Java死鎖範例以及如何分析死鎖(轉載自ImportNew)
本文由 ImportNew - 範琦琦 翻譯自 journaldev。歡迎加入翻譯小組。轉載請見文末要求。
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。在這裏我已寫好一個簡單的程序,它將會引起死鎖方案然後我們就會明白如何分析它。
Java死鎖範例
ThreadDeadlock.java
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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();
}
}
}
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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,就形成了一個依賴於資源的循環。
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。
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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,發現死鎖並將它排除是非常難的。
分析死鎖
為了分析一個死鎖,我們需要關註下應用中的Java線程轉存,在上一節中我已經解釋了如何使用VisualVM收集資料或者jstack應用程序產生線程轉存。
以下就是上述程序的線程轉存。
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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.
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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,我們需要關註死鎖狀態的線程,然後資源再等待去封鎖,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,這裏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." );
}
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- 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並非完全的對象。
- 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。
原文鏈接: journaldev 翻譯: ImportNew.com - 範琦琦
譯文鏈接: http://www.importnew.com/9668.html
[ 轉載請保留原文出處、譯者和譯文鏈接。]
關於作者: 範琦琦
Java死鎖範例以及如何分析死鎖(轉載自ImportNew)