public static ExecutorService newFixedThreadPool(int var0) {
    return new ThreadPoolExecutor(var0, var0, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue());
}
public static ExecutorService newSingleThreadExecutor() {
    return new Executors.FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue()));
}
public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, 2147483647, 60L, TimeUnit.SECONDS, new SynchronousQueue());
}
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
    return new Executors.DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1));
}
public static ScheduledExecutorService newScheduledThreadPool(int var0) {
    return new ScheduledThreadPoolExecutor(var0);
}

/**
 * Creates a new {@code ThreadPoolExecutor} with the given initial
 * parameters.
 *
 * @param corePoolSize the number of threads to keep in the pool, even
 *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
 * @param maximumPoolSize the maximum number of threads to allow in the
 *        pool
 * @param keepAliveTime when the number of threads is greater than
 *        the core, this is the maximum time that excess idle threads
 *        will wait for new tasks before terminating.
 * @param unit the time unit for the {@code keepAliveTime} argument
 * @param workQueue the queue to use for holding tasks before they are
 *        executed.  This queue will hold only the {@code Runnable}
 *        tasks submitted by the {@code execute} method.
 * @param threadFactory the factory to use when the executor
 *        creates a new thread
 * @param handler the handler to use when execution is blocked
 *        because the thread bounds and queue capacities are reached
 * @throws IllegalArgumentException if one of the following holds:<br>
 *         {@code corePoolSize < 0}<br>
 *         {@code keepAliveTime < 0}<br>
 *         {@code maximumPoolSize <= 0}<br>
 *         {@code maximumPoolSize < corePoolSize}
 * @throws NullPointerException if {@code workQueue}
 *         or {@code threadFactory} or {@code handler} is null
 */
public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

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();
    if (workerCountOf(c) < corePoolSize) { //第一步：如果線程數(shù)量小于核心線程數(shù)
        if (addWorker(command, true))//則啟動一個核心線程執(zhí)行任務
            return;
        c = ctl.get();
    }
    if (isRunning(c) && workQueue.offer(command)) {//第二步：當前線程數(shù)量大于等于核心線程數(shù)，加入任務隊列，成功的話會進行二次檢查
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);//啟動非核心線程執(zhí)行，注意這里任務是null，其實里面會去取任務隊列里的任務執(zhí)行
    }
    else if (!addWorker(command, false))//第三步：加入不了隊列（即隊列滿了），嘗試啟動非核心線程
        reject(command);//如果啟動不了非核心線程執(zhí)行，說明到達了最大線程數(shù)量的限制，會使用第7個參數(shù)拋出異常
}

// 獲取運行狀態(tài)
private static int runStateOf(int c)     { return c & ~CAPACITY; }

// 獲取活動線程數(shù)
private static int workerCountOf(int c)  { return c & CAPACITY; }

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);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }
    //前面都是線程池狀態(tài)的判斷，暫時不理會，主要看下面兩個關(guān)鍵的地方
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask); // 新建一個Worker對象，這個對象包含了待執(zhí)行的任務，并且新建一個線程
        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(); // 啟動剛創(chuàng)建的worker對象里面的thread執(zhí)行
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker. */
    public void run() {
        runWorker(this);
    }

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // The value 1 represents the locked state.

    protected boolean isHeldExclusively() {
        return getState() != 0;
    }

    protected boolean tryAcquire(int unused) {
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread());
            return true;
        }
        return false;
    }

    protected boolean tryRelease(int unused) {
        setExclusiveOwnerThread(null);
        setState(0);
        return true;
    }

    public void lock()        { acquire(1); }
    public boolean tryLock()  { return tryAcquire(1); }
    public void unlock()      { release(1); }
    public boolean isLocked() { return isHeldExclusively(); }

    void interruptIfStarted() {
        Thread t;
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

Worker(Runnable firstTask) { // worker本身實現(xiàn)了Runnable接口
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask; // 持有外部傳進來的runnable任務
        //創(chuàng)建了一個thread對象，并把自身這個runnable對象給了thread，一旦該thread執(zhí)行start方法，就會執(zhí)行worker的run方法
        this.thread = getThreadFactory().newThread(this); 
    }
在addWorker方法中執(zhí)行的t.start會去執(zhí)行worker的run方法：

public void run() {
        runWorker(this);
    }
run方法又執(zhí)行了ThreadPoolExecutor的runWorker方法，把當前worker對象傳入。

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask; // 取出worker的runnable任務
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        // 循環(huán)不斷的判斷任務是否為空，當?shù)谝粋€判斷為false的時候，即task為null，這個task啥時候為null呢？
        // 要么w.firstTask為null，還記得我們在execute方法第二步的時候，執(zhí)行addWorker的時候傳進來的runnable是null嗎？
        // 要么是執(zhí)行了一遍while循環(huán)，在下面的finally中執(zhí)行了task=null；
        // 或者執(zhí)行第二個判斷，一旦不為空就會繼續(xù)執(zhí)行循環(huán)里的代碼。
        while (task != null || (task = getTask()) != null) {
            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(); // 任務不為空，就會執(zhí)行任務的run方法，也就是runnable的run方法
                } 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; // 執(zhí)行完成置null，繼續(xù)下一個循環(huán)
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

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

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

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

        int wc = workerCountOf(c);

        // timed變量用于判斷是否需要進行超時控制。
        // allowCoreThreadTimeOut默認是false，也就是核心線程不允許進行超時；
        // wc > corePoolSize，表示當前線程池中的線程數(shù)量大于核心線程數(shù)量；
        // 對于超過核心線程數(shù)量的這些線程或者允許核心線程進行超時控制的時候，需要進行超時控制
        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        // 如果需要進行超時控制，且上次從緩存隊列中獲取任務時發(fā)生了超時（timedOut開始為false，后面的循環(huán)末尾超時時會置為true)
        // 或者當前線程數(shù)量已經(jīng)超過了最大線程數(shù)量，那么嘗試將workerCount減1,即當前活動線程數(shù)減1，
        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            // 如果減1成功，則返回null，這就意味著runWorker()方法中的while循環(huán)會被退出，其對應的線程就要銷毀了，也就是線程池中少了一個線程了
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            // 注意workQueue中的poll()方法與take()方法的區(qū)別
            //poll方式取任務的特點是從緩存隊列中取任務,最長等待keepAliveTime的時長，取不到返回null
            //take方式取任務的特點是從緩存隊列中取任務，若隊列為空,則進入阻塞狀態(tài)，直到能取出對象為止
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true; // 能走到這里說明已經(jīng)超時了
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}