站在架构的角度思考线程池的设计和原理

new Thread().start();

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
     // 判断当前工作线程数是否小于核心线程数（延迟初始化）
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))  //添加工作线程的同时，执行command
                return;
            c = ctl.get();
        }
     // workQueue.offer 添加到阻塞队列
        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);
        }
     // 如果阻塞队列满了，则添加工作线程（扩容的线程）
        else if (!addWorker(command, false))
            reject(command);  //拒绝策略
    }

 private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
      // 自旋，通过原子操作 cas 来增加活跃线程数量
        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;
                // CAS 操作更新活跃线程数，通过原子操作来增加线程数量
                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
            }
        }

        
     // 初始化工作线程
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            // 构建一个工作线程,此时还没有启动.
            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); //添加到一个容器中。
                        int s = workers.size();
                        if (s > largestPoolSize) 
                            largestPoolSize = s; // 重新更新largestPoolSize
                        workerAdded = true; //添加成功
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start(); //启动线程
                    workerStarted = true;
                }
            }
        } finally { //失败， 回滚。
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

 private final class Worker
        extends AbstractQueuedSynchronizer //Worker继承了AQS -> 实现了互斥锁
        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;
        /** Initial task to run.  Possibly null. */
        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) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            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) {
                }
            }
        }
    }

public void run() {
    runWorker(this);
}

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 循环保证当前线程不结束，直到task为null
            while (task != null || (task = getTask()) != null) {
                //表示当前线程正在运行一个任务，如果其他地方要 shutdown().你必须要等我执行完成。
                w.lock(); //Worker继承了AQS -> 实现了互斥锁
                // 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(); //是否应该触发中断
                try {
                    beforeExecute(wt, task); //空的实现
                    Throwable thrown = null;
                    try {
                        task.run(); //执行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); ; //空的实现
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

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

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

            // 如果线程池已经结束状态 或是任务队列为空，直接返回null. 需要清理掉所有的工作线程
            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);
            // 是否允许超时（allowCoreThreadTimeOut=true） 或 如果当前的工作线程数量大于核心线程数
            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c)) //cas 减少工作线程数量
                    return null; //表示要销毁当前工作线程.
                continue;
            }
            
            //获取任务的过程
            try {
                Runnable r = timed ?
                    // 如果允许超时或是工作线程大于核心线程数，则尝试取出元素，否则等待线程存活时间（keepAliveTime）后取出
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();  //工作线程小于核心线程数 且 队列为空，直接取出，如果阻塞队列没有任务，当前工作线程会阻塞在这里
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) { //中断异常
                timedOut = false;
            }
        }
    }

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

   public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

  public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

public ThreadPoolExecutor(int corePoolSize,//核心线程数
                              int maximumPoolSize,//最大线程数
                              long keepAliveTime,//存活时间
                              TimeUnit unit,//存活时间单位
                              BlockingQueue<Runnable> workQueue,//阻塞队列
                              ThreadFactory threadFactory,//线程工厂，用来创建工作线程的。 默认实现（自定义线程池中线程的名字）
                              RejectedExecutionHandler handler) {//拒绝执行策略 。默认实现
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

/**
 * Create the BlockingQueue to use for the ThreadPoolExecutor.
 * <p>A LinkedBlockingQueue instance will be created for a positive
 * capacity value; a SynchronousQueue else.
 * @param queueCapacity the specified queue capacity
 * @return the BlockingQueue instance
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.SynchronousQueue
 */
protected BlockingQueue<Runnable> createQueue(int queueCapacity) {
  if (queueCapacity > 0) {
    return new LinkedBlockingQueue<>(queueCapacity);
  }
  else {
    return new SynchronousQueue<>();
  }
}

//java.util.concurrent.LinkedBlockingQueue

/**
 * Inserts the specified element at the tail of this queue, waiting if
 * necessary for space to become available.
 *
 * @throws InterruptedException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
public void put(E e) throws InterruptedException {
    if (e == null) throw new NullPointerException();
    // Note: convention in all put/take/etc is to preset local var
    // holding count negative to indicate failure unless set.
    int c = -1;
    Node<E> node = new Node<E>(e);
    final ReentrantLock putLock = this.putLock;
    final AtomicInteger count = this.count;
    putLock.lockInterruptibly();
    try {
        /*
         * Note that count is used in wait guard even though it is
         * not protected by lock. This works because count can
         * only decrease at this point (all other puts are shut
         * out by lock), and we (or some other waiting put) are
         * signalled if it ever changes from capacity. Similarly
         * for all other uses of count in other wait guards.
         */
        while (count.get() == capacity) {
            notFull.await();
        }
        enqueue(node);
        c = count.getAndIncrement();
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        putLock.unlock();
    }
    if (c == 0)
        signalNotEmpty();
}