深入理解java线程池—ThreadPoolExecutor
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几句闲扯：首先，我想说java的线程池真的是很绕，以前一直都感觉新建几个线程一直不退出到底是怎么实现的，也就有了后来学习ThreadPoolExecutor源码。学习源码的过程中，最恶心的其实就是几种状态的转换了，这也是ThreadPoolExecutor的核心。花了将近小一周才大致的弄明白ThreadPoolExecutor的机制，遂记录下来。
线程池有多重要
线程是一个程序员一定会涉及到的一个概念，但是线程的创建和切换都是代价比较大的。所以，我们有没有一个好的方案能做到线程的复用呢？这就涉及到一个概念——线程池。合理的使用线程池能够带来3个很明显的好处： 1.降低资源消耗：通过重用已经创建的线程来降低线程创建和销毁的消耗 2.提高响应速度：任务到达时不需要等待线程创建就可以立即执行。 3.提高线程的可管理性：线程池可以统一管理、分配、调优和监控。
java多线程池的支持——ThreadPoolExecutor
java的线程池支持主要通过ThreadPoolExecutor来实现，我们使用的ExecutorService的各种线程池策略都是基于ThreadPoolExecutor实现的，所以ThreadPoolExecutor十分重要。要弄明白各种线程池策略，必须先弄明白ThreadPoolExecutor。
1. 实现原理
首先看一个线程池的流程图：
Paste_Image.pngstep1.调用ThreadPoolExecutor的execute提交线程，首先检查CorePool，如果CorePool内的线程小于CorePoolSize，新创建线程执行任务。step2.如果当前CorePool内的线程大于等于CorePoolSize，那么将线程加入到BlockingQueue。step3.如果不能加入BlockingQueue，在小于MaxPoolSize的情况下创建线程执行任务。step4.如果线程数大于等于MaxPoolSize，那么执行拒绝策略。
2.线程池的创建
线程池的创建可以通过ThreadPoolExecutor的构造方法实现：
/** * 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; }具体解释一下上述参数：
- corePoolSize 核心线程池大小
- maximumPoolSize 线程池最大容量大小
- keepAliveTime 线程池空闲时，线程存活的时间
- TimeUnit 时间单位
- ThreadFactory 线程工厂
- BlockingQueue任务队列
- RejectedExecutionHandler 线程拒绝策略
3.线程的提交
ThreadPoolExecutor的构造方法如上所示，但是只是做一些参数的初始化，ThreadPoolExecutor被初始化好之后便可以提交线程任务，线程的提交方法主要是execute和submit。这里主要说execute，submit会在后续的博文中分析。
/** * Executes the given task sometime in the future. The task * may execute in a new thread or in an existing pooled thread. * * If the task cannot be submitted for execution, either because this * executor has been shutdown or because its capacity has been reached, * the task is handled by the current {@code RejectedExecutionHandler}. * * @param command the task to execute * @throws RejectedExecutionException at discretion of * {@code RejectedExecutionHandler}, if the task * cannot be accepted for execution * @throws NullPointerException if {@code command} is null */ 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. * 如果当前的线程数小于核心线程池的大小，根据现有的线程作为第一个Worker运行的线程， * 新建一个Worker，addWorker自动的检查当前线程池的状态和Worker的数量， * 防止线程池在不能添加线程的状态下添加线程 * * 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. * 如果线程入队成功，然后还是要进行double-check的，因为线程池在入队之后状态是可能会发生变化的 * * 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. * * 如果task不能入队(队列满了)，这时候尝试增加一个新线程，如果增加失败那么当前的线程池状态变化了或者线程池已经满了 * 然后拒绝task */ int c = ctl.get(); //当前的Worker的数量小于核心线程池大小时，新建一个Worker。 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))//recheck防止线程池状态的突变，如果突变，那么将reject线程，防止workQueue中增加新线程 reject(command); else if (workerCountOf(recheck) == 0)//上下两个操作都有addWorker的操作，但是如果在workQueue.offer的时候Worker变为0， //那么将没有Worker执行新的task，所以增加一个Worker. addWorker(null, false); } //如果workQueue满了，那么这时候可能还没到线程池的maxnum，所以尝试增加一个Worker else if (!addWorker(command, false)) reject(command);//如果Worker数量到达上限，那么就拒绝此线程 }这里需要明确几个概念：
- Worker和Task的区别，Worker是当前线程池中的线程，而task虽然是runnable，但是并没有真正执行，只是被Worker调用了run方法，后面会看到这部分的实现。
- maximumPoolSize和corePoolSize的区别：这个概念很重要，maximumPoolSize为线程池最大容量，也就是说线程池最多能起多少Worker。corePoolSize是核心线程池的大小，当corePoolSize满了时，同时workQueue full（ArrayBolckQueue是可能满的） 那么此时允许新建Worker去处理workQueue中的Task，但是不能超过maximumPoolSize。超过corePoolSize之外的线程会在空闲超时后终止。
核心方法：addWorker
Worker的增加和Task的获取以及终止都是在此方法中实现的，也就是这一个方法里面包含了很多东西。在addWorker方法中提到了Status的概念，Status是线程池的核心概念，这里我们先看一段关于status的注释：
