从源码看ThreadPoolExecutor(线程池)的参数

在jdk1.8的类ThreadPoolExecutor中创建有参的构造器。在下面的代码中展示了涉及到的参数。

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;
    }

    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */

但是有时候在面试的过程中，会问到你问到这些参数的意思。上面的注释就是对各个参数的解释，在接下来的过程中将分别解释线程池的7个参数。

  /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set

corePoolSize: 即使有些线程是闲置的，都要保持池中核心线程的数量，除非你设置了allowCoreThreadTimeOut参数

* @param maximumPoolSize the maximum number of threads to allow in the
     *        pool

maximumPoolSize:参数maximumPoolSize是在池中允许的最大线程数
 

* @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.

keepAliveTime（存活时间） ：当某个线程的线程数远大于核心数时，在新任务中大量的空闲线程一直处于等待状态，那就回收它。 

* @param unit the time unit for the {@code keepAliveTime} argument

unit: keepAliveTime参数的时间单位

  * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.

workQueue（存放待执行任务的队列）:在任务执行之前提交任务到队列中使用。这个队列仅仅存放Runnable任务被execute方法提交。(又可以叫做存放处于等待状态线程的队列)

 * @param threadFactory the factory to use when the executor
     *        creates a new thread

threadFactory（线程工厂）：创建线程时工厂开始使用。

* @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached

handler（拒绝策略）：当到达最大线程数或者是队列中都是任务时，提交的任务将会被阻塞，这时候就执行拒绝策略。

这篇文章主要讲线程池的7个参数，在下一篇文章中继续讲，当我们自定义一个线程池时，线程池的参数将如何设置。