Java线程池之Executors.newSingleThreadExecutor()

Java线程池Executors.newSingleThreadExecutor()

前言：本文先就Java线程池 ThreadPoolExecutor 进行分析，然后逐步分析单线程池的源码工作流程

ThreadPoolExecutor的工作流程

我们执行以下代码：

ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
singleThreadExecutor.execute(new Runnable() {
    @Override
    public void run() {
        ...
    }
});

当线程池提交Runnable实现时singleThreadExecutor.execute(Runnable command) ，其大致的工作流程如下：

下面跟随源码进入到ThreadPoolExecutor中进行详细分析

线程池初始化

当Executors.newSingleThreadExecutor()执行时，其内部源码如下：

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

继续深入new ThreadPoolExecutor方法可以发现，对应的参数的意义分别是：

public ThreadPoolExecutor(int corePoolSize, // 核心线程数
                          int maximumPoolSize, // 最大线程数
                          long keepAliveTime, // 保活时间（这里是0，单线程池暂时用不上此参数）
                          TimeUnit unit, // 保活时间单位
                          BlockingQueue<Runnable> workQueue // 存放Runnable实现的队列) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         Executors.defaultThreadFactory(), defaultHandler);
}

往后执行，也就是线程池执行初始化的一个过程，给线程池对象的重要参数进行赋值。

线程池提交任务

先看看提交时，最外层的代码：

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    /*
     * Proceed in 3 steps:
     *
     * 1. If fewer than corePoolSize threads are running, try to
     * start a new thread with the given command as its first
     * task.  The call to addWorker atomically checks runState and
     * workerCount, and so prevents false alarms that would add
     * threads when it shouldn't, by returning false.
     *
     * 2. If a task can be successfully queued, then we still need
     * to double-check whether we should have added a thread
     * (because existing ones died since last checking) or that
     * the pool shut down since entry into this method. So we
     * recheck state and if necessary roll back the enqueuing if
     * stopped, or start a new thread if there are none.
     *
     * 3. If we cannot queue task, then we try to add a new
     * thread.  If it fails, we know we are shut down or saturated
     * and so reject the task.
     */
    int c = ctl.get();
    // 1.
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 2.
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 3.
    else if (!addWorker(command, false))
        reject(command);
}

翻译翻译Doug Lea大神写的注释:

1. 首先判断Worker的数量是否小于corePoolSize核心线程池数量，小于则new Worker对象，启动线程并以此次的Runnable任务为第一次任务进行run()执行。后一段则说有原子性的检查机制保证Worker线程不会额外创建，否则会返回false；
2. 上述条件不满足时，会将Runnale任务放入队列，如果放置成功，且会进行双重校验：先判断线程池是否是SHUTDOWN状态，然后尝试撤回Runnable任务，如果失败，则判断Worker的数量是否为0，如果为0，则新增一个非核心线程池的Worker。根据鄙人目前的理解，这里应该是避免线程池执行shutdown()方法，然后导致Runnable任务没有被正确的处理。
3. 如果Runnable放入队列失败，则添加非核心线程池的Worker。PS：LinkedBlockingQueue无论如何都可以offer()成功，SynchronousQueue的offer()是返回的false。上述两个BlockingQueue分别对应固定线程数的线程池和非固定线程数的线程池，分别对应的典型是Executors.newSingleThreadExecutor()和Executors.newCachedThreadPool()。非固定线程数的线程池放在下一期讲。

addWorker方法与Worker类

单线程的线程池的提交任务的时候，最重要的就2个步骤：addWorker(command, true)添加核心线程Worker与 workQueue.offer(command)将超出核心线程数的任务放入队列中。

那么addWorker方法里面发生了什么呢，我摘取了我认为最重要的一段：上锁、new Worker对象与启动线程

private boolean addWorker(Runnable firstTask, boolean core) {
    ...
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                int rs = runStateOf(ctl.get());
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start(); // 注意这里
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

那么，这个Worker究竟是怎么执行的呢，调用t.start()发生了什么呢。先看看Worker的类结构：

再看看Worker的成员变量有哪些，参考其构造方法：

Worker(Runnable firstTask) {
    setState(-1); // inhibit interrupts until runWorker
    this.firstTask = firstTask;
    this.thread = getThreadFactory().newThread(this);
}

接下来就是我认为关于Worker最重要核心的地方了，通过上述代码我们可以发现，Worker实现了Runnable接口，然后在构建Worker对象的时候，初始化了它的成员变量thread，并且是以Woker本身赋值进去的！！！看到这里，我们就知道了，t.start()的时候，是执行的Worker.run()方法。

下面是Worker.run()方法内部的部分源码：

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) 
            w.lock();
            // If pool is stopping, ensure thread is interr
            // if not, ensure thread is not interrupted.  T
            // requires a recheck in second case to deal wi
            // shutdownNow race while clearing interrupt
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) {
                    thrown = x; throw x;
                } catch (Error x) {
                    thrown = x; throw x;
                } catch (Throwable x) {
                    thrown = x; throw new Error(x);
                } finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

也就是说，启动Worker的之后，就是循环getTask()与task.run()，这个getTask()得到的task才是提交到线程池中的Runnable实现。
对于单线程池，getTask()最终执行的是LinkedBlockingQueue.take()方法：

• 按照队列的FIFO原则（先进先出），取出头部节点
• 如果没有头部节点，则调用notEmpty.await();方法等待，等待offer()方法时，调用notEmpty.signal();，这里涉及多线程的其它知识了，可以参考并发包中的AbstractQueuedSynchronizer相关知识点。

至此，单线程池的执行过程大概讲解了一通，现在再来看开篇的工作流程图，又有了更深地理解。