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Java死鎖_動(dòng)力節(jié)點(diǎn)Java學(xué)院整理

發(fā)布時(shí)間：2020-09-06 14:59:02 來(lái)源：腳本之家 閱讀：154 作者：mrr 欄目：編程語(yǔ)言

死鎖是兩個(gè)甚至多個(gè)線程被永久阻塞時(shí)的一種運(yùn)行局面，這種局面的生成伴隨著至少兩個(gè)線程和兩個(gè)或者多個(gè)資源。在這里我已寫好一個(gè)簡(jiǎn)單的程序，它將會(huì)引起死鎖方案然后我們就會(huì)明白如何分析它。

Java死鎖范例

ThreadDeadlock.java

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(gè)對(duì)象，這兩個(gè)對(duì)象向?qū)Ψ綄で笏梨i而且都在使用同步阻塞。

在主函數(shù)中，我使用了三個(gè)為同步線程運(yùn)行的線程，而且在其中每個(gè)線程中都有一個(gè)可共享的資源。

這些線程以向第一個(gè)對(duì)象獲取封鎖這種方式運(yùn)行。但是當(dāng)它試著像第二個(gè)對(duì)象獲取封鎖時(shí)，它就會(huì)進(jìn)入等待狀態(tài)，因?yàn)樗呀?jīng)被另一個(gè)線程封鎖住了。這樣，在線程引起死鎖的過(guò)程中，就形成了一個(gè)依賴于資源的循環(huán)。

當(dāng)我執(zhí)行上面的程序時(shí)，就產(chǎn)生了輸出，但是程序卻因?yàn)樗梨i無(wú)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(jié)果中辨認(rèn)出死鎖局面，但是在我們實(shí)際生活所用的應(yīng)用中，發(fā)現(xiàn)死鎖并將它排除是非常難的。

分析死鎖

 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.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: 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.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(gè)線程轉(zhuǎn)存的輸出清楚地說(shuō)明了死鎖環(huán)境和線程，以及包含死鎖環(huán)境的資源。

為了分析死鎖，我們需要關(guān)注死鎖狀態(tài)的線程，然后資源再等待去封鎖，每一個(gè)資源都有一個(gè)獨(dú)特的ID，有了這個(gè)ID我們就能發(fā)現(xiàn)是哪一個(gè)進(jìn)程已經(jīng)封鎖住對(duì)象。舉個(gè)例子，線程“t3”正在等待封鎖0x000000013df2f658,但是它已經(jīng)被線程“t1”封鎖住了。

當(dāng)我們分析死鎖環(huán)境的時(shí)候，如果發(fā)現(xiàn)線程正在引起死鎖，這是我們就要改變代碼來(lái)避免死鎖的產(chǎn)生。

避免死鎖

有很多方針可供我們使用來(lái)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(jīng)有一個(gè)資源了就要避免封鎖另一個(gè)資源。如果你運(yùn)行時(shí)只有一個(gè)對(duì)象封鎖，那是幾乎不可能出現(xiàn)一個(gè)死鎖局面的。例如，這里是另一個(gè)運(yùn)行中沒有嵌套封鎖的run()方法，而且程序運(yùn)行沒有死鎖局面，運(yùn)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.");
}

只對(duì)有請(qǐng)求的進(jìn)行封鎖：你應(yīng)當(dāng)只想你要運(yùn)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(duì)象資源。但是如果我們只對(duì)它所屬領(lǐng)域中的一個(gè)感興趣，那我們應(yīng)當(dāng)封鎖住那個(gè)特殊的領(lǐng)域而并非完全的對(duì)象。

避免無(wú)限期的等待：如果兩個(gè)線程正在等待對(duì)象結(jié)束，無(wú)限期的使用線程加入，如果你的線程必須要等待另一個(gè)線程的結(jié)束，若是等待進(jìn)程的結(jié)束加入最好準(zhǔn)備最長(zhǎng)時(shí)間。

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