转载：ThreadPoolExecutor 源码阅读

时间 2019-11-05

标签转载 threadpoolexecutor 源码阅读繁體版

原文原文链接

前言

　　以前研究了一下如何使用ScheduledThreadPoolExecutor动态建立定时任务(Springboot定时任务原理及如何动态建立定时任务)，简单了解了ScheduledThreadPoolExecutor相关源码。今天看了同窗写的ThreadPoolExecutor 的源码解读，甚是NB，必须转发一下。html

读了一下 ThreadPoolExecutor 的源码(JDK 11), 简单的作个笔记.java

Executor 框架

Executor

Executor 接口只有一个方法:markdown

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 的实现并不需要是异步的.app

ExecutorService

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

shutdown() 方法会启动比较柔和的关闭过程, 而且不会阻塞. ExecutorService 将会继续执行已经提交的任务, 但不会再接受新的任务. 若是 ExecutorService 已经被关闭, 则不会有附加的操做.
shutdownNow() 方法会尝试中止正在执行的任务, 再也不执行等待执行的任务, 而且返回等待执行的任务列表, 不会阻塞. 这个方法只能尝试中止任务, 典型的取消实现是经过中断来取消任务, 所以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全部任务, 当全部任务完成时返回 Future 的列表, 支持中断. 若是在此操做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, 并取消其余未完成的任务, 支持中断. 若是在此操做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.less

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

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, 只能在任务完成以后获取结果, 而且在任务完成后, 就再也不能取消或重启, 除非使用 runAndReset() 方法.ide

FutureTask 有 7 个状态:oop

NEW
COMPLETING
NORMAL
EXCEPTIONAL
CANCELLED
INTERRUPTING
INTERRUPTED

可能的状态转换:post

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, 可是状态仍是 COMPLETING
            Thread.yield();
        else if (Thread.interrupted()) {
            // 检查中断
            removeWaiter(q);
            throw new InterruptedException();
        }
        else if (q == null) {
            // 没有建立 WaitNode 节点, 若是 timed 而且 nanos 大于 0, 建立一个 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, 而后阻塞
            if (state < COMPLETING)
                LockSupport.parkNanos(this, parkNanos);
        }
        else
            // 不须要超时的阻塞
            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) 建立的
        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, 对链表中的每一个 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;
        }
    }
    // 默认实现什么都没作
    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 {
        // 取消全部已经提交的任务
        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, 即便这些线程是空闲的, 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以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 用来建立线程的工厂, 默认使用 Executors.defaultThreadFactory() 来建立一个新的工厂;
handler: 任务由于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, 而且处理队列中的任务;
SHUTDOWN: 不接受新任务, 可是处理队列中的任务, 此时仍然可能建立新的线程;
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: 全部的任务都终结了, 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, 是容许启动且不容许中止的 worker 的数量, 与实际的线程数量瞬时不一样. 用户可见的线程池大小是 Worker 集合的大小.

Worker 与任务调度

工做线程被封装在 Worker 中 , 而且存放在一个 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 获取到了须要执行的任务; getTask() 会阻塞, 并在线程须要退出时返回 null
            w.lock();
            // 检查线程池状态和线程的中断状态, 若是被中断, 表明线程池正在 STOP
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                // 从新设置中断状态
                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() 方法会在如下4种状况返回 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, 而且任务队列为空;
等待任务时超时, 而且超时的 worker 须要被终止.

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, 而且不在须要线程 (线程池已经处于 STOP 状态 或 workQueue 为空)
            decrementWorkerCount();
            return null;
        }
        int wc = workerCountOf(c);
        // 是否须要剔除超时的 worker
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            // 须要剔除当前 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须要返回 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须要返回 null
            timedOut = false;
        }
    }
}

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

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) {
            // 不是因为用户代码异常而忽然退出
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                // 不须要在添加新 worker
                return;
        }
        // 尝试添加新的 worker
        addWorker(null, false);
    }
}

提交任务

ThreadPoolExecutor 没有重写 submit() 方法, 咱们只要看一下 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 来增长一个线程(在 addWorker 方法中使用 corePoolSize 来检查是否须要增长线程), 使用 corePoolSize 做为, 并把 command 做为新线程的第一个任务
        if (addWorker(command, true))
            return;
        // 调用失败, 从新获取状态
        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, 由于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))
            // 再也不是运行中状态, 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 , 建立新的线程, 在 addWorker 方法中使用 maximumPoolSize 来检查是否须要增长线程
            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, 而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 是新建的工做线程的第一个任务; core 若是为 true , 表示用 corePoolSize 做为边界条件, 不然表示用 maximumPoolSize. 这里的 core 用布尔值是为了确保检查最新的状态.

addWorker() 主要作了这么两件事情:

是否能够在当前线程池状态和给定的边界条件(core or maximum)下建立一个新的工做线程;
若是能够, 调整 worker counter, 若是可能的话, 建立一个新的 worker 并启动它, 把 firstTask 做为这个新 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()))
            // 若是线程池状态已经进入 SHUDOWN, 而且再也不须要工做线程(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))
                // 由于状态变化致使 CAS 失败, continue retry, 重试外部循环
                continue retry;
            // 因为 workerCount 改变致使 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 成功建立了线程
            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();
                
                // 从新检查状态
                if (isRunning(c) ||
                    (runStateLessThan(c, STOP) && firstTask == null)) {
                    // 线程池在 RUNNING 状态 或 须要线程(线程池还不在 STOP 状态 而且 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();
        // 若是线程池状态尚未达到SHUTDOWN, 将线程池状态改成 SHUTDOWN
        advanceRunState(SHUTDOWN);
        // 中断空闲的工做者线程
        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();
        // 若是线程池状态尚未达到 STOP, 将线程池状态改成 STOP
        advanceRunState(STOP);
        // 中断全部 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, 若是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()))
            // 状态不须要改变 (处于 RUNNING 状态 或 已经处于 TIDYING 状态 或 (还没到达 STOP 状态, 而且 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做线程
            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 {
                    // 默认实现没有作任何事情
                    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：http://www.javashuo.com/article/p-qzkixbep-co.html