转载：ThreadPoolExecutor 源码阅读

前言

　　之前研究了一下如何使用ScheduledThreadPoolExecutor动态创建定时任务(Springboot定时任务原理及如何动态创建定时任务)，简单了解了ScheduledThreadPoolExecutor相关源码。今天看了同学写的ThreadPoolExecutor 的源码解读，甚是NB，必须转发一下。

读了一下 ThreadPoolExecutor 的源码(JDK 11), 简单的做个笔记.

Executor 框架

Executor

Executor 接口只有一个方法:

public interface Executor {
    void execute(Runnable command);
}

Executor 接口提供了一种将任务提交和任务执行机制解耦的方法. Executor 的实现并不须要是异步的.

ExecutorService

ExecutorService 在 Executor 的基础上, 提供了一些管理终止的方法和可以生成 Future 来跟踪一个或多个异步任务的进度的方法:

• shutdown() 方法会启动比较柔和的关闭过程, 并且不会阻塞. ExecutorService 将会继续执行已经提交的任务, 但不会再接受新的任务. 如果 ExecutorService 已经被关闭, 则不会有附加的操作.
• shutdownNow() 方法会尝试停止正在执行的任务, 不再执行等待执行的任务, 并且返回等待执行的任务列表, 不会阻塞. 这个方法只能尝试停止任务, 典型的取消实现是通过中断来取消任务, 因此不能响应中断的任务可能永远不会终止.
• invokeAll() 方法执行给定集合中的所有任务, 当所有任务完成时返回 Future 的列表, 支持中断. 如果在此操作正在进行时修改了给定的集合，则此方法的结果未定义.
• invokeAny() 方法会执行给定集合中的任务, 当有一个任务完成时, 返回这个任务的结果, 并取消其他未完成的任务, 支持中断. 如果在此操作正在进行时修改了给定的集合，则此方法的结果未定义.

AbstractExecutorService

AbstractExecutorService 提供了一些 ExecutorService 的执行方法的默认实现. 这个方法使用了 newTaskFor() 方法返回的 RunnableFuture (默认是 FutureTask ) 来实现 submit() 、invokeAll()、 invokeAny() 方法.

RunnableFuture 继承了 Runnable 和 Future , 在 run() 方法成功执行后, 将会设置完成状态, 并允许获取执行的结果:

public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

FutureTask

FutureTask 实现了 RunnableFuture 接口, 表示一个可取消的计算任务, 只能在任务完成之后获取结果, 并且在任务完成后, 就不再能取消或重启, 除非使用 runAndReset() 方法.

FutureTask 有 7 个状态:

• NEW
• COMPLETING
• NORMAL
• EXCEPTIONAL
• CANCELLED
• INTERRUPTING
• INTERRUPTED

可能的状态转换:

• NEW -> COMPLETING -> NORMAL
• NEW -> COMPLETING -> EXCEPTIONAL
• NEW -> CANCELLED
• NEW -> INTERRUPTING -> INTERRUPTED

FutureTask 在更新 state 、 runner、 waiters 时, 都使用了 VarHandle.compareAndSet() :

// VarHandle mechanics
private static final VarHandle STATE;
private static final VarHandle RUNNER;
private static final VarHandle WAITERS;
static {
    try {
        MethodHandles.Lookup l = MethodHandles.lookup();
        STATE = l.findVarHandle(FutureTask.class, "state", int.class);
        RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
        WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
    } catch (ReflectiveOperationException e) {
        throw new ExceptionInInitializerError(e);
    }

    // Reduce the risk of rare disastrous classloading in first call to
    // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
    Class<?> ensureLoaded = LockSupport.class;
}

protected void set(V v) {
    if (STATE.compareAndSet(this, NEW, COMPLETING)) {
        outcome = v;
        STATE.setRelease(this, NORMAL); // final state
        finishCompletion();
    }
}

