面试题：线程池的底层工作原理

线程池的几个重要的参数：

1、corePoolSize：线程池的核心线程数（也是默认线程数）

2、maximumPoolSize：最大线程数

3、keepAliveTime：允许的线程最大空闲时间（单位/秒）

线程池内部是通过队列+线程实现的，当我们利用线程池执行任务时：

• 如果此时线程池中的线程数量小于corePoolSize，即使线程池中的线程都处于空闲状态，也要创建新的线程来处理被添加的任务，

• 如里此时线程池中的线程数量等于corePoolSize，但是缓冲队列workQueue未满，那么任务被放入缓冲队列。

• 如果此时线程池中的线程数量大于等于corePoolSize，缓中队列workQueue满，并且线程池中的数量小于maximumPooISize，建新的线程来外理被添加的任务。

• 如果此时线程池中的线程数量大于corePoosize，缓冲队列workQueue满，并目线程池中的数量等于maximumPoosize，那么通过 handler所指定的策略来处理此任务。

• 当线程池中的线程数量大于 corePoolsize时，如果某线程空闲时间超过keepAliveTime，线程将被终止。这样，线程可以动态的调整池中的线程数。

线程池的拒绝策略：指的是RejectedExecutionHandler接口的实现类。

在线程池接口的源代码中，可以发现ThreadPoolExecutor接口内置了4种拒绝策略。

package java.util.concurrent;


public class ThreadPoolExecutor extends AbstractExecutorService {

    /**
     * A handler for rejected tasks that runs the rejected task
     * directly in the calling thread of the {@code execute} method,
     * unless the executor has been shut down, in which case the task
     * is discarded.
     */
    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();
            }
        }
    }

    /**
     * A handler for rejected tasks that throws a
     * {@code RejectedExecutionException}.
     */
    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());
        }
    }

    /**
     * A handler for rejected tasks that silently discards the
     * rejected task.
     */
    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) {
        }
    }

    /**
     * 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);
            }
        }
    }

}

四种拒绝策略的相关说明：

1、AbortPolicy：

默认。拒绝这个任务，并且抛出RejectedExecutionException异常。

2、DiscardPolicy：

队列满了，丢掉任务，不会抛出异常！

3、DiscardOldestPolicy
队列满了，尝试去和最早的竞争，也不会抛出异常！
抛弃最老任务策略，也就是说如果队列满了，就会将最早进入队列的任务抛弃，从队列中腾出空间，再尝试加入队列。因为队列是队尾进、队头出，队头元素是最老的，所以每次都是移除队头元素后再尝试入队。

4、CallerRunsPolicy
调用者执行策略。在新任务被添加到线程池时，如果添加失败，那么提交任务的线程会自己去执行该任务，不会使用线程池中的线程去执行新任务。