可共享的资源。

这些线程以向第一个对象获取封锁这种方式运行。但是当它试着像第二个对象获取封锁时，它就会进入等待状态，因为它已经被另一个线程封锁住了。这样，在线程引起死锁的过程中，就形成了一个依赖于资源的循环。

当我执行上面的程序时，就产生了输出，但是程序却因为死锁无法停止。

t1 acquiring lock on java.lang.Object@2e92b791
    t1 acquired lock on java.lang.Object@2e92b791
    t2 acquiring lock on java.lang.Object@12d687a2
    t2 acquired lock on java.lang.Object@12d687a2
    t3 acquiring lock on java.lang.Object@3b1c2f38
    t3 acquired lock on java.lang.Object@3b1c2f38
    t1 acquiring lock on java.lang.Object@12d687a2
    t2 acquiring lock on java.lang.Object@3b1c2f38
    t3 acquiring lock on java.lang.Object@2e92b791

在此我们可以清楚地在输出结果中辨认出死锁局面，但是在我们实际生活所用的应用中，发现死锁并将它排除是非常难的。

# 分析死锁 #

为了分析一个死锁，这里我使用了VisualVM收集资料，当然你也可以使用jstack来看该信息

以下就是上述程序的线程转存。

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.

这三个线程转存的输出清楚地说明了死锁环境和线程，以及包含死锁环境的资源。

为了分析死锁，我们需要关注死锁状态的线程，然后资源再等待去封锁，每一个资源都有一个独特的ID，有了这个ID我们就能发现是哪一个进程已经封锁住对象。举个例子，线程“t3”正在等待封锁0x000000013df2f658,但是它已经被线程“t1”封锁住了。

当我们分析死锁环境的时候，如果发现线程正在引起死锁，这是我们就要改变代码来避免死锁的产生。

visualvm的截图如下：

![Center][]

![Center 1][]

jstack的截图如下：

![Center 2][]

![Center 3][]

# 避免死锁 #

有很多方针可供我们使用来避免死锁的局面。

*  **避免嵌套封锁**：这是死锁最主要的原因的，如果你已经有一个资源了就要避免封锁另一个资源。如果你运行时只有一个对象封锁，那是几乎不可能出现一个死锁局面的。例如，这里是另一个运行中没有嵌套封锁的run()方法，而且程序运行没有死锁局面，运行得很成功。
 *      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.");
        }

*  **只对有请求的进行封锁**：你应当只想你要运行的资源获取封锁，比如在上述程序中我在封锁的完全的对象资源。但是如果我们只对它所属领域中的一个感兴趣，那我们应当封锁住那个特殊的领域而并非完全的对象。
 *  **避免无限期的等待**：如果两个线程正在等待对象结束，无限期的使用线程加入，如果你的线程必须要等待另一个线程的结束，若是等待进程的结束加入最好准备最长时间。（[设置线程超时时间之 Callable接口和Future接口][Callable_Future]）

[Center]: /images/20220709/ff6b937027394c4dad977cc8cc8a7196.png
[Center 1]: /images/20220709/a832b02f6e5a4a018d5ae885ed75c23a.png
[Center 2]: /images/20220709/a5ef525ff5304450a35e1cf1465d7aa2.png
[Center 3]: /images/20220709/2992c0db9f784a49aebc0c12e8b9e1da.png
[Callable_Future]: http://blog.csdn.net/li12412414/article/details/54577034