摘要：是所有線程池實(shí)現(xiàn)的父類，我們先看看構(gòu)造函數(shù)構(gòu)造參數(shù)線程核心數(shù)最大線程數(shù)線程空閑后，存活的時(shí)間，只有線程數(shù)大于的時(shí)候生效存活時(shí)間的單位任務(wù)的阻塞隊(duì)列創(chuàng)建線程的工程，給線程起名字當(dāng)線程池滿了，選擇新加入的任務(wù)應(yīng)該使用什么策略，比如拋異常丟棄當(dāng)前

ThreadPoolExecutor是所有線程池實(shí)現(xiàn)的父類，我們先看看構(gòu)造函數(shù)

corePoolSize：線程核心數(shù)

maximumPoolSize：最大線程數(shù)

keepAliveTime：線程空閑后，存活的時(shí)間，只有線程數(shù)大于corePoolSize的時(shí)候生效

unit：存活時(shí)間的單位

workQueue：任務(wù)的阻塞隊(duì)列

threadFactory：創(chuàng)建線程的工程，給線程起名字

handler：當(dāng)線程池滿了，選擇新加入的任務(wù)應(yīng)該使用什么策略，比如拋異常、丟棄當(dāng)前任務(wù)、丟棄阻塞隊(duì)列的最老任務(wù)等，也可以自定義。

判斷是否超過線程核心數(shù)corePoolSize，沒超過創(chuàng)建線程

超過線程核心數(shù)，則判斷隊(duì)列是否已滿，沒有滿，放入隊(duì)列

隊(duì)列也滿了，判斷是否超過maximumPoolSize，沒有就創(chuàng)建線程

超過了，根據(jù)策略執(zhí)行

//32為，前3位作為線程池的狀態(tài)，后三位是線程數(shù)
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的二進(jìn)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，還是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為11100000000000000000000000000000，與完就是線程的狀態(tài)
private static int workerCountOf(int c)  { return c & CAPACITY; }//與完，是線程的數(shù)量
private static int ctlOf(int rs, int wc) { return rs | wc; }
private static boolean isRunning(int c) {
    return c < SHUTDOWN;//小于0，說明是RUNNING，RUNNING=-1
}

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {//如果線程數(shù)少于線程核心數(shù)
        if (addWorker(command, true))//增加任務(wù)成功，返回true，沒成功，繼續(xù)往下
            return;
        c = ctl.get();
    }
    //判斷隊(duì)列
    if (isRunning(c) && workQueue.offer(command)) {//如果線程池還在跑，并且可以插入隊(duì)列
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))//線程池不是運(yùn)行狀態(tài)，就移除剛剛插入的任務(wù)
            reject(command);//執(zhí)行策略
        else if (workerCountOf(recheck) == 0)//
            addWorker(null, false);
    }
    //隊(duì)列也滿了，判斷最大線程數(shù)
    else if (!addWorker(command, false))
        reject(command);//執(zhí)行策略
}

private boolean addWorker(Runnable firstTask, boolean core) {//core為true，使用corePoolSize判斷，否則使用maximumPoolSize
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);//獲取當(dāng)前線程狀態(tài)

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && // 就是STOP、TIDYING、TERMINATED，此時(shí)不讓任務(wù)進(jìn)來
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))//
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;//超過了線程核心數(shù)或最大線程數(shù)，不讓新增
            if (compareAndIncrementWorkerCount(c))//返回true，說明成功了，跳出retry循環(huán)
                break retry;
            //失敗了，說明被其他符號條件的線程占了，就再判斷線程狀態(tài)是否跟之前一樣，不一樣重新獲取，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();//獲取鎖
            try {
                int rs = runStateOf(ctl.get());//獲取線程池的狀態(tài)

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // 沒通過start來啟動run的
                        throw new IllegalThreadStateException();
                    workers.add(w);//加點(diǎn)hashset
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;//更新當(dāng)前最大值
                    workerAdded = true;//增加成功
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start();//啟動線程
                workerStarted = true;//啟動成功
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);//失敗，線程數(shù)-1，從hashset移除，并嘗試Terminate
    }
    return workerStarted;
}

上面執(zhí)行 t.start();的時(shí)候，就會通過run方法調(diào)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) {//任務(wù)不為空或者獲取的任務(wù)也不為空
            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();//調(diào)用run方法，這里沒有通過start，也就是說沒有啟動新線程
                } 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++;//完成任務(wù)數(shù)加1
                w.unlock();//釋放
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);//移除w，在task為空的時(shí)候，比如線程池狀態(tài)停止或者啟動的線程太多
    }
}

getTask方法
當(dāng)Worker第一次啟動的時(shí)候，調(diào)用run方法，后面就一直從隊(duì)列里獲取任務(wù)

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

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);//獲取當(dāng)前線程池狀態(tài)

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {//
            decrementWorkerCount();//線程數(shù)量-1
            return null;
        }

        int wc = workerCountOf(c);//線程數(shù)
        //allowCoreThreadTimeOut為true，說明線程數(shù)要根據(jù)是否超過核心線程數(shù)判斷keepAliveTime
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;//是否超過核心線程數(shù)

        if ((wc > maximumPoolSize || (timed && timedOut))//超過了最大線程數(shù)
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))//線程數(shù)-1
                return null;//返回空
            continue;
        }

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