java并发编程学习之线程池-ThreadPoolExecutor(三)

摘要：是所有线程池实现的父类，我们先看看构造函数构造参数线程核心数最大线程数线程空闲后，存活的时间，只有线程数大于的时候生效存活时间的单位任务的阻塞队列创建线程的工程，给线程起名字当线程池满了，选择新加入的任务应该使用什么策略，比如抛异常丢弃当前

ThreadPoolExecutor是所有线程池实现的父类，我们先看看构造函数

corePoolSize：线程核心数

maximumPoolSize：最大线程数

keepAliveTime：线程空闲后，存活的时间，只有线程数大于corePoolSize的时候生效

unit：存活时间的单位

workQueue：任务的阻塞队列

threadFactory：创建线程的工程，给线程起名字

handler：当线程池满了，选择新加入的任务应该使用什么策略，比如抛异常、丢弃当前任务、丢弃阻塞队列的最老任务等，也可以自定义。

判断是否超过线程核心数corePoolSize，没超过创建线程

超过线程核心数，则判断队列是否已满，没有满，放入队列

队列也满了，判断是否超过maximumPoolSize，没有就创建线程

超过了，根据策略执行

//32为，前3位作为线程池的状态，后三位是线程数
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;//28
private static final int CAPACITY   = (1 << COUNT_BITS) - 1;00011111 11111111 11111111 11111110
//-1的二进制是11111111 11111111 11111111 11111111
private static final int RUNNING    = -1 << COUNT_BITS;//-1如上，左移28位后，就是111000000 00000000 00000000 00000000
private static final int SHUTDOWN   =  0 << COUNT_BITS;//0左移28位，还是0，00000000 00000000 00000000 00000000
private static final int STOP       =  1 << COUNT_BITS;//00100000 00000000 00000000 00000000
private static final int TIDYING    =  2 << COUNT_BITS;//01000000 00000000 00000000 00000000
private static final int TERMINATED =  3 << COUNT_BITS;//01100000 00000000 00000000 00000000
private static int runStateOf(int c)     { return c & ~CAPACITY; }//~CAPACITY为11100000000000000000000000000000，与完就是线程的状态
private static int workerCountOf(int c)  { return c & CAPACITY; }//与完，是线程的数量
private static int ctlOf(int rs, int wc) { return rs | wc; }
private static boolean isRunning(int c) {
    return c < SHUTDOWN;//小于0，说明是RUNNING，RUNNING=-1
}

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {//如果线程数少于线程核心数
        if (addWorker(command, true))//增加任务成功，返回true，没成功，继续往下
            return;
        c = ctl.get();
    }
    //判断队列
    if (isRunning(c) && workQueue.offer(command)) {//如果线程池还在跑，并且可以插入队列
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))//线程池不是运行状态，就移除刚刚插入的任务
            reject(command);//执行策略
        else if (workerCountOf(recheck) == 0)//
            addWorker(null, false);
    }
    //队列也满了，判断最大线程数
    else if (!addWorker(command, false))
        reject(command);//执行策略
}

private boolean addWorker(Runnable firstTask, boolean core) {//core为true，使用corePoolSize判断，否则使用maximumPoolSize
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);//获取当前线程状态

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && // 就是STOP、TIDYING、TERMINATED，此时不让任务进来
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))//
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;//超过了线程核心数或最大线程数，不让新增
            if (compareAndIncrementWorkerCount(c))//返回true，说明成功了，跳出retry循环
                break retry;
            //失败了，说明被其他符号条件的线程占了，就再判断线程状态是否跟之前一样，不一样重新获取，跳到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);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();//获取锁
            try {
                int rs = runStateOf(ctl.get());//获取线程池的状态

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // 没通过start来启动run的
                        throw new IllegalThreadStateException();
                    workers.add(w);//加点hashset
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;//更新当前最大值
                    workerAdded = true;//增加成功
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start();//启动线程
                workerStarted = true;//启动成功
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);//失败，线程数-1，从hashset移除，并尝试Terminate
    }
    return workerStarted;
}

上面执行 t.start();的时候，就会通过run方法调用下面的方法

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) {//任务不为空或者获取的任务也不为空
            w.lock();           
            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();//调用run方法，这里没有通过start，也就是说没有启动新线程
                } 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++;//完成任务数加1
                w.unlock();//释放
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);//移除w，在task为空的时候，比如线程池状态停止或者启动的线程太多
    }
}

getTask方法
当Worker第一次启动的时候，调用run方法，后面就一直从队列里获取任务

private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);//获取当前线程池状态

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {//
            decrementWorkerCount();//线程数量-1
            return null;
        }

        int wc = workerCountOf(c);//线程数
        //allowCoreThreadTimeOut为true，说明线程数要根据是否超过核心线程数判断keepAliveTime
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;//是否超过核心线程数

        if ((wc > maximumPoolSize || (timed && timedOut))//超过了最大线程数
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))//线程数-1
                return null;//返回空
            continue;
        }

        try {
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();//获取任务
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}