死鎖是兩個甚至多個線程被永久阻塞時的一種運行局面，這種局面的生成伴隨著至少兩個線程和兩個或者多個資源。在這里我已寫好一個簡單的程序，它將會引起死鎖方案然后我們就會明白如何分析它。
Java死鎖范例
ThreadDeadlock.java
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package com.bjpowernode.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(); } }} |
在上面的程序中同步線程正完成Runnable的接口，它工作的是兩個對象，這兩個對象向對方尋求死鎖而且都在使用同步阻塞。
在主函數中，我使用了三個為同步線程運行的線程，而且在其中每個線程中都有一個可共享的資源。
這些線程以向第一個對象獲取封鎖這種方式運行。但是當它試著像第二個對象獲取封鎖時，它就會進入等待狀態，因為它已經被另一個線程封鎖住了。這樣，在線程引起死鎖的過程中，就形成了一個依賴于資源的循環。
當我執行上面的程序時，就產生了輸出，但是程序卻因為死鎖無法停止。
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
在此我們可以清楚地在輸出結果中辨認出死鎖局面，但是在我們實際生活所用的應用中，發現死鎖并將它排除是非常難的。
分析死鎖
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2012-12-27 19:08:34Full 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.bjpowernode.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.bjpowernode.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.bjpowernode.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: 114Found 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.bjpowernode.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.bjpowernode.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.bjpowernode.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. |
這三個線程轉存的輸出清楚地說明了死鎖環境和線程，以及包含死鎖環境的資源。
為了分析死鎖，我們需要關注死鎖狀態的線程，然后資源再等待去封鎖，每一個資源都有一個獨特的ID，有了這個ID我們就能發現是哪一個進程已經封鎖住對象。舉個例子，線程“t3”正在等待封鎖0x000000013df2f658,但是它已經被線程“t1”封鎖住了。
當我們分析死鎖環境的時候，如果發現線程正在引起死鎖，這是我們就要改變代碼來避免死鎖的產生。
避免死鎖
有很多方針可供我們使用來避免死鎖的局面。
避免嵌套封鎖：這是死鎖最主要的原因的，如果你已經有一個資源了就要避免封鎖另一個資源。如果你運行時只有一個對象封鎖，那是幾乎不可能出現一個死鎖局面的。例如，這里是另一個運行中沒有嵌套封鎖的run()方法，而且程序運行沒有死鎖局面，運行得很成功。
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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.");} |
只對有請求的進行封鎖：你應當只想你要運行的資源獲取封鎖，比如在上述程序中我在封鎖的完全的對象資源。但是如果我們只對它所屬領域中的一個感興趣，那我們應當封鎖住那個特殊的領域而并非完全的對象。
避免無限期的等待：如果兩個線程正在等待對象結束，無限期的使用線程加入，如果你的線程必須要等待另一個線程的結束，若是等待進程的結束加入最好準備最長時間。
