Executors线程池源码分析

本节学习Executors类源码

首先看一下常用的线程池的几种创建方式

    /**
     * 创建一个线程池，该线程池重用固定数量的线程，在共享的无界队列中运行
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue());
    }

    /**
     * 创建一个线程池，该线程池只有一个工作者线程运行在无界队列中
     */
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue()));
    }

    /**
     * 创建一个线程池，如果需要就创建线程, 但是会重用之前已创建的可用线程
     */
    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue());
    }

    /**
     * 创建一个可定时执行任务的线程池
     */
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

下面看一下ThreadPoolExecutor类

有几个核心的参数：

    /**
     * 阻塞队列，保存任务并交给工作者线程
     */
    private final BlockingQueue workQueue;

    /**
     * 重入锁
     */
    private final ReentrantLock mainLock = new ReentrantLock();

    /**
     * 工作者线程集合
     */
    private final HashSet workers = new HashSet();

    /**
     * Wait condition to support awaitTermination
     */
    private final Condition termination = mainLock.newCondition();

    /**
     * 跟踪最大线程池大小
     */
    private int largestPoolSize;

    /**
     * 已执行完成的线程个数
     */
    private long completedTaskCount;

    /**
     * 创建线程的工厂
     */
    private volatile ThreadFactory threadFactory;

    /**
     * 饱和策略执行者
     */
    private volatile RejectedExecutionHandler handler;

    /**
     * 空闲工作线程的空闲时间；超过corePoolSize的线程或者allowCoreThreadTimeOut为true的主线程使用这个作为超时时间；
否则线程一直等待任务或关闭
     */
    private volatile long keepAliveTime;

    /**
     * 如果为false，核心线程会一直存活
     * 如果为true，核心线程使用keepAliveTime等待空闲工作者线程超时
     */
    private volatile boolean allowCoreThreadTimeOut;

    /**
     * 核心线程个数
     */
    private volatile int corePoolSize;

    /**
     * 最大线程个数
     */
    private volatile int maximumPoolSize;

    /**
     * 默认的拒绝策略
     */
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

    /**
     * 构造函数，创建线程池
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();//非法校验
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);//毫秒
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

下面看一下线程池执行任务的过程：

    /**
     * 执行任务
     */
    public void execute(Runnable command) {
        if (command == null)//如果任务为空，抛异常
            throw new NullPointerException();
        /*
         * 1. 如果工作者线程个数少于corePoolSize，添加一个工作者线程
         * 2. 否则，如果线程池处于运行状态则尝试插入工作队列中，如果插入成功，再次检查线程池状态，如果非运行，则队列中移除任务，且拒绝该任务，如果工作者线程为0，则添加工作者线程
         * 3. 如果不能插入工作队列中，则尝试创建新的线程，如果失败，则拒绝任务。
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();//重新检查，如果线程数超过最大，则拒绝任务，从队列中移除任务
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

    /**
     * 创建新的线程
     */
    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);//获取工作线程个数
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))//如果超过最大或者核心线程数，则返回false
                    return false;
                if (compareAndIncrementWorkerCount(c))//线程个数+1
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);//创建新的线程
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();//获取锁
                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);//添加到工作集合中
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;//设置最大线程数
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();//释放锁
                }
                if (workerAdded) {
                    t.start();//执行任务
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);//任务启动失败，则从任务队列中移除该任务，ctl减一
        }
        return workerStarted;
    }

    /**
     *拒绝任务
     */
    final void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }

默认使用的是AbortPolicy，直接抛出RejectedExecutionException异常

还有三种策略：

1.CallerRunsPolicy：如果线程池非中断状态，则直接在调用拒绝的线程上执行该任务，否则丢弃任务。

2.DiscardOldestPolicy：如果线程池在非中断状态，则从工作队列中移除最近的任务，且执行新的任务，否则丢弃任务

3. DiscardPolicy ： 丢弃任务，什么也不做。

下面看一下任务的执行

    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) {//如果task为空，则从队列中获取
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // 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();//任务执行
                    } 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);//从workers移除
        }
    }