Java 线程池学习

 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();
        // 1 worker 数量小于核心线程
        if (workerCountOf(c) < corePoolSize) {
            // 尝试添加，成功就返回，否则继续
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        // 判断还没有挂壁那么就往阻塞队列塞数据
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            // 再次 check 一边，没有运行且移除掉这个 task 之后 触发 reject
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0) // 如果此时 worker 为 0 了，创建一个新的 worker， 这里 task 为 null 也是允许的只是说没有第一个任务而已，因为上边已经加入任务到队列了，这里在 runWorker 的时候还会去队列里面取数据，这里如果不为空会重复执行 task
                addWorker(null, false);
        }
        // 最终把这个 task 添加到非核心线程中，这里如果 worker count 大于等于 maximumPoolSize 就会触发拒绝
        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.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                // 我把 BlockingQueue 的 size 设置的比较小的时候，这里极容易发生 reject 原因是 wc 大于等于 maximumPoolSize
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                // 这里的 break retry 就直接不会进到 for 循环中，c 经过 cas 之后 +1 成功就会推出这个双重循环
                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) {
                // 这里是共享一把可重入锁，因为要操作 workers
                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();
                        // 这里就算添加 worker 到列表了
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    // 这里启动 worker 中的 thread
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            // add 失败会减一并且尝试清除任务，并且触发终止线程池操作，很多信号会调用 tryTerminate ，他的名字是 try 不一定立刻终止，因为有任务在的话还是会保证任务执行完毕的
            if (! workerStarted)
                addWorkerFailed(w);
        }
        // 添加成功返回 true
        return workerStarted;
    }


    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;

        /** Thread this worker is running in.  Null if factory fails. */
        final Thread thread;
        // 可空的第一个 task ，这里为空在 runWorker 的时候会从 getTask 获取
        /** Initial task to run.  Possibly null. */
        Runnable firstTask;
        // 保存每个 worker 完成的 task 数量
        /** 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) {
            // 这里自己先加锁
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        /** Delegates main run loop to outer runWorker. */
        public void run() {
            // 这里会释放锁先，为了可以被打断
            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) {
        // 这里为啥取当前 Thread 而不是拿 worker 的 Thread 呢，是因为调用到这里的时候必然在 worker 里面的 thread 执行的，因此这里和拿 worker thread 效果一样
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            // 这里 task 如果为空，那么就通过 getTask 去从队列里面取
            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 {
                        // 执行拿到的 task 
                        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;
                    // worker 的执行成功加一
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            // 对 worker 的收尾
            processWorkerExit(w, completedAbruptly);
        }
    }