深入理解Java线程池ThreadPoolExcutor实现原理、数据结构和算法（源码解析）

public class ThreadPoolExecutor extends AbstractExecutorService {
...
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int COUNT_MASK = (1 << COUNT_BITS) - 1;
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;
    private final BlockingQueue<Runnable> workQueue;
    private final ReentrantLock mainLock = new ReentrantLock();
    private final HashSet<Worker> workers = new HashSet<>();
    private final Condition termination = mainLock.newCondition();
    private int largestPoolSize;
    private long completedTaskCount;
    private volatile ThreadFactory threadFactory;
    private volatile RejectedExecutionHandler handler;
    private volatile long keepAliveTime;
    private volatile boolean allowCoreThreadTimeOut;
    private volatile int corePoolSize;
    private volatile int maximumPoolSize;
...
}

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    ...
}

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
    
   
    int c = ctl.get();
    //通过位运算，获取当前线程数，如果线程数小于corePoolSize，则执行addWorker(),创建新线程执行任务
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        // 若添加工作任务执行任务不成功，尝试添加到阻塞队列中
        c = ctl.get();
    }
    // 线程数超过核心线程数，任务添加到阻塞队列中
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // 添加到阻塞队列，但若线程池状态不是运行中，则从队列中取出
        if (! isRunning(recheck) && remove(command))
            // 并且执行拒绝策略
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 阻塞队列已满，则若工作线程数小于最大线程数 创建线程执行任务
    else if (!addWorker(command, false))
        // 若线程数已达最大线程数，否则执行拒绝策略
        reject(command);
}

private final class Worker extends AbstractQueuedSynchronizer implements Runnable {
    private static final long serialVersionUID = 6138294804551838833L;
    //当前Worker所处于的线程
    final Thread thread;
    //待执行的任务
    Runnable firstTask;
    //任务计数器
    volatile long completedTasks;
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }
}

private boolean addWorker(Runnable firstTask, boolean core) {
    // 自旋，判断线程池状态，并对线程数量执行原子+1操作
    retry:
    for (;;) {
        int c = ctl.get();
        // 获取线程池状态
        int rs = runStateOf(c);
        // 如果线程池已经关闭，则直接返回false
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;
        for (;;) {
            // 判断线程数是否已达上限，根据传入参数core的不同，判断corePoolSize或者maximumPoolSize。
            // 如果线程数已达上限，直接返回false
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // 执行原子操作，对线程数+1
            if (compareAndIncrementWorkerCount(c))
                // 执行原子操作成功，则退出自旋 并 创建工作线程执行任务
                break retry;
            c = ctl.get();  
            if (runStateOf(c) != rs)
                continue retry;
        }
    }
    // ctl变量操作成功，执行Worker相关逻辑
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        // 创建一个新的Worker，传入待执行的任务
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // 加锁后，再次判断线程池状态
                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);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            // 如果创建了新的Worker，则调用其start方法立即执行
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    // 指向线程池调execute添加的任务
    Runnable task = w.firstTask;
    w.firstTask = null;
    // 首先释放锁，允许中断
    w.unlock(); 
    boolean completedAbruptly = true;
    
    try {
        // 从worker中取第1个任务，若任务为空则从阻塞队列中取任务，直到返回null，这里达到线程复用的效果，实现线程处理多个任务。
        while (task != null || (task = getTask()) != null) {
            // 执行任务前先加锁
            w.lock();
            // 如果线程池已经终止，则中断该线程。保存了线程池在STOP状态下线程中断的，非STOP状态下线程没有被中断
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                // 记录正在运行的任务
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    // 执行任务（调任务的run方法）
                    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 {
                    // 执行完任务，清除记录信息
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                // 当前Worker计数器+1，统计worker执行了多少任务，最后累加进completedTaskCount变量，可以调用相应方法返回一些统计信息。
                w.completedTasks++;
                // 释放锁
                w.unlock();
            }
        }
        // 表示worker是否异常终止，执行到这里代表执行正常，后续的方法需要这个变量
        completedAbruptly = false;
    } finally {
        // completedTasks累加到completedTaskCount变量中
        processWorkerExit(w, completedAbruptly);
    }
}

private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }
        int wc = workerCountOf(c);
        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
        if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }
        try {
            Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

private void processWorkerExit(ThreadPoolExecutor.Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        decrementWorkerCount();
    final ReentrantLock mainLock = this.mainLock;
    
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks;
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }
   
    tryTerminate();
    int c = ctl.get();
    // 判断状态是否小于STOP
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
        addWorker(null, false);
    }
}

public void shutdown() {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        advanceRunState(SHUTDOWN);
        interruptIdleWorkers();
        onShutdown(); // hook for ScheduledThreadPoolExecutor
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
}

public List<Runnable> shutdownNow() {
    List<Runnable> tasks;
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        advanceRunState(STOP);
        interruptWorkers();
        tasks = drainQueue();
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
    return tasks;
}

final void tryTerminate() {
    for (;;) {
        int c = ctl.get();
        if (isRunning(c) ||
                runStateAtLeast(c, TIDYING) ||
                (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
            return;
        if (workerCountOf(c) != 0) { // Eligible to terminate
            interruptIdleWorkers(ONLY_ONE);
            return;
        }
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                try {
                    terminated();
                } finally {
                    ctl.set(ctlOf(TERMINATED, 0));
                    termination.signalAll();
                }
                return;
            }
        } finally {
            mainLock.unlock();
        }
        // else retry on failed CAS
    }
}

public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        for (;;) {
            if (runStateAtLeast(ctl.get(), TERMINATED))
                return true;
            if (nanos <= 0)
                return false;
            nanos = termination.awaitNanos(nanos);
        }
    } finally {
        mainLock.unlock();
    }
}