Java ThreadPoolExecutor的參數深入理解

更新時間：2017年03月22日 15:06:57 作者：Lubby

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

Java ThreadPoolExecutor的參數深入理解

一、使用Executors創(chuàng)建線程池

之前創(chuàng)建線程的時候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()這三個方法。當然Executors也是用不同的參數去new ThreadPoolExecutor

1. newFixedThreadPool()

創(chuàng)建線程數固定大小的線程池。由于使用了LinkedBlockingQueue所以maximumPoolSize 沒用，當corePoolSize滿了之后就加入到LinkedBlockingQueue隊列中。每當某個線程執(zhí)行完成之后就從LinkedBlockingQueue隊列中取一個。所以這個是創(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表示線程數大小為1,滿了就放入隊列中，執(zhí)行完了就從隊列取一個。

 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所以任務會放入SynchronousQueue隊列中，SynchronousQueue只能存放大小為1，所以會立刻新起線程，由于maxumumPoolSize為Integer.MAX_VALUE所以可以認為大小為2147483647。受內存大小限制。

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的構造函數

 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：

1、corePoolSize核心線程數大小，當線程數<corePoolSize ，會創(chuàng)建線程執(zhí)行runnable

2、maximumPoolSize 最大線程數，當線程數 >= corePoolSize的時候，會把runnable放入workQueue中

3、keepAliveTime 保持存活時間，當線程數大于corePoolSize的空閑線程能保持的最大時間。

4、unit 時間單位

5、workQueue 保存任務的阻塞隊列

6、threadFactory 創(chuàng)建線程的工廠

7、handler 拒絕策略

任務執(zhí)行順序：

1、當線程數小于corePoolSize時，創(chuàng)建線程執(zhí)行任務。

2、當線程數大于等于corePoolSize并且workQueue沒有滿時，放入workQueue中

3、線程數大于等于corePoolSize并且當workQueue滿時，新任務新建線程運行，線程總數要小于maximumPoolSize

4、當線程總數等于maximumPoolSize并且workQueue滿了的時候執(zhí)行handler的rejectedExecution。也就是拒絕策略。

ThreadPoolExecutor默認有四個拒絕策略：

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

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

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

4、ThreadPoolExecutor.DiscardOldestPolicy() 丟棄在隊列中隊首的任務

當然可以自己繼承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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