/** * 首先ctl是一个原子量，同时它里面包含了两个field，一个是workerCount，另一个是runState * workerCount表示当前有效的线程数，也就是Worker的数量 * runState表示当前线程池的状态 * The main pool control state, ctl, is an atomic integer packing * two conceptual fields * workerCount, indicating the effective number of threads * runState, indicating whether running, shutting down etc * * 两者是怎么结合的呢？首先workerCount是占据着一个atomic integer的后29位的，而状态占据了前3位 * 所以，workerCount上限是(2^29)-1。 * In order to pack them into one int, we limit workerCount to * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2 * billion) otherwise representable. If this is ever an issue in * the future, the variable can be changed to be an AtomicLong, * and the shift/mask constants below adjusted. But until the need * arises, this code is a bit faster and simpler using an int. * * The workerCount is the number of workers that have been * permitted to start and not permitted to stop. The value may be * transiently different from the actual number of live threads, * for example when a ThreadFactory fails to create a thread when * asked, and when exiting threads are still performing * bookkeeping before terminating. The user-visible pool size is * reported as the current size of the workers set. * * runState是整个线程池的运行生命周期，有如下取值： * 1. RUNNING：可以新加线程，同时可以处理queue中的线程。 * 2. SHUTDOWN：不增加新线程，但是处理queue中的线程。 * 3.STOP 不增加新线程，同时不处理queue中的线程。 * 4.TIDYING 所有的线程都终止了（queue中），同时workerCount为0，那么此时进入TIDYING * 5.terminated()方法结束，变为TERMINATED * The runState provides the main lifecyle control, taking on values: * * RUNNING: Accept new tasks and process queued tasks * SHUTDOWN: Don't accept new tasks, but process queued tasks * STOP: Don't accept new tasks, don't process queued tasks, * and interrupt in-progress tasks * TIDYING: All tasks have terminated, workerCount is zero, * the thread transitioning to state TIDYING * will run the terminated() hook method * TERMINATED: terminated() has completed * * The numerical order among these values matters, to allow * ordered comparisons. The runState monotonically increases over * time, but need not hit each state. The transitions are: * 状态的转化主要是： * RUNNING -> SHUTDOWN（调用shutdown()） * On invocation of shutdown(), perhaps implicitly in finalize() * (RUNNING or SHUTDOWN) -> STOP(调用shutdownNow()) * On invocation of shutdownNow() * SHUTDOWN -> TIDYING（queue和pool均empty） * When both queue and pool are empty * STOP -> TIDYING（pool empty，此时queue已经为empty） * When pool is empty * TIDYING -> TERMINATED(调用terminated()) * When the terminated() hook method has completed * * Threads waiting in awaitTermination() will return when the * state reaches TERMINATED. * * Detecting the transition from SHUTDOWN to TIDYING is less * straightforward than you'd like because the queue may become * empty after non-empty and vice versa during SHUTDOWN state, but * we can only terminate if, after seeing that it is empty, we see * that workerCount is 0 (which sometimes entails a recheck -- see * below). */下面是状态的代码：
