摘要：耐心看完的你或多或少會有收獲的解釋在了解線程池之前，希望你已經了解了內存模型和前位表示運行狀態，后面位存儲當前運行最大容量實際線程池大小還是由決定以下為線程池的幾個狀態官方注釋在最上方接受新的任務不接受新的任務，但是已在

耐心看完的你或多或少會有收獲！

在了解線程池之前，希望你已經了解了 Java內存模型 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 3 位表示運行狀態，后面 29 位存儲當前運行 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. 實際線程池大小還是由 CAPACITY 決定
     */
    private volatile int maximumPoolSize;
    
    // 以下為線程池的幾個狀態 官方注釋在最上方
    // 接受新的任務
    private static final int RUNNING    = -1 << COUNT_BITS; // 11100000000000000000000000000000
    
    // 不接受新的任務，但是已在隊列中的任務，還會繼續處理
    private static final int SHUTDOWN   =  0 << COUNT_BITS; // 00000000000000000000000000000000
    
    // 不接受，不處理新的任務，且中斷正在進行中的任務
    private static final int STOP       =  1 << COUNT_BITS; // 00100000000000000000000000000000
    
    // 所有任務已停止，workerCount 清零，注意 workerCount 是由 workerCountOf(int c) 計算得出的
    private static final int TIDYING    =  2 << COUNT_BITS; // 01000000000000000000000000000000
    
    // 所有任務已完成
    private static final int TERMINATED =  3 << COUNT_BITS; // 01100000000000000000000000000000
    
    // 線程池運行狀態和已工作的 workerCount 初始化為 RUNNING 和 0
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    
    // 計算當前 state
    // ~CAPACITY 為 11100000000000000000000000000000 & c（假如前三位為 000 說明線程池已經 SHUTDOWN）
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    
    // 同時拿到 state workerCount
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    
    // 可以計算出當前工作的 workerCount
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    
    // 線程入列
    public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
       
            // 獲得當前 state 和 workerCount
            // 判斷是否滿足加入核心線程
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                // 以核心線程的方式加入隊列
                if (addWorker(command, true))
                    return;
                // 添加失敗 獲取最新的線程池 state 和 workerCount
                c = ctl.get();
            }
            // 在運行且成功加入隊列
            if (isRunning(c) && workQueue.offer(command)) {
                int recheck = ctl.get();
                // 再檢查一次，不在運行就拒絕任務
                if (!isRunning(recheck) && remove(command))
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    // 加入一個 null
                    addWorker(null, false);
            }
            // 加入失敗就拒絕任務
            else if (!addWorker(command, false))
                reject(command);
        }
    
    // 實際的操作
    private boolean addWorker(Runnable firstTask, boolean core) {
            retry:
            for (;;) {
                // 獲得當前 state 和 workerCount
                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 (;;) {
                    // 計算 workerCount
                    int wc = workerCountOf(c);
                    if (wc >= CAPACITY ||
                        wc >= (core ? corePoolSize : maximumPoolSize))
                        return false;
                    // 成功了就退出
                    if (compareAndIncrementWorkerCount(c))
                        break retry;
                    c = ctl.get();  // Re-read ctl
                    if (runStateOf(c) != rs)
                        // 走到這一步說明 rs 為 RUNNING 或 SHUTDOWN 可以重新嘗試加入
                        continue retry;
                    // else CAS failed due to workerCount change; retry inner loop
                }
            }
    
            boolean workerStarted = false;
            boolean workerAdded = false;
            Worker w = null;
            try {
                // 統一線程的名字
                // 設置 daemon 和 priority
                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());
    
                        // 異常檢查
                        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 啟動線程
                    if (workerAdded) {
                        t.start();
                        workerStarted = true;
                    }
                }
            } finally {
                // 加入失敗 
                if (! workerStarted)
                    addWorkerFailed(w);
            }
            return workerStarted;
        }
        
    // 加入失敗 做一些掃尾清理
    private void addWorkerFailed(Worker w) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (w != null)
                    workers.remove(w);
                // workerCount-1
                decrementWorkerCount();
                // 嘗試更新狀態 何為嘗試，即需要滿足一定條件，而不是冒然去做某事
                tryTerminate();
            } finally {
                mainLock.unlock();
            }
        }

寫得好的源碼，注釋一定要好好看一遍

線程池的狀態和工作線程數量用 32 位二進制數表示，然后通過二進制的位運算獲取狀態和數量，這種設計實在是太過精妙

膜拜大師