19. Java JUC源码分析系列笔记-RejectedExecutionHandler

1. RejectedExecutionHandler是什么

corePoolSize满了，blockingQueue也满了，maxPoolSize也满了，那么新的任务该怎么处理，这就看RejectedExecutionHandler

2. 分类

2.1. CallerRunsPolicy

处理策略：不管线程池死活，来任务了，我就执行，不管你能不能真的执行。

/**
 * 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) {
        //只要线程池没有关闭那么直接执行
        if (!e.isShutdown()) {
            r.run();
        }
    }
}
    

2.2. AbortPolicy

处理策略：抛异常。这个是默认处理策略。

/**
 * 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) {
        //直接抛出异常
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +
                                             e.toString());
    }
}

2.3. DiscardPolicy

处理策略：什么都不做。相当于放弃了当前任务的执行

/**
 * 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) {
        //无视这个新任务
    }
}

2.4. DiscardOldestPolicy

处理策略：移出任务队列头部的任务，然后执行当前线程。相当于放弃了任务队列头部的任务。

/**
 * 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) {
        //线程池没有关闭的话，移除队首
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }
    }
}

3. 参考

• 线程池有哪些RejectedExecutionHandler，分别对应的使用场景？ - 简书