//利用ctl来保证当前线程池的状态和当前的线程的数量。ps：低29位为线程池容量，高3位为线程状态。 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); //设定偏移量 private static final int COUNT_BITS = Integer.SIZE - 3; //确定最大的容量2^29-1 private static final int CAPACITY = (1 << COUNT_BITS) - 1; //几个状态，用Integer的高三位表示 // runState is stored in the high-order bits //111 private static final int RUNNING = -1 << COUNT_BITS; //000 private static final int SHUTDOWN = 0 << COUNT_BITS; //001 private static final int STOP = 1 << COUNT_BITS; //010 private static final int TIDYING = 2 << COUNT_BITS; //011 private static final int TERMINATED = 3 << COUNT_BITS; //获取线程池状态，取前三位 // Packing and unpacking ctl private static int runStateOf(int c) { return c & ~CAPACITY; } //获取当前正在工作的worker,主要是取后面29位 private static int workerCountOf(int c) { return c & CAPACITY; } //获取ctl private static int ctlOf(int rs, int wc) { return rs | wc; }接下来贴上addWorker方法看看：
/** * Checks if a new worker can be added with respect to current * pool state and the given bound (either core or maximum). If so, * the worker count is adjusted accordingly, and, if possible, a * new worker is created and started running firstTask as its * first task. This method returns false if the pool is stopped or * eligible to shut down. It also returns false if the thread * factory fails to create a thread when asked, which requires a * backout of workerCount, and a recheck for termination, in case * the existence of this worker was holding up termination. * * @param firstTask the task the new thread should run first (or * null if none). Workers are created with an initial first task * (in method execute()) to bypass queuing when there are fewer * than corePoolSize threads (in which case we always start one), * or when the queue is full (in which case we must bypass queue). * Initially idle threads are usually created via * prestartCoreThread or to replace other dying workers. * * @param core if true use corePoolSize as bound, else * maximumPoolSize. (A boolean indicator is used here rather than a * value to ensure reads of fresh values after checking other pool * state). * @return true if successful */ private boolean addWorker(Runnable firstTask, boolean core) { retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. /** * rs!=Shutdown || fistTask！=null || workCount.isEmpty * 如果当前的线程池的状态>SHUTDOWN 那么拒绝Worker的add 如果=SHUTDOWN * 那么此时不能新加入不为null的Task，如果在WorkCount为empty的时候不能加入任何类型的Worker， * 如果不为empty可以加入task为null的Worker,增加消费的Worker */ 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 } } Worker w = new Worker(firstTask); Thread t = w.thread; 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 c = ctl.get(); int rs = runStateOf(c); /** * rs!=SHUTDOWN ||firstTask!=null * * 同样检测当rs>SHUTDOWN时直接拒绝减小Wc，同时Terminate，如果为SHUTDOWN同时firstTask不为null的时候也要Terminate */ if (t == null || (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null))) { decrementWorkerCount(); tryTerminate(); return false; } workers.add(w); int s = workers.size(); if (s > largestPoolSize) largestPoolSize = s; } finally { mainLock.unlock(); } t.start(); // It is possible (but unlikely) for a thread to have been // added to workers, but not yet started, during transition to // STOP, which could result in a rare missed interrupt, // because Thread.interrupt is not guaranteed to have any effect // on a non-yet-started Thread (see Thread#interrupt). //Stop或线程Interrupt的时候要中止所有的运行的Worker if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted()) t.interrupt(); return true; }addWorker中首先进行了一次线程池状态的检测：
