线程池（ThreadPoolExecutor）的几个状态

• RUNNING：Accept new tasks and process queued tasks（可接受新的任务并且执行已经加入队列的任务）
• SHUTDOWN：Don't accept new tasks, but process queued tasks（不再接受新的任务，但是可以继续把已经加入队列的任务执行完成）
• STOP：Don't accept new tasks, don't process queued tasks,and interrupt in-progress tasks（不接收新任务，不执行已经加入到队列中的任务，中断当前正在执行的任务）
• TIDYING：All tasks have terminated, workerCount is zero,the thread transitioning to state TIDYING will run the terminated() hook method（所有的任务都结束了，workerCount 变为0，线程状态变为TIDYING ，然后会执行terminated()方法）
• TERMINATED：terminated() has completed（terminated()方法已经执行完成）

线程池的状态切换过程

• RUNNING -> SHUTDOWN：On invocation of shutdown(), perhaps implicitly in finalize()
• (RUNNING or SHUTDOWN) -> STOP：On invocation of shutdownNow()
• SHUTDOWN -> TIDYING：When both queue and pool are empty
• STOP -> TIDYING：When pool is empty
• TIDYING -> TERMINATED：When the terminated() hook method has completed

线程池的构造函数参数

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;
    }

• corePoolSize：核心线程数，即当前线程池中维持运行状态的线程数量，可以为0，也可以大于0，如果大于0就代表当前线程允许多个线程并发执行，如果等于0就代表线程池中的线程任务运行结束后就不在有线程处于运行状态，如果等于1就代表当前线程池在同一时间只允许一个线程在运行，类似于：Executors.newSingleThreadExecutor()
• maximumPoolSize：线程池中最大可接受的线程数量，或者说任务数量
• keepAliveTime：如果当前线程中中接受的线程数量已经超过和核心线程数，次数空闲线程在等待接收新线程可最多维持keepAliveTime的单位时间，超过这个时间，空闲线程将会终止
• workQueue：任务队列，是阻塞队列，BlockingQueue
• handler：当新put进来的任务被reject之后就会转给handler去处理，默认实现为抛出RejectedExecutionException异常

线程池的execute方法

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) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        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);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

• isRunning方法判断当前的线程池是否存在已经在运行的线程，workQueue是任务队列，负责存放execute进来的command任务对象
• 这里有两个地方会被线程池reject，也就是会抛出RejectedExecutionException异常，一个是当线程池已经被shutdown掉了，另一个当workQueue已经满了，而且当前线程池中正在工作的线程数量已经超过maximumPoolSize限制的数量了，这时候也会拒绝接受任务了
• 如果当前线程池正在运行，并且线程池中的活跃线程数量还小于核心线程数corePoolSize，此时会直接start一个线程作为核心线程，线程池中运行的核心线程都是并发的
• 如果当前线程池中的核心线程数已经达到限制，这时候会尝试吧任务加到任务队列中，等核心线程去遍历取出任务队列中的任务来执行
• 线程池之所以说能够合理的管理线程资源，不乱开线程的原因就是控制核心线程数的运行数量，在线程池中并发的核心线程是有一个最大上线的（corePoolSize），所有外部execute进来的任务在核心线程数达到上线之后都会放进workQueue，这样做的好处就是开启的线程数最多只有corePoolSize，而其他多余的线程任务将以对象的方式存放到workQueue等待执行，所以申请了线程创建的只有corePoolSize个线程，而其他的任务只是占用了对象内存而已，避免了每一个线程都去start，既占用了对象内存，又申请了线程资源，对于java来说运行一个线程需要去申请线程的运行内存的，在底层是用pthread_create去创建一个linux线程，代价相对来说比较大，所以由线程池管理线程的这种方式限制了线程start的个数，避免造成过于的并发线程占用了大量的内存资源而导致OOM的问题

下面继续分析线程池如何管理运行线程，其实就一句话，维护一个线程队列，然后对这个线程队列进行存取操作

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;
    }

每一个运行起来的核心线程都会被封装成一个Worker对象然后保存到workers列表中，核心线程start之后会执行Worker中的runWorker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) {
                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);
        }
    }

这里面是一个while循环，反复的去workQueue队列里面去取任务出来执行，在同一个核心线程中，只有等上一个任务执行完成之后才能继续去执行下一个任务，所以同一个核心线程中的任务执行并不是并发的，但是不同核心线程中的任务执行是并发的，继续分析getTask方法：

