【Java】线程池 参数

* @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

几个关键参数：

1.corePoolSize：核心线程数，将会永久存在线程池中；

2.maximumPoolSize：最大线程数，如果任务队列满了，新来的任务将会额外创建临时线程来处理，但是总的线程数目不会超过这个值；

3.keepAliveTime：指的是额外创建的临时线程过期阈值；

线程池的参数作用：如果线程数小于corePoolSize，那么每一个任务进来都会创建线程；如果大于等于corePoolSize，那么新来的任务将会进入队列等待处理；如果队列超过阈值，那么就会新建临时线程，直到线程数到达maximumPoolSize。如果到达maximumPoolSize，那么会根据拒绝策略拒绝后来的任务。

线程池原本的模型就是池化一定数目的线程，然后处理用户请求，如果用户请求多了，那么就扔进队列里，等待池化的线程处理完手头工作再处理 新的请求。那么如果队列也满了，说明系统不堪重负，此时需要临时添加一下线程救急，一旦过了高峰期，这些临时线程就会被回收，这就是线程池的概念模型。