int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. //判断当前线程池的状态是不是已经shutdown，如果shutdown了拒绝线程加入 //(rs!=SHUTDOWN || first!=null || workQueue.isEmpty()) //如果rs不为SHUTDOWN，此时状态是STOP、TIDYING或TERMINATED，所以此时要拒绝请求 //如果此时状态为SHUTDOWN，而传入一个不为null的线程，那么需要拒绝 //如果状态为SHUTDOWN，同时队列中已经没任务了，那么拒绝掉 if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false;其实是比较难懂的，主要在线程池状态判断条件这里：
- 如果是runing，那么跳过if。
- 如果rs>=SHUTDOWN,同时不等于SHUTDOWN，即为SHUTDOWN以上的状态，那么不接受新线程。
如果rs>=SHUTDOWN，同时等于SHUTDOWN，同时first！=null，那么拒绝新线程，如果first==null，那么可能是新增加线程消耗Queue中的线程。但是同时还要检测workQueue是否isEmpty()，如果为Empty，那么队列已空，不需要增加消耗线程，如果队列没有空那么运行增加first=null的Worker。
从这里是可以看出一些策略的
首先，在rs>SHUTDOWN时，拒绝一切线程的增加，因为STOP是会终止所有的线程，同时移除Queue中所有的待执行的线程的，所以也不需要增加first=null的Worker了
其次，在SHUTDOWN状态时，是不能增加first！=null的Worker的，同时即使first=null，但是此时Queue为Empty也是不允许增加Worker的，SHUTDOWN下增加的Worker主要用于消耗Queue中的任务。
SHUTDOWN状态时，是不允许向workQueue中增加线程的，isRunning(c) && workQueue.offer(command) 每次在offer之前都要做状态检测，也就是线程池状态变为>=SHUTDOWN时不允许新线程进入线程池了。for (;;) { int wc = workerCountOf(c); //如果当前的数量超过了CAPACITY，或者超过了corePoolSize和maximumPoolSize（试core而定） if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; //CAS尝试增加线程数，如果失败，证明有竞争，那么重新到retry。 if (compareAndIncrementWorkerCount(c)) break retry; c = ctl.get(); // Re-read ctl //判断当前线程池的运行状态 if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop }这段代码做了一个兼容，主要是没有到corePoolSize 或maximumPoolSize上限时，那么允许添加线程，CAS增加Worker的数量后，跳出循环。
接下来实例化Worker,实例化Worker其实是很关键的，后面会说。
因为workers是HashSet线程不安全的，那么此时需要加锁，所以mainLock.lock(); 之后重新检查线程池的状态，如果状态不正确，那么减小Worker的数量，为什么tryTerminate（）目前不大清楚。如果状态正常，那么添加Worker到workers。最后：if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted()) t.interrupt();注释说的很清楚，为了能及时的中断此Worker，因为线程存在未Start的情况，此时是不能响应中断的，如果此时status变为STOP，则不能中断线程。此处用作中断线程之用。
接下来我们看Worker的方法：/** * Creates with given first task and thread from ThreadFactory. * @param firstTask the first task (null if none) */ Worker(Runnable firstTask) { this.firstTask = firstTask; this.thread = getThreadFactory().newThread(this); }这里可以看出Worker是对firstTask的包装，并且Worker本身就是Runnable的，看上去真心很流氓的感觉~~~
通过ThreadFactory为Worker自己构建一个线程。
因为Worker是Runnable类型的，所以是有run方法的,上面也看到了会调用t.start() 其实就是执行了run方法：/** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); }调用了runWorker:
/** * Main worker run loop. Repeatedly gets tasks from queue and * executes them, while coping with a number of issues: * 1 Worker可能还是执行一个初始化的task——firstTask。 * 但是有时也不需要这个初始化的task（可以为null）,只要pool在运行，就会 * 通过getTask从队列中获取Task，如果返回null，那么worker退出。 * 另一种就是external抛出异常导致worker退出。 * 1. We may start out with an initial task, in which case we * don't need to get the first one. Otherwise, as long as pool is * running, we get tasks from getTask. If it returns null then the * worker exits due to changed pool state or configuration * parameters. Other exits result from exception throws in * external code, in which case completedAbruptly holds, which * usually leads processWorkerExit to replace this thread. * * * 2 在运行任何task之前，都需要对worker加锁来防止other pool中断worker。 * clearInterruptsForTaskRun保证除了线程池stop，那么现场都没有中断标志 * 2. Before running any task, the lock is acquired to prevent * other pool interrupts while the task is executing, and * clearInterruptsForTaskRun called to ensure that unless pool is * stopping, this thread does not have its interrupt set. * * 3. Each task run is preceded by a call to beforeExecute, which * might throw an exception, in which case we cause thread to die * (breaking loop with completedAbruptly true) without processing * the task. * * 4. Assuming beforeExecute completes normally, we run the task, * gathering any of its thrown exceptions to send to * afterExecute. We separately handle RuntimeException, Error * (both of which the specs guarantee that we trap) and arbitrary * Throwables. Because we cannot rethrow Throwables within * Runnable.run, we wrap them within Errors on the way out (to the * thread's UncaughtExceptionHandler). Any thrown exception also * conservatively causes thread to die. * * 5. After task.run completes, we call afterExecute, which may * also throw an exception, which will also cause thread to * die. According to JLS Sec 14.20, this exception is the one that * will be in effect even if task.run throws. * * The net effect of the exception mechanics is that afterExecute * and the thread's UncaughtExceptionHandler have as accurate * information as we can provide about any problems encountered by * user code. * * @param w the worker */ final void runWorker(Worker w) { Runnable task = w.firstTask; w.firstTask = null; //标识线程是不是异常终止的 boolean completedAbruptly = true; try { //task不为null情况是初始化worker时，如果task为null，则去队列中取线程--->getTask() while (task != null || (task = getTask()) != null) { w.lock(); //获取woker的锁，防止线程被其他线程中断 clearInterruptsForTaskRun();//清楚所有中断标记 try { beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现 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);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现 } } finally { task = null;//运行过的task标null w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { //处理worker退出的逻辑 processWorkerExit(w, completedAbruptly); } }从上面代码可以看出，execute的Task是被“包装 ”了一层，线程启动时是内部调用了Task的run方法。
接下来所有的核心集中在getTask()方法上：/** * Performs blocking or timed wait for a task, depending on * current configuration settings, or returns null if this worker * must exit because of any of: * 1. There are more than maximumPoolSize workers (due to * a call to setMaximumPoolSize). * 2. The pool is stopped. * 3. The pool is shutdown and the queue is empty. * 4. This worker timed out waiting for a task, and timed-out * workers are subject to termination (that is, * {@code allowCoreThreadTimeOut || workerCount > corePoolSize}) * both before and after the timed wait. * * @return task, or null if the worker must exit, in which case * workerCount is decremented * * * 队列中获取线程 */ private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. //当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } boolean timed; // Are workers subject to culling? for (;;) { int wc = workerCountOf(c); timed = allowCoreThreadTimeOut || wc > corePoolSize; if (wc <= maximumPoolSize && ! (timedOut && timed)) break; if (compareAndDecrementWorkerCount(c)) return null; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }这段代码十分关键，首先看几个局部变量：
boolean timedOut = false;
主要是判断后面的poll是否要超时
boolean timed;
主要是标识着当前Worker超时是否要退出。wc > corePoolSize时需要减小空闲的Worker数，那么timed为true，但是wc <= corePoolSize时，不能减小核心线程数timed为false。
timedOut初始为false，如果timed为true那么使用poll取线程。如果正常返回，那么返回取到的task。如果超时，证明worker空闲，同时worker超过了corePoolSize，需要删除。返回r=null。则 timedOut = true。此时循环到wc <= maximumPoolSize && ! (timedOut && timed)时，减小worker数，并返回null，导致worker退出。如果线程数<= corePoolSize，那么此时调用 workQueue.take()，没有线程获取到时将一直阻塞，知道获取到线程或者中断，关于中断后面Shutdown的时候会说。至此线程执行过程就分析完了~~~~
