Executors类的newFixedThreadPool, newCachedThreadPool, newScheduledThreadPool

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搬砖会快乐 2022-11-15 15:01:57 ©著作权

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Executors 类对 ThreadPoolExecutor 的构造函数进行了封装，使用该类可方便地创建线程池。

1. newFixedThreadPool

public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}

// 对应的ThreadPoolExecutor设置如下：
this.corePoolSize = nThreads;
this.maximumPoolSize = nThreads;
this.workQueue = new LinkedBlockingQueue<Runnable>(); 
this.keepAliveTime = TimeUnit.MILLISECONDS.toNanos(0L);
this.threadFactory = Executors.defaultThreadFactory();
this.handler = defaultHandler;

2. newCachedThreadPool

public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}

// 对应的ThreadPoolExecutor设置如下：
this.corePoolSize = 0;
this.maximumPoolSize = Integer.MAX_VALUE;
this.workQueue = new SynchronousQueue<Runnable>();
this.keepAliveTime = TimeUnit.SECONDS.toNanos(60L); //60秒
this.threadFactory = Executors.defaultThreadFactory();
this.handler = defaultHandler;

分析任务入队和出队，分别对应 ThreadPoolExecutor 类的 execute 方法和 getTask 方法：

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();

    int c = ctl.get();
    // corePoolSize=0，所以不会走这个分支
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 把任务放进队列。
    // newCachedThreadPool使用的队列是SynchronousQueue，当没有线程因take阻塞时，offer返回false。
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        //如果工作线程数为0，创建工作线程
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 如果入队失败，则创建工作线程
    else if (!addWorker(command, false))
        reject(command);
}

cachedThreadPool 的一个特点是：工作线程执行完任务后，继续从工作队列获取任务（poll），等待60秒，超时则返回 null。task为 null时，工作线程就退出了 while 循环，也就是说这个线程要死了。

while (task != null || (task = getTask()) != null) {...}

3. newScheduledThreadPool

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
}
//对应ThreadPoolExecutor的设置
this.corePoolSize = corePoolSize;
this.maximumPoolSize = Integer.MAX_VALUE;
this.workQueue = new DelayedWorkQueue();
this.keepAliveTime = TimeUnit.NANOSECONDS.toNanos(0);
this.threadFactory = Executors.defaultThreadFactory();
this.handler = defaultHandler;

分析定时器延迟队列的 take 方法：DelayedWorkQueue 的底层是堆，访问堆顶的任务，如果任务的时间到了，则返回，否则等待直到时间到来。

// ScheduledThreadPoolExecutor.DelayedWorkQueue
public RunnableScheduledFuture take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        for (;;) {
            RunnableScheduledFuture first = queue[0];
            if (first == null)
                available.await();
            else {
                long delay = first.getDelay(TimeUnit.NANOSECONDS);
                if (delay <= 0)
                    return finishPoll(first);
                else if (leader != null)
                    available.await();
                else {
                    Thread thisThread = Thread.currentThread();
                    leader = thisThread;
                    try {
                        available.awaitNanos(delay);
                    } finally {
                        if (leader == thisThread)
                            leader = null;
                    }
                }
            }
        }
    } finally {
        if (leader == null && queue[0] != null)
            available.signal();
        lock.unlock();
    }
}

可以看出，3种线程池的主要区别是使用的队列不同。

4. DefaultThreadFactory

static class DefaultThreadFactory implements ThreadFactory {
    private static final AtomicInteger poolNumber = new AtomicInteger(1);
    private final ThreadGroup group;
    private final AtomicInteger threadNumber = new AtomicInteger(1);
    private final String namePrefix;

    DefaultThreadFactory() {
        SecurityManager s = System.getSecurityManager();
        group = (s != null) ? s.getThreadGroup() :
                              Thread.currentThread().getThreadGroup();
        namePrefix = "pool-" +
                      poolNumber.getAndIncrement() +
                     "-thread-";
    }

    public Thread newThread(Runnable r) {
        Thread t = new Thread(group, r,
                              namePrefix + threadNumber.getAndIncrement(),
                              0);
        if (t.isDaemon())
            t.setDaemon(false); //设置为前台线程
        if (t.getPriority() != Thread.NORM_PRIORITY)
            t.setPriority(Thread.NORM_PRIORITY);
        return t;
    }
}