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Java中常見死鎖與活鎖的實例詳解

更新時間：2018年11月26日 11:37:20 作者：爬蜥

這篇文章主要介紹了Java中常見死鎖與活鎖的實例詳解，小編覺得挺不錯的，現(xiàn)在分享給大家，也給大家做個參考。一起跟隨小編過來看看吧

本文介紹了Java中常見死鎖與活鎖的實例詳解，分享給大家，具體如下：

順序死鎖：過度加鎖，導(dǎo)致由于執(zhí)行順序的原因，互相持有對方正在等待的鎖
資源死鎖：多個線程在相同的資源上發(fā)生等待

由于調(diào)用順序而產(chǎn)生的死鎖

public class Test {
  Object leftLock = new Object();
  Object rightLock = new Object();
  public static void main(String[] args) {
    final Test test = new Test();
    Thread a = new Thread(new Runnable() {
      @Override      public void run() {
        int i=0;
        while (i<10)
        {
          test.leftRight();
          i++;
        }
      }
    },"aThread");
    Thread b = new Thread(new Runnable() {
      @Override      public void run() {
        int i=0;
        while (i<10)
        {
          test.rightleft();
          i++;
        }
      }
    },"bThread");
    a.start();
    b.start();
  }

  public void leftRight(){
    synchronized (leftLock){
      System.out.println(Thread.currentThread().getName()+":leftRight:get left");
      synchronized (rightLock){
        System.out.println(Thread.currentThread().getName()+":leftRight:get right");
      }
    }
  }

  public void rightleft(){
    synchronized (rightLock){
      System.out.println(Thread.currentThread().getName()+":rightleft: get right");
      synchronized (leftLock){
        System.out.println(Thread.currentThread().getName()+":rightleft: get left");
      }
    }
  }

}

運(yùn)行后輸出如下

aThread:leftRight:get left
bThread:rightleft: get right

可以通過jstack發(fā)現(xiàn)死鎖的痕跡

"bThread" prio=5 tid=0x00007fabb2001000 nid=0x5503 waiting for monitor entry [0x000000011d54b000]
  java.lang.Thread.State: BLOCKED (on object monitor)
  at main.lockTest.Test.rightleft(Test.java:52)
  - waiting to lock <0x00000007aaee5748> (a java.lang.Object)
  - locked <0x00000007aaee5758> (a java.lang.Object)
  at main.lockTest.Test$2.run(Test.java:30)
  at java.lang.Thread.run(Thread.java:745)

  Locked ownable synchronizers:
  - None

"aThread" prio=5 tid=0x00007fabb2801000 nid=0x5303 waiting for monitor entry [0x000000011d448000]
  java.lang.Thread.State: BLOCKED (on object monitor)
  at main.lockTest.Test.leftRight(Test.java:43)
  - waiting to lock <0x00000007aaee5758> (a java.lang.Object)
  - locked <0x00000007aaee5748> (a java.lang.Object)
  at main.lockTest.Test$1.run(Test.java:19)
  at java.lang.Thread.run(Thread.java:745)

  Locked ownable synchronizers:
  - None

可以看到bThread持有鎖0x00000007aaee5758,同時等待0x00000007aaee5748,然而恰好aThread持有鎖0x00000007aaee5748并等待0x00000007aaee5758,從而形成了死鎖

線程饑餓死鎖

public class ExecutorLock {
  private static ExecutorService single=Executors.newSingleThreadExecutor();
  public static class AnotherCallable implements Callable<String>{

    @Override    public String call() throws Exception {
      System.out.println("in AnotherCallable");
      return "annother success";
    }
  }


  public static class MyCallable implements Callable<String>{

    @Override    public String call() throws Exception {
      System.out.println("in MyCallable");
      Future<String> submit = single.submit(new AnotherCallable());
      return "success:"+submit.get();
    }
  }
  public static void main(String[] args) throws ExecutionException, InterruptedException {
    MyCallable task = new MyCallable();
    Future<String> submit = single.submit(task);
    System.out.println(submit.get());
    System.out.println("over");
    single.shutdown();
  }
}

執(zhí)行的輸出只有一行

in MyCallable

通過jstack觀察可以看到如下

"main" prio=5 tid=0x00007fab3f000000 nid=0x1303 waiting on condition [0x0000000107d63000]
  java.lang.Thread.State: WAITING (parking)
  at sun.misc.Unsafe.park(Native Method)
  - parking to wait for <0x00000007aaeed1d8> (a java.util.concurrent.FutureTask)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
  at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425)
  at java.util.concurrent.FutureTask.get(FutureTask.java:187)
  at main.lockTest.ExecutorLock.main(ExecutorLock.java:32)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:606)
  at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

  Locked ownable synchronizers:
  - None
..
"pool-1-thread-1" prio=5 tid=0x00007fab3f835800 nid=0x5303 waiting on condition [0x00000001199ee000]
  java.lang.Thread.State: WAITING (parking)
  at sun.misc.Unsafe.park(Native Method)
  - parking to wait for <0x00000007ab0f8698> (a java.util.concurrent.FutureTask)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
  at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425)
  at java.util.concurrent.FutureTask.get(FutureTask.java:187)
  at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:26)
  at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:20)
  at java.util.concurrent.FutureTask.run(FutureTask.java:262)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
  at java.lang.Thread.run(Thread.java:745)

  Locked ownable synchronizers:
  - <0x00000007aaeed258> (a java.util.concurrent.ThreadPoolExecutor$Worker)

主線程在等待一個FutureTask完成，而線程池中一個線程也在等待一個FutureTask完成。
從代碼實現(xiàn)可以看到，主線程往線程池中扔了一個任務(wù)A，任務(wù)A又往同一個線程池中扔了一個任務(wù)B，并等待B的完成，由于線程池中只有一個線程，這將導(dǎo)致B會被停留在阻塞隊列中，而A還得等待B的完成，這也就是互相等待導(dǎo)致了死鎖的反生

這種由于正在執(zhí)行的任務(wù)線程都在等待其它工作隊列中的任務(wù)而阻塞的現(xiàn)象稱為線程饑餓死鎖

活鎖

并未產(chǎn)生線程阻塞，但是由于某種問題的存在，導(dǎo)致無法繼續(xù)執(zhí)行的情況。

1、消息重試。當(dāng)某個消息處理失敗的時候，一直重試，但重試由于某種原因，比如消息格式不對，導(dǎo)致解析失敗，而它又被重試

這種時候一般是將不可修復(fù)的錯誤不要重試，或者是重試次數(shù)限定

2、相互協(xié)作的線程彼此響應(yīng)從而修改自己狀態(tài)，導(dǎo)致無法執(zhí)行下去。比如兩個很有禮貌的人在同一條路上相遇，彼此給對方讓路，但是又在同一條路上遇到了?；ハ嘀g反復(fù)的避讓下去

這種時候可以選擇一個隨機(jī)退讓，使得具備一定的隨機(jī)性

以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

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