ThreadPoolExecutor-线程池

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                                        阻塞队列为空
                shutdown()           线程池工作线程数为0
            +--------------> SHUTDOWN ----------------+
            |                                         |              terminated()
RUNNING ----+                                         +----> TIDYING ------------> TERMINATED
            | shutdownNow()          线程池工作线程数为0 |
            +-------------->   STOP   ----------------+

构造方法

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public ThreadPoolExecutor(int corePoolSize,
                            int maximumPoolSize,
                            long keepAliveTime,
                            TimeUnit unit,
                            BlockingQueue<Runnable> workQueue,
                            ThreadFactory threadFactory,
                            RejectedExecutionHandler handler) {
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

参数解析

  • corePoolSize：核心线程池的大小。创建了线程池后，默认情况下线程池中并没有任何线程，而是等待有任务到来才创建线程去执行任务。当有任务来之后，就会创建一个线程去执行任务，当线程池中的线程数目达到 corePoolSize 后，就会把到达的任务放到缓存队列当中；
  • maximumPoolSize：线程池最大线程数；
  • keepAliveTime：表示线程没有任务执行时最多保持多久时间会终止；
  • unit：参数 keepAliveTime 的时间单位（DAYS、HOURS、MINUTES、SECONDS 等）
  • workQueue：阻塞队列，用来存储等待执行的任务
    • ArrayBlockingQueue，有界队列
    • LinkedBlockingQueue，无界队列
    • SynchronousQueue
  • threadFactory：线程工厂，主要用来创建线程
  • handler：拒绝处理任务的策略
    • AbortPolicy：丢弃任务并抛出 RejectedExecutionException 异常，默认策略
    • DiscardPolicy：丢弃任务，但不抛出异常
    • DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务，重复此过程
    • CallerRunsPolicy：由调用线程处理该任务

预定义线程池

FixedThreadPool

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public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>());
}
  • corePoolSize 与 maximumPoolSize 相等，即其线程全为核心线程，是一个固定大小的线程池，是其优势；
  • keepAliveTime = 0 该参数默认对核心线程无效，而 FixedThreadPool 全部为核心线程；
  • workQueue 为 LinkedBlockingQueue（无界阻塞队列），队列最大值为 Integer.MAX_VALUE。如果任务提交速度持续大余任务处理速度，会造成队列大量阻塞。因为队列很大，很有可能在拒绝策略前，内存溢出。是其劣势；
  • FixedThreadPool 的任务执行是无序的；

适用场景：可用于 Web 服务瞬时削峰，但需注意长时间持续高峰情况造成的队列阻塞。

CachedThreadPool

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public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                    60L, TimeUnit.SECONDS,
                                    new SynchronousQueue<Runnable>());
}
  • corePoolSize = 0，maximumPoolSize = Integer.MAX_VALUE，即线程数量几乎无限制；
  • keepAliveTime = 60s，线程空闲 60s 后自动结束。
  • workQueue 为 SynchronousQueue 同步队列，这个队列类似于一个接力棒，入队出队必须同时传递，因为 CachedThreadPool 线程创建无限制，不会有队列等待，所以使用 SynchronousQueue；

适用场景：快速处理大量耗时较短的任务，如 Netty 的 NIO 接受请求时，可使用 CachedThreadPool。

SingleThreadExecutor

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public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

这里多了一层 FinalizableDelegatedExecutorService 包装，这一层有什么用呢，写个 dome 来解释一下：

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    public static void main(String[] args) {
        ExecutorService fixedExecutorService = Executors.newFixedThreadPool(1);
        ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) fixedExecutorService;
        System.out.println(threadPoolExecutor.getMaximumPoolSize());
        threadPoolExecutor.setCorePoolSize(8);

        ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
//      运行时异常 java.lang.ClassCastException
//      ThreadPoolExecutor threadPoolExecutor2 = (ThreadPoolExecutor) singleExecutorService;
    }

对比可以看出，FixedThreadPool 可以向下转型为 ThreadPoolExecutor，并对其线程池进行配置，而 SingleThreadExecutor 被包装后，无法成功向下转型。因此，SingleThreadExecutor 被定以后，无法修改，做到了真正的 Single。

ScheduledThreadPool

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public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
}
  • newScheduledThreadPool 调用的是 ScheduledThreadPoolExecutor 的构造方法，而 ScheduledThreadPoolExecutor 继承了 ThreadPoolExecutor，构造是还是调用了其父类的构造方法。
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public ScheduledThreadPoolExecutor(int corePoolSize) {
    super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
            new DelayedWorkQueue());
}

对于 ScheduledThreadPool 本文不做描述，其特性请关注后续篇章。

自定义线程池

以下是自定义线程池，使用了有界队列，自定义 ThreadFactory 和拒绝策略的 demo

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public class ThreadTest {

    public static void main(String[] args) throws InterruptedException, IOException {
        int corePoolSize = 2;
        int maximumPoolSize = 4;
        long keepAliveTime = 10;
        TimeUnit unit = TimeUnit.SECONDS;
        BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
        ThreadFactory threadFactory = new NameTreadFactory();
        RejectedExecutionHandler handler = new MyIgnorePolicy();
        ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit,
                workQueue, threadFactory, handler);
        executor.prestartAllCoreThreads(); // 预启动所有核心线程

        for (int i = 1; i <= 10; i++) {
            MyTask task = new MyTask(String.valueOf(i));
            executor.execute(task);
        }

        System.in.read(); //阻塞主线程
    }

    static class NameTreadFactory implements ThreadFactory {

        private final AtomicInteger mThreadNum = new AtomicInteger(1);

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
            System.out.println(t.getName() + " has been created");
            return t;
        }
    }

    public static class MyIgnorePolicy implements RejectedExecutionHandler {

        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            doLog(r, e);
        }

        private void doLog(Runnable r, ThreadPoolExecutor e) {
            // 可做日志记录等
            System.err.println( r.toString() + " rejected");
//          System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
        }
    }

    static class MyTask implements Runnable {
        private String name;

        public MyTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            try {
                System.out.println(this.toString() + " is running!");
                Thread.sleep(3000); //让任务执行慢点
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return "MyTask [name=" + name + "]";
        }
    }
}

输出结果如下：

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my-thread-1 has been created
my-thread-2 has been created
my-thread-3 has been created
MyTask[name=1]is running!
MyTask[name=2]is running!
my-thread-4 has been created
MyTask[name=3]is running!
MyTask[name=6]is running!
MyTask[name=7]is rejected
MyTask[name=8]is rejected
MyTask[name=9]is rejected
MyTask[name=10]is rejected
MyTask[name=4]is running!
MyTask[name=5]is running!

1，由于线程预启动，首先创建了 1，2 号线程，然后 task1，task2 被执行；
2，但任务提交没有结束，此时任务 task3，task6 到达发现核心线程已经满了，进入等待队列；
3，等待队列满后创建任务线程 3，4 执行任务 task3，task6，同时 task4，task5 进入队列；
4，此时创建线程数（4）等于最大线程数，且队列已满，所以 7，8，9，10 任务被拒绝；
5，任务执行完毕后回头来执行 task4，task5，队列清空。