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Java ThreadPoolExecutor的參數(shù)深入理解

更新時(shí)間：2017年03月22日 15:06:57 作者：Lubby

這篇文章主要介紹了Java ThreadPoolExecutor的參數(shù)深入理解的相關(guān)資料,需要的朋友可以參考下

Java ThreadPoolExecutor的參數(shù)深入理解

一、使用Executors創(chuàng)建線程池

之前創(chuàng)建線程的時(shí)候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()這三個(gè)方法。當(dāng)然Executors也是用不同的參數(shù)去new ThreadPoolExecutor

1. newFixedThreadPool()

創(chuàng)建線程數(shù)固定大小的線程池。由于使用了LinkedBlockingQueue所以maximumPoolSize 沒用，當(dāng)corePoolSize滿了之后就加入到LinkedBlockingQueue隊(duì)列中。每當(dāng)某個(gè)線程執(zhí)行完成之后就從LinkedBlockingQueue隊(duì)列中取一個(gè)。所以這個(gè)是創(chuàng)建固定大小的線程池。

 public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                   0L, TimeUnit.MILLISECONDS,
                   new LinkedBlockingQueue<Runnable>());
  }
 public ThreadPoolExecutor(int corePoolSize,
               int maximumPoolSize,
               long keepAliveTime,
               TimeUnit unit,
               BlockingQueue<Runnable> workQueue) {
   this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
       Executors.defaultThreadFactory(), defaultHandler);
  }

2.newSingleThreadPool()

創(chuàng)建線程數(shù)為1的線程池，由于使用了LinkedBlockingQueue所以maximumPoolSize 沒用，corePoolSize為1表示線程數(shù)大小為1,滿了就放入隊(duì)列中，執(zhí)行完了就從隊(duì)列取一個(gè)。

 public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
      (new ThreadPoolExecutor(1, 1,
                  0L, TimeUnit.MILLISECONDS,
                  new LinkedBlockingQueue<Runnable>()));
  }

3.newCachedThreadPool()

創(chuàng)建可緩沖的線程池。沒有大小限制。由于corePoolSize為0所以任務(wù)會(huì)放入SynchronousQueue隊(duì)列中，SynchronousQueue只能存放大小為1，所以會(huì)立刻新起線程，由于maxumumPoolSize為Integer.MAX_VALUE所以可以認(rèn)為大小為2147483647。受內(nèi)存大小限制。

public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                   60L, TimeUnit.SECONDS,
                   new SynchronousQueue<Runnable>());
  }
public ThreadPoolExecutor(int corePoolSize,
             int maximumPoolSize,
             long keepAliveTime,
             TimeUnit unit,
             BlockingQueue<Runnable> workQueue) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
       Executors.defaultThreadFactory(), defaultHandler);
  }

二、使用ThreadPoolExecutor創(chuàng)建線程池

ThreadPoolExecutor的構(gòu)造函數(shù)

 public ThreadPoolExecutor(int corePoolSize,
               int maximumPoolSize,
               long keepAliveTime,
               TimeUnit unit,
               BlockingQueue<Runnable> workQueue,
               ThreadFactory threadFactory,
               RejectedExecutionHandler handler) {
    if (corePoolSize < 0 ||
      maximumPoolSize <= 0 ||
      maximumPoolSize < corePoolSize ||
      keepAliveTime < 0)
      throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
      throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
  }

參數(shù)：

1、corePoolSize核心線程數(shù)大小，當(dāng)線程數(shù)<corePoolSize ，會(huì)創(chuàng)建線程執(zhí)行runnable

2、maximumPoolSize 最大線程數(shù)，當(dāng)線程數(shù) >= corePoolSize的時(shí)候，會(huì)把runnable放入workQueue中

3、keepAliveTime 保持存活時(shí)間，當(dāng)線程數(shù)大于corePoolSize的空閑線程能保持的最大時(shí)間。

4、unit 時(shí)間單位

5、workQueue 保存任務(wù)的阻塞隊(duì)列

6、threadFactory 創(chuàng)建線程的工廠

7、handler 拒絕策略

任務(wù)執(zhí)行順序：

1、當(dāng)線程數(shù)小于corePoolSize時(shí)，創(chuàng)建線程執(zhí)行任務(wù)。

2、當(dāng)線程數(shù)大于等于corePoolSize并且workQueue沒有滿時(shí)，放入workQueue中

3、線程數(shù)大于等于corePoolSize并且當(dāng)workQueue滿時(shí)，新任務(wù)新建線程運(yùn)行，線程總數(shù)要小于maximumPoolSize

4、當(dāng)線程總數(shù)等于maximumPoolSize并且workQueue滿了的時(shí)候執(zhí)行handler的rejectedExecution。也就是拒絕策略。

ThreadPoolExecutor默認(rèn)有四個(gè)拒絕策略：

1、ThreadPoolExecutor.AbortPolicy() 直接拋出異常RejectedExecutionException

2、ThreadPoolExecutor.CallerRunsPolicy() 直接調(diào)用run方法并且阻塞執(zhí)行

3、ThreadPoolExecutor.DiscardPolicy() 直接丟棄后來的任務(wù)

4、ThreadPoolExecutor.DiscardOldestPolicy() 丟棄在隊(duì)列中隊(duì)首的任務(wù)

當(dāng)然可以自己繼承RejectedExecutionHandler來寫拒絕策略.

int corePoolSize = 1;
 int maximumPoolSize = 2;
 int keepAliveTime = 10;
// BlockingQueue<Runnable> workQueue = new LinkedBlockingQueue<Runnable>();
 BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<Runnable>(5);
 ThreadFactory threadFactory = Executors.defaultThreadFactory();
 //線程池和隊(duì)列滿了之后的處理方式
 //1.跑出異常
 RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy(); 
 RejectedExecutionHandler handler2 = new ThreadPoolExecutor.CallerRunsPolicy();
 RejectedExecutionHandler handler3 = new ThreadPoolExecutor.DiscardPolicy();
 RejectedExecutionHandler handler4 = new ThreadPoolExecutor.DiscardOldestPolicy();

 
 ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, TimeUnit.SECONDS, workQueue, threadFactory, handler2);
 
 
 for (int j = 1; j < 15; j++) {
  threadPoolExecutor.execute(new Runnable() {
  
  public void run() {
   
   try {
   System.out.println(Thread.currentThread().getName());
   TimeUnit.SECONDS.sleep(1);
   } catch (InterruptedException e) {
   e.printStackTrace();
   }
   
   
  }
  });
 }
 
 System.out.println(threadPoolExecutor);
 
 }

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