来看一下 get() 方法:

public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
    if (unit == null)
        throw new NullPointerException();
    int s = state;
    if (s <= COMPLETING &&
        (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
        throw new TimeoutException();
    return report(s);
}

private int awaitDone(boolean timed, long nanos)
    throws InterruptedException {
    long startTime = 0L;    
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
        int s = state;
        if (s > COMPLETING) {
            // 已经在终结状态, 返回状态
            if (q != null)
                q.thread = null;
            return s;
        }
        else if (s == COMPLETING)
            // 已经完成了, 但是状态还是 COMPLETING
            Thread.yield();
        else if (Thread.interrupted()) {
            // 检查中断
            removeWaiter(q);
            throw new InterruptedException();
        }
        else if (q == null) {
            // 没有创建 WaitNode 节点, 如果 timed 并且 nanos 大于 0, 创建一个 WaitNode
            if (timed && nanos <= 0L)
                return s;
            q = new WaitNode();
        }
        else if (!queued)
            // 将新的 WaitNode 放到链表头部, 并尝试 cas 到 waiters
            queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
        else if (timed) {
            final long parkNanos;
            if (startTime == 0L) { // first time
                startTime = System.nanoTime();
                if (startTime == 0L)
                    startTime = 1L;
                parkNanos = nanos;
            } else {
                long elapsed = System.nanoTime() - startTime;
                if (elapsed >= nanos) {
                    // 超时了
                    removeWaiter(q);
                    return state;
                }
                // park 的时间
                parkNanos = nanos - elapsed;
            }
            // nanos 比较慢, 再次检查, 然后阻塞
            if (state < COMPLETING)
                LockSupport.parkNanos(this, parkNanos);
        }
        else
            // 不需要超时的阻塞
            LockSupport.park(this);
    }
}

再来看下 run() 方法:

public void run() {
    if (state != NEW ||
        !RUNNER.compareAndSet(this, null, Thread.currentThread()))
        // 不在 NEW 状态, 或者 runner 不为 null
        return;
    try {
        // callable 是在构造器中指定的或用 Executors.callable(runnable, result) 创建的
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                // 设置异常状态和异常结果
                setException(ex);
            }
            if (ran)
                // 正常完成, 设置完成状态和结果
                set(result);
        }
    } finally {
        // runner must be non-null until state is settled to
        // prevent concurrent calls to run()
        runner = null;
        // state must be re-read after nulling runner to prevent
        // leaked interrupts
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

protected void set(V v) {
    if (STATE.compareAndSet(this, NEW, COMPLETING)) {
        outcome = v;
        STATE.setRelease(this, NORMAL); // final state
        finishCompletion();
    }
}

private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {
        if (WAITERS.weakCompareAndSet(this, q, null)) {
            // cas 移除 waiters, 对链表中的每个 Node 的线程 unpark
            for (;;) {
                Thread t = q.thread;
                if (t != null) {
                    q.thread = null;
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }
    // 默认实现什么都没做
    done();
    callable = null;        // to reduce footprint
}

AbstractExecutorService 的执行方法

来看下 AbstractExecutorService 实现的几个执行方法, 这里就只放上以 Callable 为参数的方法:

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
    return new FutureTask<T>(callable);
}

public <T> Future<T> submit(Callable<T> task) {
    if (task == null) throw new NullPointerException();
    RunnableFuture<T> ftask = newTaskFor(task);
    execute(ftask);
    return ftask;
}

public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException {
    try {
        return doInvokeAny(tasks, false, 0);
    } catch (TimeoutException cannotHappen) {
        assert false;
        return null;
    }
}

private <T> T doInvokeAny(Collection<? extends Callable<T>> tasks,
                              boolean timed, long nanos)
        throws InterruptedException, ExecutionException, TimeoutException {
  if (tasks == null)
        throw new NullPointerException();
    int ntasks = tasks.size();
    if (ntasks == 0)
        throw new IllegalArgumentException();
    ArrayList<Future<T>> futures = new ArrayList<>(ntasks);
    ExecutorCompletionService<T> ecs =
        new ExecutorCompletionService<T>(this);
    try {
        ExecutionException ee = null;
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        Iterator<? extends Callable<T>> it = tasks.iterator();
        // 提交一个任务到 ecs
        futures.add(ecs.submit(it.next()));
        --ntasks;
        int active = 1;

        for (;;) {
            // 尝试获取第一个完成的任务的 Future
            Future<T> f = ecs.poll();
            if (f == null) {
                // 没有完成的任务
                if (ntasks > 0) {
                    // 还有没提交的任务, 再提交一个到 ecs
                    --ntasks;
                    futures.add(ecs.submit(it.next()));
                    ++active;
                }
                else if (active == 0)
                    // 没有还没提交的任务和正在执行的任务了
                    break;
                else if (timed) {
                    f = ecs.poll(nanos, NANOSECONDS);
                    if (f == null)
                        throw new TimeoutException();
                    nanos = deadline - System.nanoTime();
                }
                else
                    f = ecs.take();
            }
            if (f != null) {
                // 存在已经完成的任务
                --active;
                try {
                    // 获取结果并返回
                    return f.get();
                } catch (ExecutionException eex) {
                    ee = eex;
                } catch (RuntimeException rex) {
                    ee = new ExecutionException(rex);
                }
            }
        }
        
