ThreadPoolExecutor浅谈

摘要：耐心看完的你或多或少会有收获的解释在了解线程池之前，希望你已经了解了内存模型和前位表示运行状态，后面位存储当前运行最大容量实际线程池大小还是由决定以下为线程池的几个状态官方注释在最上方接受新的任务不接受新的任务，但是已在

耐心看完的你或多或少会有收获！

在了解线程池之前，希望你已经了解了 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 3 位表示运行状态，后面 29 位存储当前运行 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;
    
    // 以下为线程池的几个状态 官方注释在最上方
    // 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 和 0
    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; }
    
    // 线程入列
    public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
       
            // 获得当前 state 和 workerCount
            // 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 state 和 workerCount
                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);
        }
    
    // 实际的操作
    private boolean addWorker(Runnable firstTask, boolean core) {
            retry:
            for (;;) {
                // 获得当前 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 (;;) {
                    // 计算 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 {
                // 统一线程的名字
                // 设置 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());
    
                        // 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();
            }
        }

写得好的源码，注释一定要好好看一遍

线程池的状态和工作线程数量用 32 位二进制数表示，然后通过二进制的位运算获取状态和数量，这种设计实在是太过精妙

膜拜大师