public void execute(Runnable command) {
    if (command == null)
    // 三种情况：
    int c = ctl.get();
    //1，线程数 少于 核心线程 直接创建线程
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    //线程数数超过 核心线程，但是blockqueue 未满，enqueue.
    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);
    }
    // queue 已经满，直接创建线程（超过max  reject）
    else if (!addWorker(command, false))
        reject(command);
}

// CAPACITY= 00011111111111111111111111111111(29个1)
// 获取当前线程数
// 线程池的最大数就是2^29-1
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
            }
        }

        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();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

Worker 
Worker(Runnable firstTask) {
    setState(-1); // inhibit interrupts until runWorker
    this.firstTask = firstTask;
    // 注意：这里，这个线程 传的runnable 是this, 也就是 worker本身， 所以start()后进入runnable状态，等到获取时间片后，就执行 run方法。
    this.thread = getThreadFactory().newThread(this);
}
}

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

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();try {
                    Throwable thrown = null;
                    try {
                        task.run();
                    }
　　　　　　　　　　　　afterExecute(task, thrown);

                    } finally { task = null;

　　　　　　　　　　　　// 统计完成任务数
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            //回收工作线程，尝试更新状态。
            processWorkerExit(w, completedAbruptly);
        }
    }