关于终止线程池
我个人认为，如果想了解明白线程池，那么就一定要理解好各个状态之间的转换，想理解转换，线程池的终止机制是很好的一个途径。对于关闭线程池主要有两个方法shutdown()和shutdownNow():
首先从shutdown()方法开始：/** * Initiates an orderly shutdown in which previously submitted * tasks are executed, but no new tasks will be accepted. * Invocation has no additional effect if already shut down. * * <p>This method does not wait for previously submitted tasks to * complete execution. Use {@link #awaitTermination awaitTermination} * to do that. * * @throws SecurityException {@inheritDoc} */ public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //判断是否可以操作目标线程 checkShutdownAccess(); //设置线程池状态为SHUTDOWN,此处之后，线程池中不会增加新Task advanceRunState(SHUTDOWN); //中断所有的空闲线程 interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } //转到Terminate tryTerminate(); }shutdown做了几件事：
1. 检查是否能操作目标线程
2. 将线程池状态转为SHUTDOWN
3. 中断所有空闲线程
这里就引发了一个问题，什么是空闲线程？
这需要接着看看interruptIdleWorkers是怎么回事。private void interruptIdleWorkers(boolean onlyOne) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); //这里的意图很简单，遍历workers 对所有worker做中断处理。 // w.tryLock()对Worker加锁，这保证了正在运行执行Task的Worker不会被中断，那么能中断哪些线程呢？ 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(); } }这里主要是为了中断worker，但是中断之前需要先获取锁，这就意味着正在运行的Worker不能中断。但是上面的代码有w.tryLock()，那么获取不到锁就不会中断，shutdown的Interrupt只是对所有的空闲Worker（正在从workQueue中取Task，此时Worker没有加锁）发送中断信号。
while (task != null || (task = getTask()) != null) { w.lock(); //获取woker的锁，防止线程被其他线程中断 clearInterruptsForTaskRun();//清楚所有中断标记 try { beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现 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);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现 } } finally { task = null;//运行过的task标null w.completedTasks++; w.unlock(); } }在runWorker中，每一个Worker getTask成功之后都要获取Worker的锁之后运行，也就是说运行中的Worker不会中断。因为核心线程一般在空闲的时候会一直阻塞在获取Task上，也只有中断才可能导致其退出。这些阻塞着的Worker就是空闲的线程（当然，非核心线程，并且阻塞的也是空闲线程）。在getTask方法中：
private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. //当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } boolean timed; // Are workers subject to culling? for (;;) { //allowCoreThreadTimeOu是判断CoreThread是否会超时的，true为会超时，false不会超时。默认为false int wc = workerCountOf(c); timed = allowCoreThreadTimeOut || wc > corePoolSize; if (wc <= maximumPoolSize && ! (timedOut && timed)) break; if (compareAndDecrementWorkerCount(c)) return null; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }会有两阶段的Worker：
- 刚进入getTask()，还没进行状态判断。
- block在poll或者take上的Worker。
当调用ShutDown方法时，首先设置了线程池的状态为ShutDown，此时1阶段的worker进入到状态判断时会返回null，此时Worker退出。
因为getTask的时候是不加锁的，所以在shutdown时可以调用worker.Interrupt.此时会中断退出，Loop到状态判断时，同时workQueue为empty。那么抛出中断异常，导致重新Loop，在检测线程池状态时，Worker退出。如果workQueue不为null就不会退出，此处有些疑问，因为没有看见中断标志位清除的逻辑，那么这里就会不停的循环直到workQueue为Empty退出。
这里也能看出来SHUTDOWN只是清除一些空闲Worker，并且拒绝新Task加入，对于workQueue中的线程还是继续处理的。
对于shutdown中获取mainLock而addWorker中也做了mainLock的获取，这么做主要是因为Works是HashSet类型的，是线程不安全的，我们也看到在addWorker后面也是对线程池状态做了判断，将Worker添加和中断逻辑分离开。
接下来做了tryTerminate()操作，这操作是进行了后面状态的转换，在shutdownNow后面说。
