线程池

【 线程池的作用，Executors创建线程池，常见工作队列】：https://www.jianshu.com/p/6c6f396fc88e

【Exectros方式创建线程池和ThreadPoolExecutor的构造函数创建线程池】：https://blog.csdn.net/hollis_chuang/article/details/83743723

拒绝策略：

一：AbortPolicy (默认)【抛出异常，自行处理】

public static class AbortPolicy implements RejectedExecutionHandler {
    /**
     * Creates an {@code AbortPolicy}.
     */
    public AbortPolicy() { }

    /**
     * Always throws RejectedExecutionException.
     *
     * @param r the runnable task requested to be executed
     * @param e the executor attempting to execute this task
     * @throws RejectedExecutionException always
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +
                                             e.toString());
    }
}

 二：CallerRunsPolicy 【在executor未被关闭的情况下，一直重试】

public static class CallerRunsPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code CallerRunsPolicy}.
     */
    public CallerRunsPolicy() { }

    /**
     * Executes task r in the caller's thread, unless the executor
     * has been shut down, in which case the task is discarded.
     *
     * @param r the runnable task requested to be executed
     * @param e the executor attempting to execute this task
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            r.run();
        }
    }
}

 三：DiscardPolicy【直接将任务丢弃】

public static class DiscardPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code DiscardPolicy}.
     */
    public DiscardPolicy() { }

    /**
     * Does nothing, which has the effect of discarding task r.
     *
     * @param r the runnable task requested to be executed
     * @param e the executor attempting to execute this task
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
    }
}

 四：DiscardOldestPolicy 【当executor还在运行时：会丢弃位于队列头部的任务，即最早进入队列中的任务（最旧），然后一直重试（如果不成功，队头任务还会被丢弃），直至execuotr被关闭。】

* <li>In {@link ThreadPoolExecutor.DiscardOldestPolicy}, if the
* executor is not shut down, the task at the head of the work queue
* is dropped, and then execution is retried (which can fail again,
* causing this to be repeated.) </li>

/**
 * A handler for rejected tasks that discards the oldest unhandled
 * request and then retries {@code execute}, unless the executor
 * is shut down, in which case the task is discarded.
 */

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
    /**
     * Creates a {@code DiscardOldestPolicy} for the given executor.
     */
    public DiscardOldestPolicy() { }

    /**
     * Obtains and ignores the next task that the executor
     * would otherwise execute, if one is immediately available,
     * and then retries execution of task r, unless the executor
     * is shut down, in which case task r is instead discarded.
     *
     * @param r the runnable task requested to be executed
     * @param e the executor attempting to execute this task
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }
    }
}