[Java 并发] ThreadPoolExecutor 是如何做到线程复用的

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
		
    int c = ctl.get();
    // 当前线程数小于 corePoolSize 时,调用 addWorker 创建核心线程来执行任务
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 当前线程数不小于 corePoolSize ,就将任务添加到 workQueue 中
    if (isRunning(c) && workQueue.offer(command)) {
    	// 获取到当前线程的状态,赋值给 recheck ,是为了重新检查状态
        int recheck = ctl.get();
        // 如果 isRunning 返回 false ,那就 remove 掉这个任务,然后执行拒绝策略,也就是回滚重新排队
        if (! isRunning(recheck) && remove(command))
            reject(command);
        // 线程池处于 running 状态,但是没有线程,那就创建线程执行任务
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 如果放入 workQueue 失败,尝试通过创建非核心线程来执行任务
    // 如果还是失败,说明线程池已经关闭或者已经饱和,会拒绝执行该任务
    else if (!addWorker(command, false))
        reject(command);
}

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
		// 线程池状态 >= SHUTDOWN 时,不再接受新的任务,直接返回 false
		// 如果 rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty() 同样不接受新的任务,返回 false
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
                firstTask == null &&
                ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
			// wc >= CAPACITY 说明线程数不够,所以就返回 false
			// wc >= (core ? corePoolSize : maximumPoolSize) 是在做判断
				// 如果 core 为 true ,说明要创建的线程是核心线程,接下来判断 wc 是否大于 核心线程数 ,如果大于返回 false
				// 如果 core 为 false ,说明要创建的线程是非核心线程,接下来判断 wc 是否大于 最大线程数 ,如果大于返回 false
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
			// CAS 操作增加 workerCount 的值,如果成功跳出循环
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
			// 判断线程池状态有没有变化,如果有变化,则重试
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }

	// workerCount 增加成功之后开始走下面的代码
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
		// 创建一个 worker 对象
        w = new Worker(firstTask);
		// 实例化一个 Thread 对象
        final Thread t = w.thread;
        if (t != null) {
			// 接下来的操作需要加锁进行
            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();
					// 将任务线程添加到线程池中
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
				// 启动任务线程,开始执行任务
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
			// 如果任务线程启动失败调用 addWorkerFailed 
			// addWorkerFailed 方法里面主要做了两件事:将该线程从线程池中移除;将 workerCount 的值减 1
            addWorkerFailed(w);
    }
    return workerStarted;
}

private final HashSet<Worker> workers = new HashSet<Worker>();

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;

       // 处理任务的线程
       final Thread thread;
       // worker 传入的任务
       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) {
       	// 将 state 设为 -1 ,避免 worker 在执行前被中断
           setState(-1); // inhibit interrupts until runWorker
           this.firstTask = firstTask;
		// 创建一个线程,来执行任务
           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) {
               }
           }
       }
   }

final void runWorker(Worker w) {
       Thread wt = Thread.currentThread();
       Runnable task = w.firstTask;
       w.firstTask = null;
       // 允许中断
       w.unlock(); // allow interrupts
       boolean completedAbruptly = true;
       try {
       	// 判断 task 是否为空,如果不为空直接执行
       	// 如果 task 为空,调用 getTask() 方法,从 workQueue 中取出新的 task 执行
           while (task != null || (task = getTask()) != null) {
           	// 加锁,防止被其他线程中断
               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
               // 检查线程池的状态,如果线程池处于 stop 状态,则需要中断当前线程
               if ((runStateAtLeast(ctl.get(), STOP) ||
                    (Thread.interrupted() &&
                     runStateAtLeast(ctl.get(), STOP))) &&
                   !wt.isInterrupted())
                   wt.interrupt();
               try {
               	// 执行 beforeExecute 
                   beforeExecute(wt, task);
                   Throwable thrown = null;
                   try {
                   	// 执行任务
                       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 {
                   	// 执行 afterExecute 方法
                       afterExecute(task, thrown);
                   }
               } finally {
               	// 将 task 设置为 null ,循环操作
                   task = null;
                   w.completedTasks++;
                   // 释放锁
                   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);

           // Are workers subject to culling?
           // allowCoreThreadTimeOut 变量默认为 false ,也就是核心线程就算是空闲也不会被销毁
           // 如果为 true ,核心线程在 keepAliveTime 内是空闲的,就会被销毁
           boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

           // 如果运行线程数大于最大线程数,但是缓存队列已经空了,此时递减 worker 数量
           // 如果有设置允许线程超时或者线程数量超过了核心线程数量,并且线程在规定时间内没有 poll 到任务并且队列为空,此时也递减 worker 数量
           if ((wc > maximumPoolSize || (timed && timedOut))
               && (wc > 1 || workQueue.isEmpty())) {
               if (compareAndDecrementWorkerCount(c))
                   return null;
               continue;
           }

           try {
               // 如果 timed 为 true ,会调用 workQueue 的 poll 方法
               	// 超时时间为 keepAliveTime ,如果超过 keepAliveTime 时长的话, poll 就会返回 null 
               	// 如果返回为 null ,在 runWorker 中 
               	// while (task != null || (task = getTask()) != null) 循环条件被打破,从而跳出循环,此时线程执行完毕
               // 如果 timed 为 false ( allowCoreThreadTimeOut 为 false ,并且 wc > corePoolSize 为 false )
               	// 会调用 workQueue 的 take 方法阻塞到当前
               	// 当队列中有任务加入时,线程被唤醒, take 方法返回任务,开始执行
               Runnable r = timed ?
                   workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                   workQueue.take();
               if (r != null)
                   return r;
               timedOut = true;
           } catch (InterruptedException retry) {
               timedOut = false;
           }
       }
   }