ThreadPoolExecutor浅谈

在了解线程池之前，希望你能够理解 Java 内存模型 和 AQS CAS

     /**                    
     * The runState provides the main lifecycle 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:
     *
     * 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 terminated()
     */
    // 前三位表示运行状态，后面存储当前运行 workerCount
    private static final int COUNT_BITS = Integer.SIZE - 3; // 32 - 3
    //  最大容量
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1; // 00011111111111111111111111111111
    /**
     * Maximum pool size. Note that the actual maximum is internally
     * bounded by CAPACITY. 实际线程池大小还是由 CAPACITY 决定
     */
    private volatile int maximumPoolSize;
    // 线程池的几个状态
    // 接受新的任务
    private static final int RUNNING    = -1 << COUNT_BITS; // 11100000000000000000000000000000
    // 不接受新的任务，但是已在队列中的任务，还会继续处理
    private static final int SHUTDOWN   =  0 << COUNT_BITS; // 00000000000000000000000000000000
    // 不接受，不处理新的任务，且中断正在进行中的任务
    private static final int STOP       =  1 << COUNT_BITS; // 00100000000000000000000000000000
    // 所有任务已停止，workerCount 清零，注意 workerCount 是由 workerCountOf(int c) 计算得出的
    private static final int TIDYING    =  2 << COUNT_BITS; // 01000000000000000000000000000000
    // 所有任务已完成
    private static final int TERMINATED =  3 << COUNT_BITS; // 01100000000000000000000000000000
    // 线程池运行状态和已工作的 workerCount 初始化为 RUNNING
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    
    // 计算当前 state
    // ~CAPACITY 为 11100000000000000000000000000000 & c（假如前三位为 000 说明线程池已经 SHUTDOWN）
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    // 同时拿到 state workerCount
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    // 可以计算出当前工作的 workerCount
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    
    public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
       
            // 判断是否满足加入核心线程
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                // 以核心线程的方式加入队列
                if (addWorker(command, true))
                    return;
                // 添加失败 获取最新的线程池状态和 size 
                c = ctl.get();
            }
            // 在运行且成功加入队列
            if (isRunning(c) && workQueue.offer(command)) {
                int recheck = ctl.get();
                // 再检查一次，不在运行就拒绝任务
                if (!isRunning(recheck) && remove(command))
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    // 加入一个 null
                    addWorker(null, false);
            }
            // 加入失败就拒绝任务
            else if (!addWorker(command, false))
                reject(command);
        }
    
    private boolean addWorker(Runnable firstTask, boolean core) {
            retry:
            for (;;) {
                // 获得当前状态和 size
                int c = ctl.get();
                int rs = runStateOf(c);
    
                // 大于 SHUTDOWN 即 STOP TIDYING TERMINATED
                // Check if queue empty only if necessary.
                if (rs >= SHUTDOWN &&
                    ! (rs == SHUTDOWN &&
                       firstTask == null &&
                       ! workQueue.isEmpty()))
                    return false;
    
                for (;;) {
                    // 计算 workerCount
                    int wc = workerCountOf(c);
                    if (wc >= CAPACITY ||
                        wc >= (core ? corePoolSize : maximumPoolSize))
                        return false;
                    // 成功了就退出自旋
                    if (compareAndIncrementWorkerCount(c))
                        break retry;
                    c = ctl.get();  // Re-read ctl
                    if (runStateOf(c) != rs)
                        // 走到这一步说明 rs 为 RUNNING 或 SHUTDOWN 可以重新尝试加入
                        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());
    
                        // 异常检查
                        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();
                    }
                    // 添加成功 启动线程
                    if (workerAdded) {
                        t.start();
                        workerStarted = true;
                    }
                }
            } finally {
                // 加入失败 
                if (! workerStarted)
                    addWorkerFailed(w);
            }
            return workerStarted;
        }
        
    // 加入失败 做一些扫尾清理
    private void addWorkerFailed(Worker w) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (w != null)
                    workers.remove(w);
                // workerCount-1
                decrementWorkerCount();
                // 尝试更新状态 何为尝试，即需要满足一定条件，而不是冒然去做某事
                tryTerminate();
            } finally {
                mainLock.unlock();
            }
        }

原文地址：https://www.cnblogs.com/eahau/p/11003102.html