我們?cè)谑褂肊xecutorService的時(shí)候知道，在ExecutorService中有個(gè)一個(gè)Queue來保存提交的任務(wù)，通過不同的構(gòu)造函數(shù)，我們可以創(chuàng)建無界的隊(duì)列（ExecutorService.newCachedThreadPool）和有界的隊(duì)列(ExecutorService newFixedThreadPool(int nThreads))。

無界隊(duì)列很好理解，我們可以無限制的向ExecutorService提交任務(wù)。那么對(duì)于有界隊(duì)列來說，如果隊(duì)列滿了該怎么處理呢？

今天我們要介紹一下java中ExecutorService的飽和策略(reject policy)。

以ExecutorService的具體實(shí)現(xiàn)ThreadPoolExecutor來說，它定義了4種飽和策略。分別是AbortPolicy，DiscardPolicy，DiscardOldestPolicy和CallerRunsPolicy。

如果要在ThreadPoolExecutor中設(shè)定飽和策略可以調(diào)用setRejectedExecutionHandler方法，如下所示：

ThreadPoolExecutor threadPoolExecutor= new ThreadPoolExecutor(5, 10, 10, TimeUnit.SECONDS, new LinkedBlockingDeque<Runnable>(20)); threadPoolExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy() );

上面的例子中我們定義了一個(gè)初始5個(gè)，最大10個(gè)工作線程的Thread Pool，并且定義其中的Queue的容量是20。如果提交的任務(wù)超出了容量，則會(huì)使用AbortPolicy策略。

AbortPolicy

AbortPolicy意思是如果隊(duì)列滿了，最新的提交任務(wù)將會(huì)被拒絕，并拋出RejectedExecutionException異常：

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()); } }

上面的代碼中，rejectedExecution方法中我們直接拋出了RejectedExecutionException異常。

DiscardPolicy

DiscardPolicy將會(huì)悄悄的丟棄提交的任務(wù)，而不報(bào)任何異常。

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) { } }

DiscardOldestPolicy

DiscardOldestPolicy將會(huì)丟棄最老的任務(wù)，保存最新插入的任務(wù)。

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); } } }

我們看到在rejectedExecution方法中，poll了最老的一個(gè)任務(wù)，然后使用ThreadPoolExecutor提交了一個(gè)最新的任務(wù)。

CallerRunsPolicy

CallerRunsPolicy和其他的幾個(gè)策略不同，它既不會(huì)拋棄任務(wù)，也不會(huì)拋出異常，而是將任務(wù)回退給調(diào)用者，使用調(diào)用者的線程來執(zhí)行任務(wù)，從而降低調(diào)用者的調(diào)用速度。我們看下是怎么實(shí)現(xiàn)的：

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(); } } }

在rejectedExecution方法中，直接調(diào)用了 r.run()方法，這會(huì)導(dǎo)致該方法直接在調(diào)用者的主線程中執(zhí)行，而不是在線程池中執(zhí)行。從而導(dǎo)致主線程在該任務(wù)執(zhí)行結(jié)束之前不能提交任何任務(wù)。從而有效的阻止了任務(wù)的提交。

使用Semaphore

如果我們并沒有定義飽和策略，那么有沒有什么方法來控制任務(wù)的提交速度呢？考慮下之前我們講到的Semaphore，我們可以指定一定的資源信號(hào)量來控制任務(wù)的提交，如下所示：

public class SemaphoreUsage { private final Executor executor; private final Semaphore semaphore; public SemaphoreUsage(Executor executor, int count) { this.executor = executor; this.semaphore = new Semaphore(count); } public void submitTask(final Runnable command) throws InterruptedException { semaphore.acquire(); try { executor.execute(() -> { try { command.run(); } finally { semaphore.release(); } } ); } catch (RejectedExecutionException e) { semaphore.release(); } }}

本文的例子可參考https://github.com/ddean2009/learn-java-concurrency/tree/master/rejectPolicy

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