        // 出错, 抛出
        if (ee == null)
            ee = new ExecutionException();
        throw ee;

    } finally {
        // 取消所有已经提交的任务
        cancelAll(futures);
    }
}
    
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException {
    if (tasks == null)
        throw new NullPointerException();
    ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
    try {
        for (Callable<T> t : tasks) {
            // 提交任务
            RunnableFuture<T> f = newTaskFor(t);
            futures.add(f);
            execute(f);
        }
        for (int i = 0, size = futures.size(); i < size; i++) {
            Future<T> f = futures.get(i);
            if (!f.isDone()) {
                // 任务没有完成, get() 等待任务完成
                try { f.get(); }
                catch (CancellationException | ExecutionException ignore) {}
            }
        }
        return futures;
    } catch (Throwable t) {
        cancelAll(futures);
        throw t;
    }
}

构造器

ThreadPoolExecutor 一共有4个构造器, 这里就只放上两个构造器:

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         Executors.defaultThreadFactory(), defaultHandler);
}

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

参数说明:

• corePoolSize: 在线程池中保持的线程的数量, 即使这些线程是空闲的, 除非 allowCoreThreadTimeOut 被设置为 true;
• maximumPoolSize: 线程池中最大线程数量;
• keepAliveTime: 多余空闲线程在终止之前等待新任务的最长时间;
• unit: keepAliveTime 的时间单位;
• workQueue: 任务的等待队列, 用于存放等待执行的任务. 仅包含 execute() 方法提交的 Runnable;
• threadFactory: executor 用来创建线程的工厂, 默认使用 Executors.defaultThreadFactory() 来创建一个新的工厂;
• handler: 任务因为达到了线程边界和队列容量而被阻止时的处理程序, 默认使用 AbortPolicy.

状态

ThreadPoolExecutor 有5个状态:

• RUNNING: 接受新任务, 并且处理队列中的任务;
• SHUTDOWN: 不接受新任务, 但是处理队列中的任务, 此时仍然可能创建新的线程;
• STOP: 不接受新任务, 处理队列中的任务, 中断正在运行的任务;
• TIDYING: 所有的任务都终结了, workCount 的值是0, 将状态转换为 TIDYING 的线程会执行 terminated() 方法;
• TERMINATED: terminated() 方法执行完毕.

状态转换:

• RUNNING -> SHUTDOWN , On invocation of shutdown()
• (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

workCount 和 state 被打包在一个 AtomicInteger 中, 其中的高三位用于表示线程池状态( state ), 低 29 位用于表示 workCount:

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;

// runState is stored in the high-order bits
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;

// Packing and unpacking ctl
private static int runStateOf(int c)     { return c & ~COUNT_MASK; }
private static int workerCountOf(int c)  { return c & COUNT_MASK; }
private static int ctlOf(int rs, int wc) { return rs | wc; }

workCount 表示有效的线程数量, 是允许启动且不允许停止的 worker 的数量, 与实际的线程数量瞬时不同. 用户可见的线程池大小是 Worker 集合的大小.

Worker 与任务调度

工作线程被封装在 Worker 中 , 并且存放在一个 HashSet (workers) 中由 mainLock 保护:

/**
 * Set containing all worker threads in pool. Accessed only when
 * holding mainLock.
 */
private final HashSet<Worker> workers = new HashSet<>();

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    final Thread thread;
    Runnable firstTask;
    volatile long completedTasks;

    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker. */
    public void run() {
        runWorker(this);
    }
    
    ...
}

Worker.run()方法很简单, 直接调用了 runWorker() 方法, 来看一下这个方法的源码:

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            // task 不为 null 或 获取到了需要执行的任务; getTask() 会阻塞, 并在线程需要退出时返回 null
            w.lock();
            // 检查线程池状态和线程的中断状态, 如果被中断, 代表线程池正在 STOP
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                // 重新设置中断状态
                wt.interrupt();
            try {
                // 执行前的钩子
                beforeExecute(wt, task);
                try {
                    // 执行任务
                    task.run();
                    // 执行后的钩子
                    afterExecute(task, null);
                } catch (Throwable ex) {
                    // 执行后的钩子
                    afterExecute(task, ex);
                    throw ex;
                }
            } finally {
                // 更新状态, 准备处理下一个任务
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        // 处理 Worker 的退出
        processWorkerExit(w, completedAbruptly);
    }
}

getTask() 方法会在以下4种情况返回 null :

• workCount 大于 maximumPoolSize;
• 线程池已经处于 STOP 状态;
• 线程池已经处于 SHUTDOWN 状态, 并且任务队列为空;
• 等待任务时超时, 并且超时的 worker 需要被终止.

private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    for (;;) {
        int c = ctl.get();
        if (runStateAtLeast(c, SHUTDOWN)
            && (runStateAtLeast(c, STOP) || workQueue.isEmpty())) {
            // 线程池已经处于 SHUTDOWN 状态, 并且不在需要线程 (线程池已经处于 STOP 状态 或 workQueue 为空)
            decrementWorkerCount();
            return null;
        }
        int wc = workerCountOf(c);
        // 是否需要剔除超时的 worker
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            // 需要剔除当前 worker, 尝试调整 workerCount
            if (compareAndDecrementWorkerCount(c))
                // 成功 返回 null
                return null;
            continue;
        }

        try {
            // 阻塞获取任务
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            // 设置超时标记, 下一次循环中检查是否需要返回 null
            timedOut = true;
        } catch (InterruptedException retry) {
            // 被中断, 设置超时标记, 下一次循环中检查是否需要返回 null
            timedOut = false;
        }
    }
}

processWorkerExit() 方法负责垂死 worker 的清理和簿记, 只会被工作线程调用:

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        decrementWorkerCount();

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        // 更新线程池完成的任务数量
        completedTaskCount += w.completedTasks;
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }
    
    // 尝试转换线程池状态到终止
    tryTerminate();

    int c = ctl.get();
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            // 不是由于用户代码异常而突然退出
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                // 不需要在添加新 worker
                return;
        }
        // 尝试添加新的 worker
        addWorker(null, false);
    }
}

提交任务

ThreadPoolExecutor 没有重写 submit() 方法, 我们只要看一下 execute() 就够了:

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {
        // 有效线程数量小于 corePoolSize 尝试调用 addWorker 来增加一个线程(在 addWorker 方法中使用 corePoolSize 来检查是否需要增加线程), 使用 corePoolSize 作为, 并把 command 作为新线程的第一个任务
        if (addWorker(command, true))
            return;
        // 调用失败, 重新获取状态
        c = ctl.get();
    }
    if (isRunning(c) && workQueue.offer(command)) {
        // 线程池仍然在运行, 将 command 加入 workQueue 成功, 再次检查状态, 因为此时线程池状态可能已经改变, 按照新的状态拒绝 command 或尝试添加新的线程
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            // 不再是运行中状态, 尝试从队列移除 command(还会尝试将线程池状态转换为 TERMINATED), 拒绝command
            reject(command);
        else if (workerCountOf(recheck) == 0)
            // 有效线程数量为 0 , 创建新的线程, 在 addWorker 方法中使用 maximumPoolSize 来检查是否需要增加线程
            addWorker(null, false);
    }
    else if (!addWorker(command, false))
        // 将任务放入队列失败或线程池不在运行状态, 并且尝试添加线程失败(此时线程池已经 shutdown 或饱和), 拒绝任务
        reject(command);
}

addWorker() 方法有两个参数 Runnable firstTask 和 boolean core . firstTask 是新建的工作线程的第一个任务; core 如果为 true , 表示用 corePoolSize 作为边界条件, 否则表示用 maximumPoolSize. 这里的 core 用布尔值是为了确保检查最新的状态.

addWorker() 主要做了这么两件事情:

• 是否可以在当前线程池状态和给定的边界条件(core or maximum)下创建一个新的工作线程;
• 如果可以, 调整 worker counter, 如果可能的话, 创建一个新的 worker 并启动它, 把 firstTask 作为这个新 worker 的第一个任务;

来看下 addWorker() 方法的源码:

private boolean addWorker(Runnable firstTask, boolean core) {
    // 重试标签
    retry:
    for (int c = ctl.get();;) {
        // 获取最新的状态, 检查状态
        if (runStateAtLeast(c, SHUTDOWN)
            && (runStateAtLeast(c, STOP)
                || firstTask != null
                || workQueue.isEmpty()))
            // 如果线程池状态已经进入 SHUDOWN, 并且不再需要工作线程(已经进入 STOP 状态 或 firstTask 不为 null 或 workQueue为空) 返回 false
            return false;

        for (;;) {
            if (workerCountOf(c)
                >= ((core ? corePoolSize : maximumPoolSize) & COUNT_MASK))
                // 有效线程数量大于边界条件, 返回 false
                return false;
            if (compareAndIncrementWorkerCount(c))
                // 调整 workerCount, break retry, 退出外部循环
                break retry;
            c = ctl.get();  // Re-read ctl
            if (runStateAtLeast(c, SHUTDOWN))
                // 因为状态变化导致 CAS 失败, continue retry, 重试外部循环
                continue retry;
            // 由于 workerCount 改变导致 CAS 失败, 重试内嵌循环
        }
    }

    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        // 新建 Worker
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            // threadFactory 成功创建了线程
            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 c = ctl.get();
                
                // 重新检查状态
                if (isRunning(c) ||
                    (runStateLessThan(c, STOP) && firstTask == null)) {
                    // 线程池在 RUNNING 状态 或 需要线程(线程池还不在 STOP 状态 并且 firstTask 为 null)
                    // 检查线程是否可启动
                    if (t.isAlive()) 
                        throw new IllegalThreadStateException();
                    // 将 worker 添加到 workers
                    workers.add(w);
                    // 更新 largestPoolSize
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    // 更新 worker 添加的标记
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                // 启动线程, 更新启动标记
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            // 失败回滚
            addWorkerFailed(w);
    }
    return workerStarted;
}

private void addWorkerFailed(Worker w) {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        // 从 workers 中移除 worker
        if (w != null)
            workers.remove(w);
        // 调整 workerCount()
        decrementWorkerCount();
        // 尝试将线程池状态改变为 TERMINATED
        tryTerminate();
    } finally {
        mainLock.unlock();
    }
}

线程池关闭

来看一下线程池的关闭方法:

public void shutdown() {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        // 如果线程池状态还没有达到SHUTDOWN, 将线程池状态改为 SHUTDOWN
        advanceRunState(SHUTDOWN);
        // 中断空闲的工作者线程
        interruptIdleWorkers();
        // 钩子
        onShutdown(); // hook for ScheduledThreadPoolExecutor
    } finally {
        mainLock.unlock();
    }
    // 尝试转换状态到终止
    tryTerminate();
}

public List<Runnable> shutdownNow() {
    List<Runnable> tasks;
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        // 如果线程池状态还没有达到 STOP, 将线程池状态改为 STOP
        advanceRunState(STOP);
        // 中断所有 worker
        interruptWorkers();
        // 获取任务队列中的任务, 并将这些任务从任务队列中删除
        tasks = drainQueue();
    } finally {
        mainLock.unlock();
    }
    // 尝试转换状态到终止
    tryTerminate();
    return tasks;
}

public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        // 等待线程池终止或超时
        while (runStateLessThan(ctl.get(), TERMINATED)) {
            if (nanos <= 0L)
                // 剩余时间小于 0 , 超时
                return false;
            nanos = termination.awaitNanos(nanos);
        }
        return true;
    } finally {
        mainLock.unlock();
    }
}

tryTerminate() 方法中, 如果成功将线程池状态转换到了 TERMINATED, 将会termination.signalAll() 来唤醒等待线程池终结的线程:

final void tryTerminate() {
    for (;;) {
        int c = ctl.get();
        if (isRunning(c) ||
            runStateAtLeast(c, TIDYING) ||
            (runStateLessThan(c, STOP) && ! workQueue.isEmpty()))
            // 状态不需要改变 (处于 RUNNING 状态 或 已经处于 TIDYING 状态 或 (还没到达 STOP 状态, 并且 workQueue 不为空))
            return;
        if (workerCountOf(c) != 0) { // Eligible to terminate
            // 中断一个空闲的 worker, 以传播关闭状态到工作线程
            interruptIdleWorkers(ONLY_ONE);
            return;
        }

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                // 将状态成功更新为 TIDYING
                try {
                    // 默认实现没有做任何事情
                    terminated();
                } finally {
                    // 将线程池状态更新为 TERMINATED
                    ctl.set(ctlOf(TERMINATED, 0));
                    // 唤醒等待终结的线程
                    termination.signalAll();
                }
                return;
            }
        } finally {
            mainLock.unlock();
        }
        // else retry on failed CAS
    }
}

　　原文出处：https://www.cnblogs.com/FJH1994/p/10362452.html