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);

            // 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;
            }
        }
    }

getTask方法就是去workQueue任务队列中去取任务，这个方法里面还会对当前线程池的状态做判断，如果线程池被shutdown并且任务队列已经空了，或者线程池被shutdownNow了，这时候都会返回null，要是线程池还是在运行状态，那么这时候就会去判断如果当前核心线程数已经达到上线，或者限制核心线程数不能无限等待（allowCoreThreadTimeOut = true），会从workQueue中以poll的方式取任务，并且等待keepAliveTime时间，如果超过这个时间还没取出任务，此时poll方法返回null，然后进入下一次循环，但是最后如果是核心线程数已经达到上线，workQueue已经为空，那么此时getTask也会返回null，然后线程池会结束所有的核心线程，不再维持任何核心线程；这边举一个例子，假设keepAliveTime为0，那么当线程池中执行完所有的任务后，核心线程逐个马上就会结束并且释放，这时候要是有新的任务进来需要再重新开一个核心线程去执行，如果keepAliveTime为60秒，那么在线程池中完成所有任务后，核心线程会再等待60秒后才会结束运行，在这60秒之内如果还有新的任务进来就直接取出来接着执行，所以关于核心线程的维持活跃状态的实现是通过阻塞队列去实现的

shutdown和shutdownNow方法

shutdown方法在调用完了之后，线程池会把workQueue中剩下的任务都执行完了之后才会完全结束，而shutdownNow执行完了之后，二者都不允许再接收新的任务，workQueue中剩下的任务将不会再执行，具体的区别实现在一下几个实现

• getTask方法中：

if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

shutdown之后线程池的状态是SHUTDOWN，shutdownNow后线程池的状态是STOP，所以shutdownNow之后getTask会直接返回null，而shutdown之后如果workQueue不为空getTask还会取任务

• processWorkerExit方法：

private void processWorkerExit(Worker w, boolean completedAbruptly) {
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }

        tryTerminate();

        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false);
        }
    }

if (runStateLessThan(c, STOP))这个条件判断就是说要是只调用了shutdown，那就是满足的，次数会继续执行addWorker(null, false)来再次出发workQueue的任务存取执行

• shutDown方法

public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }

private void interruptIdleWorkers() {
        interruptIdleWorkers(false);
    }

private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

在shutdown调用后会去workers遍历里面的Worker对象，上文已经提到了，workers里面维护的Worker对象是核心线程的包装对象，所以这里是去中断核心线程，但是只是中断空闲的核心线程，什么叫空闲？w.tryLock()可以获取到锁的核心线程就是空闲的，因为在Worker运行的时候会先lock获取锁，在执行完成之后才unlock，所以要是Worker对象还在执行任务，这时候就不中断

• shutdownNow

public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(STOP);
            interruptWorkers();
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }

private void interruptWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers)
                w.interruptIfStarted();
        } finally {
            mainLock.unlock();
        }
    }

shutdownNow方法就是直接遍历workers中的Worker对象并且进行中断操作，不管Worker是否是空闲状态，这里的线程中断就是调用Thread的interrupt方法，而在Worker的runWorker会去判断该Thread的的中断状态，根据中断状态来判断是否接着执行任务

总结

• 线程池的好处就是合理的管理线程，避免造成过多的并发线程占用了大量内存资源，从何导致OOM
• 线程池所谓的并发执行并不一定是并发，因为同一个核心线程中执行的任务是按照顺序执行的，并不是并发的，只有不同的核心线程的任务执行才是并发的
• 多线程的实现可以用队列加上一个核心线程的方式去实现，可以开启一个核心线程负责遍历队列中的任务，然后在核心线程中去run，注意。如果核心线程只有一个的话，那么就无法达到并发效果，类似newSingleThreadExecutor方法创建出来的ThreadPoolExecutor，这时候execute多次，一定要等上一次的任务执行完成之后才能执行下一次的任务
• Java线程start起来之后会有两个部分的内存占用，一个是JVM部分的线程对象资源，而真正开线程的其实是用linux的pthread_create去开启了一个linux线程实现异步的，所以这里还占用了系统底层的线程资源，因此开启一个Java线程的代价比直接用native方法去开启一个linux线程要大，占用的内存资源要多，所以要谨慎的使用java多线程，避免造成线程泄漏，或者OOM