Java中ThreadPoolExecutor的参数理解

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一、使用Executors创建线程池

之前创建线程的时候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()这三个方法。当然Executors也是用不同的参数去new ThreadPoolExecutor

1. newFixedThreadPool()

创建线程数固定大小的线程池。由于使用了LinkedBlockingQueue所以maximumPoolSize 没用，当corePoolSize满了之后就加入到LinkedBlockingQueue队列中。每当某个线程执行完成之后就从LinkedBlockingQueue队列中取一个。所以这个是创建固定大小的线程池。

 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()

创建线程数为1的线程池，由于使用了LinkedBlockingQueue所以maximumPoolSize 没用，corePoolSize为1表示线程数大小为1,满了就放入队列中，执行完了就从队列取一个。

 public static ExecutorService newSingleThreadExecutor() {

        return new FinalizableDelegatedExecutorService

            (new ThreadPoolExecutor(1, 1,

                                    0L, TimeUnit.MILLISECONDS,

                                    new LinkedBlockingQueue<Runnable>()));

}

3.newCachedThreadPool()

创建可缓冲的线程池。没有大小限制。由于corePoolSize为0所以任务会放入SynchronousQueue队列中，SynchronousQueue只能存放大小为1，所以会立刻新起线程，由于maxumumPoolSize为Integer.MAX_VALUE所以可以认为大小为2147483647。受内存大小限制。

 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创建线程池

ThreadPoolExecutor的构造函数

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  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;
    }

参数：

1、corePoolSize核心线程数大小，当线程数<corePoolSize ，会创建线程执行runnable

2、maximumPoolSize 最大线程数，当线程数 >= corePoolSize的时候，会把runnable放入workQueue中

3、keepAliveTime 保持存活时间，当线程数大于corePoolSize的空闲线程能保持的最大时间。

4、unit 时间单位

5、workQueue 保存任务的阻塞队列

6、threadFactory 创建线程的工厂

7、handler 拒绝策略

任务执行顺序：

1、当线程数小于corePoolSize时，创建线程执行任务。

2、当线程数大于等于corePoolSize并且workQueue没有满时，放入workQueue中

3、线程数大于等于corePoolSize并且当workQueue满时，新任务新建线程运行，线程总数要小于maximumPoolSize

4、当线程总数等于maximumPoolSize并且workQueue满了的时候执行handler的rejectedExecution。也就是拒绝策略。

ThreadPoolExecutor默认有四个拒绝策略：

1、ThreadPoolExecutor.AbortPolicy() 直接抛出异常RejectedExecutionException

2、ThreadPoolExecutor.CallerRunsPolicy() 直接调用run方法并且阻塞执行

3、ThreadPoolExecutor.DiscardPolicy() 直接丢弃后来的任务

4、ThreadPoolExecutor.DiscardOldestPolicy() 丢弃在队列中队首的任务

当然可以自己继承RejectedExecutionHandler来写拒绝策略.

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        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();
        //线程池和队列满了之后的处理方式
        //1.跑出异常
        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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