August 2021
Beginner to intermediate
450 pages
5h 40m
Chinese
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8.3.2 管理队列任务
在有限的线程池中会限制可并发执行的任务数量。（单线程的Executor是一种值得注意的特例：它们能确保不会有任务并发执行，因为它们通过线程封闭来实现线程安全性。）
在6.1.2节中曾介绍，如果无限制地创建线程，那么将导致不稳定性，并通过采用固定大小的线程池（而不是每收到一个请求就创建一个新线程）来解决这个问题。然而，这个方案并不完整。在高负载情况下，应用程序仍可能耗尽资源，只是出现问题的概率较小。如果新请求的到达速率超过了线程池的处理速率，那么新到来的请求将累积起来。在线程池中，这些请求会在一个由Executor管理的Runnable队列中等待，而不会像线程那样去竞争CPU资源。通过一个Runnable和一个链表节点来表现一个等待中的任务，当然比使用线程来表示的开销低很多，但如果客户提交给服务器请求的速率超过了服务器的处理速率，那么仍可能会耗尽资源。
即使请求的平均到达速率很稳定，也仍然会出现请求突增的情况。尽管队列有助于缓解任务的突增问题，但如果任务持续高速地到来，那么最终还是会抑制请求的到达率以避免耗尽内存。[1]甚至在耗尽内存之前，响应性能也将随着任务队列的增长而变得越来越糟。
ThreadPoolExecutor允许提供一个BlockingQueue来保存等待执行的任务。基本的任务排队方法有3种：无界队列、有界队列和同步移交（Synchronous Handoff）。队列的选择与其他的配置参数有关，例如线程池的大小等。
newFixedThreadPool和newSingleThreadExecutor在默认情况下将使用一个无界的LinkedBlockingQueue。如果所有工作者线程都处于忙碌状态，那么任务将在队列中等候。如果任务持续快速地到达，并且超过了线程池处理它们的速度，那么队列将无限制地增加。 ...
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