java中有界队列的饱和策略(reject policy)

文章目录

AbortPolicy
DiscardPolicy
DiscardOldestPolicy
CallerRunsPolicy
使用Semaphore

我们在使用ExecutorService的时候知道，在ExecutorService中有个一个Queue来保存提交的任务，通过不同的构造函数，我们可以创建无界的队列（ExecutorService.newCachedThreadPool）和有界的队列(ExecutorService newFixedThreadPool(int nThreads))。

无界队列很好理解，我们可以无限制的向ExecutorService提交任务。那么对于有界队列来说，如果队列满了该怎么处理呢？

今天我们要介绍一下java中ExecutorService的饱和策略(reject policy)。

以ExecutorService的具体实现ThreadPoolExecutor来说，它定义了4种饱和策略。分别是AbortPolicy，DiscardPolicy，DiscardOldestPolicy和CallerRunsPolicy。

如果要在ThreadPoolExecutor中设定饱和策略可以调用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());

上面的例子中我们定义了一个初始5个，最大10个工作线程的Thread Pool，并且定义其中的Queue的容量是20。如果提交的任务超出了容量，则会使用AbortPolicy策略。

AbortPolicy

AbortPolicy意思是如果队列满了，最新的提交任务将会被拒绝，并抛出RejectedExecutionException异常：

   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());}}

上面的代码中，rejectedExecution方法中我们直接抛出了RejectedExecutionException异常。

DiscardPolicy

DiscardPolicy将会悄悄的丢弃提交的任务，而不报任何异常。

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将会丢弃最老的任务，保存最新插入的任务。

   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);}}}

我们看到在rejectedExecution方法中，poll了最老的一个任务，然后使用ThreadPoolExecutor提交了一个最新的任务。

CallerRunsPolicy

CallerRunsPolicy和其他的几个策略不同，它既不会抛弃任务，也不会抛出异常，而是将任务回退给调用者，使用调用者的线程来执行任务，从而降低调用者的调用速度。我们看下是怎么实现的：

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，直接调用了 r.run()方法，这会导致该方法直接在调用者的主线程中执行，而不是在线程池中执行。从而导致主线程在该任务执行结束之前不能提交任何任务。从而有效的阻止了任务的提交。

使用Semaphore

如果我们并没有定义饱和策略，那么有没有什么方法来控制任务的提交速度呢？考虑下之前我们讲到的Semaphore，我们可以指定一定的资源信号量来控制任务的提交，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();}}}

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

更多精彩内容且看：

区块链从入门到放弃系列教程-涵盖密码学,超级账本,以太坊,Libra,比特币等持续更新
Spring Boot 2.X系列教程:七天从无到有掌握Spring Boot-持续更新
Spring 5.X系列教程:满足你对Spring5的一切想象-持续更新
java程序员从小工到专家成神之路（2020版）-持续更新中,附详细文章教程

更多内容请访问 flydean的博客