接下来看看shutdownNow：/** * Attempts to stop all actively executing tasks, halts the * processing of waiting tasks, and returns a list of the tasks * that were awaiting execution. These tasks are drained (removed) * from the task queue upon return from this method. * * <p>This method does not wait for actively executing tasks to * terminate. Use {@link #awaitTermination awaitTermination} to * do that. * * <p>There are no guarantees beyond best-effort attempts to stop * processing actively executing tasks. This implementation * cancels tasks via {@link Thread#interrupt}, so any task that * fails to respond to interrupts may never terminate. * * @throws SecurityException {@inheritDoc} */ 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; }shutdownNow和shutdown代码类似，但是实现却很不相同。首先是设置线程池状态为STOP，前面的代码我们可以看到，是对SHUTDOWN有一些额外的判断逻辑，但是对于>=STOP,基本都是reject，STOP也是比SHUTDOWN更加严格的一种状态。此时不会有新Worker加入，所有刚执行完一个线程后去GetTask的Worker都会退出。
之后调用interruptWorkers：/** * Interrupts all threads, even if active. Ignores SecurityExceptions * (in which case some threads may remain uninterrupted). */ private void interruptWorkers() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { for (Worker w : workers) { try { w.thread.interrupt(); } catch (SecurityException ignore) { } } } finally { mainLock.unlock(); } }这里可以看出来，此方法目的是中断所有的Worker，而不是像shutdown中那样只中断空闲线程。这样体现了STOP的特点，中断所有线程，同时workQueue中的Task也不会执行了。所以接下来drainQueue：
/** * Drains the task queue into a new list, normally using * drainTo. But if the queue is a DelayQueue or any other kind of * queue for which poll or drainTo may fail to remove some * elements, it deletes them one by one. */ private List<Runnable> drainQueue() { BlockingQueue<Runnable> q = workQueue; List<Runnable> taskList = new ArrayList<Runnable>(); q.drainTo(taskList); if (!q.isEmpty()) { for (Runnable r : q.toArray(new Runnable[0])) { if (q.remove(r)) taskList.add(r); } } return taskList; }获取所有没有执行的Task，并且返回。
这也体现了STOP的特点：
拒绝所有新Task的加入，同时中断所有线程，WorkerQueue中没有执行的线程全部抛弃。所以此时Pool是空的，WorkerQueue也是空的。
这之后就是进行到TIDYING和TERMINATED的转化了：/** * Transitions to TERMINATED state if either (SHUTDOWN and pool * and queue empty) or (STOP and pool empty). If otherwise * eligible to terminate but workerCount is nonzero, interrupts an * idle worker to ensure that shutdown signals propagate. This * method must be called following any action that might make * termination possible -- reducing worker count or removing tasks * from the queue during shutdown. The method is non-private to * allow access from ScheduledThreadPoolExecutor. */ final void tryTerminate() { for (;;) { int c = ctl.get(); if (isRunning(c) || runStateAtLeast(c, TIDYING) || (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty())) return; if (workerCountOf(c) != 0) { // Eligible to terminate interruptIdleWorkers(ONLY_ONE); return; } final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) { try { terminated(); } finally { ctl.set(ctlOf(TERMINATED, 0)); termination.signalAll(); } return; } } finally { mainLock.unlock(); } // else retry on failed CAS } }上面的代码其实很有意思有几种状态是不能转化到TIDYING的:
- RUNNING状态
- TIDYING或TERMINATED
- SHUTDOWN状态，但是workQueue不为空
也说明了两点：
1. SHUTDOWN想转化为TIDYING，需要workQueue为空，同时workerCount为0。
2. STOP转化为TIDYING，需要workerCount为0
如果满足上面的条件（一般一定时间后都会满足的），那么CAS成TIDYING，TIDYING也只是个过度状态，最终会转化为TERMINATED。至此，ThreadPoolExecutor一些核心思想就介绍完了，想分析清楚实在是不容易，对于ThreadPoolExecutor我还是有些不懂地方，以上只是我对源码的片面的见解，如果有不正确之处，希望大神能不吝赐教。同时也希望给正在研究ThreadPoolExecutor的童鞋提供一点帮助。
勿忘初心，方得始终。晚安~~
作者：一只小哈
链接：http://www.jianshu.com/p/ade771d2c9c0
來源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
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