ThreadPoolExecutor源码解析

Doug Lea大神的设计思路

1、线程池状态跟线程数合并到一个32位的int变量中，头三位为状态，剩下的为worker数。
2、线程封装成Worker对象，启动线程的时候优先执行这个任务firstTask。小于设置的核心线程数量或者最大线程数量则添加worker，否则丢到任务队列中，
3、默认Worker是不会终止的，一直循环去任务队列中取任务。

核心代码

成员变量

// * RUNNING<SHUTDOWN<STOP<TIDYING<TERMINATED
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    //worker数量使用的位数，29位
    private static final int COUNT_BITS = Integer.SIZE - 3;
    // worker数量，2的29次方个 29个1，最高三位为状态位
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    //接受新任务和执行队列任务            11100000_00000000_00000000_00000000
    private static final int RUNNING    = -1 << COUNT_BITS;
    //不接受新任务但执行队列任务            0
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    //不接受新任务不执行队列任务，中断执行中的任务     00100000_00000000_00000000_00000000
    private static final int STOP       =  1 << COUNT_BITS;
    //worker清零，将会调用terminated方法     01000000_00000000_00000000_00000000
    private static final int TIDYING    =  2 << COUNT_BITS;
    //terminated方法执行完毕                 01100000_00000000_00000000_00000000
    private static final int TERMINATED =  3 << COUNT_BITS;

    // Packing and unpacking ctl
    //~CAPACITY = 高位3个1，剩下全为0，与运算算出当前状态
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    //29个1与运算算出当前的worker数量
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    //状态与数量合并
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    
    //运行状态小于比较状态
    private static boolean runStateLessThan(int c, int s) {
        return c < s;
    }
    //至少是某个状态
    private static boolean runStateAtLeast(int c, int s) {
        return c >= s;
    }
    //线程池是否是运行状态，小于SHUTDOWN状态的只有RUNNING了
    private static boolean isRunning(int c) {
        return c < SHUTDOWN;
    }

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        //worker数目小于设置的数量
        if (workerCountOf(c) < corePoolSize) {
            //小于coresize增加worker，
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //RUNNING状态,添加任务到工作队列，所有worker都会从任务队列中取任务
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //再次检查，不是RUNNING状态，不再接受新任务
            if (! isRunning(recheck) && remove(command))
                reject(command);//执行 RejectedExecutionHandler 设置的reject方法
            else if (workerCountOf(recheck) == 0)//worker数为0，小于maxsize添加worker
                addWorker(null, false);
        }
        else if (!addWorker(command, false))//根据maxsize添加worker,失败则执行reject策略
            reject(command);
    }

 private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            //大于等于SHUTDOWN 状态 并且 如果是SHUTDOWN状态，且正在执行中的任务是空，待执行任务队列是空，返回false
            if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                //超过允许的最大容量或者设置的容量，返回false
                if (wc >= CAPACITY ||
                        wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //增加worker数量
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                //检查线程池状态不一致，返回retry重试
                if (runStateOf(c) != rs)
                    continue retry;
            }
        }

        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 {
                    int rs = runStateOf(ctl.get());
                    //RUNNING状态或者 是 SHUTDOWN状态，并且要执行的任务是空
                    if (rs < SHUTDOWN ||
                            (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) //线程已经启动了抛出异常
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        //设置线程池中的当前线程数
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    //线程已经启动
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

    private void addWorkerFailed(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null)
                workers.remove(w);
            //减少线程数
            decrementWorkerCount();
            tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

  final void tryTerminate() {
        for (;;) {
            int c = ctl.get();
            //运行中 或者 是处于TIDYING不是TERMINATED 状态 或者 是SHUTDOWN状态，但是待执行任务没执行完，不关闭线程池
            if (isRunning(c) ||
                    runStateAtLeast(c, TIDYING) ||
                    (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
                return;
            // worker不为0，尝试1次唤醒空闲线程
            if (workerCountOf(c) != 0) {
                interruptIdleWorkers(ONLY_ONE);
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        //TIDYING状态执行terminated方法
                        terminated();
                    } finally {
                        ctl.set(ctlOf(TERMINATED, 0));
                        termination.signalAll();
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

 final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            //任务不为空，从队列里面继续取任务
            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 {
                        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;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }