ThreadPoolExecutor的运行状态有5种，分别为：

image.png

其生命周期转换如下入所示：

图片

从数值来看running<shutdown<stop<tidying<terminated.

获取线程池状态方法：

    private static int runStateOf(int c)     { return c & ~CAPACITY; }

从代码和数值上看就是取高三位的值，也就是用高三位来确定线程池状态

这个理解清楚以后我们再来看任务如何提交：

image.png

看看源码：

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
       //默认情况ctl的初始值是RUNNING，然后获取当前的工作线程，如果工作线程<核心线程，则添加work
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
     //判断线程池当前处于running状态，且能往队列里插入任务
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
           //再次检查，如果线程池不是running状态，需要任务从队列中移除（非running状态不再接收task）
            if (! isRunning(recheck) && remove(command))
               //然后直接拒绝
                reject(command);
            else if (workerCountOf(recheck) == 0)
               //如果工作线程为0，需要新启线程执行队列里的任务
                addWorker(null, false);
        }
     //队列满了，需要启动非核心线程来执行任务
        else if (!addWorker(command, false))
            reject(command);
    }

这个是任务提交的整体逻辑。
我们再来关注下addworker的逻辑

image.png

看下源码：

/core 为true 核心线程
private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

           //线程池状态>=shutdown不再接收新的任务
            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;
    }

从addWorker逻辑里可以看到会启动线程执行任务，会调用Worker里的runWork方法，看下流程：

    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();
                // 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();
                    } 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);
        }
    }

上面有一个getTask逻辑是一个doWhile循环，线程池中线程的销毁依赖JVM自动的回收，线程池做的工作是根据当前线程池的状态维护一定数量的线程引用，防止这部分线程被JVM回收，当线程池决定哪些线程需要回收时，只需要将其引用消除即可。Worker被创建出来后，就会不断地进行轮询，然后获取任务去执行，核心线程可以无限等待获取任务，非核心线程要限时获取任务。当Worker无法获取到任务，也就是获取的任务为空时，循环会结束，Worker会主动消除自身在线程池内的引用。
再来看一卡getTask的逻辑：

image.png

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

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

            // 线程池已停止
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

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