线程池拒绝策略

线程池四种拒绝策略

在线程池配置文件中配置RejectedExecutionHandler属性，可以为线程池指定拒绝策略。

在ThreadPoolExecutor类中，有四个内部类为JDK自带的四种拒绝策略。CallerRunsPolicy，AbortPolicy，DiscardPolicy，DiscardOldestPolicy。
也可以自定义拒绝策略，实现RejectedExecutionHandler的rejectedExecution方法。

CallerRunsPolicy

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();
        }
    }
}

从源码上可以看出直接在 execute 方法中同步调用被拒绝的任务。

AbortPolicy

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());
    }
}

直接抛出异常RejectedExecutionException，丢弃任务。（jdk默认策略，队列满并线程满时直接拒绝添加新任务，并抛出异常）

DiscardPolicy

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) {
    }
}

可以看出这种策略什么都没做，直接丢弃任务。

DiscardOldestPolicy

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);
        }
    }
}

获取当前线程池的队列并调用poll()方法删除队列头的任务（即下一个执行的任务），然后添加该任务。