Java 线程池

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发布于: 2020 年 07 月 27 日

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什么是线程池（Thread Pool）﻿
一种线程使用模式，解决线程频繁开启、关闭带来的性能开销问题。
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Java 提供 4 种线程池1. FixedThreadPool固定的线程数据，保持长活，阻塞任务。
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ExecutorService service = Executors.newFixedThreadPool(10);
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2. CachedThreadPool同步队列一次处理一个任务，线程池中的线程存活 60s，无线程可用时创建新的线程。
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ExecutorService service = Executors.newCachedThreadPool();
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3. ScheduledThreadPool任务提交到延迟队列，定时消费任务。如果需要会创建更多的线程。
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ExecutorService service = Executors.newScheduledThreadPool();
// task to run after 10 second delay
service.schedule(() -> {}, 10, TimeUnit.SECONDS);
// task to run repeatedly every 10 seconds
service.scheduledAtFixedRate(() -> {}, 15, 10 TimeUnit.SECONDS);
// task to run repeatedly 10 seconds after previous task completes
service.scheduledWithFixedDelay(() -> {}, 15, 10 TimeUnit.SECONDS)
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4. SingleThreadedExecutor只创建一个线程，任务阻塞，如果线程因为任务而关闭，重新创建线程。
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ExecutorService service = Executors.newSingleThreadExecutor();
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5. ThreadPoolExecutornewFixedThreadPool 实现
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 public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
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ThreadPoolExcetuor 函数
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 public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        ...
    }
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以下两种情况会进入 reject 回调
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阻塞队列满了并且，线程都在执行。
线程池已关闭
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线程被无法提交后有四个处理策略
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6. shutdown调用 shutdown() 之后，线程池会在已提交线程执行完后关闭。
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awaitTermination(long timeout, TimeUnit unit)， 判断线程池在线程执行完或指定时间内是否关闭。
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Callable/Future﻿
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﻿ReferenceJava ExecutorService
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