线程池再探

线程池源码之execute

execute：

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
       //workerCountOf 获取线程数量，isRunning获取线程池状态
        int c = ctl.get();
        //如果小于核心线程池数量直接添加到线程池里
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //大于核心线程数量，加入队列，会做一个dobbule check,
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //如果当前线程池状态不是RUNNING则从队列删除任务，并执行拒绝策略
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //再次尝试添加线程，失败的话执行拒绝策略
        else if (!addWorker(command, false))
            reject(command);
    }

总体来说就是：

• 如果线程的数量小于线程池的核心线程数直接创建线程执行；
• 如果加入线程池失败，说明超过核心线程数量尝试加入阻塞队列；
• 如果加入阻塞队列失败了，尝试重新加入线程池如果也失败了就说明超过了最大线程数量，执行拒绝策略。

addWorker：

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

            // check 线程池的状态和阻塞队列是否为空
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;
            //这段for循环的代码含义是：往线程池里面添加一个线程
            for (;;) {
                int wc = workerCountOf(c);
                // check 线程数量是否超出
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                // 添加成功直接退出循环，添加失败的话再次尝试
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }
        //这段代码的含义是：把当前线程加入到hashSet里面，加入成功后启动线程，加入失败的话，从set里面删除，并且尝试停止线程池
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            //把线程封装成Worker，worker是继承了Aqs的
            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());
                    // check 线程池状态
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                       //加入到set里
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                //加入成功启动线程
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            //加入失败，删除线程，尝试停止线程池
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

总体来说分为两步：

• 第一步：尝试给线程池的数量加一，失败继续尝试，成功退出。
• 第二步：线程池数量成功加一之后，采用加锁的方式把线程加入到set里，然后启动线程，如果启动失败删除线程，尝试停止线程池。

来看一下work对象：

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable{
      
        final Thread thread;
        Runnable firstTask;
        volatile long completedTasks;
        //构造方法
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        public void run() {
            runWorker(this);
        }

        protected boolean isHeldExclusively() {
            return getState() != 0;
        }

        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock()        { acquire(1); }
        public boolean tryLock()  { return tryAcquire(1); }
        public void unlock()      { release(1); }
        public boolean isLocked() { return isHeldExclusively(); }

        void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

• 首先，这个work对象实现了runnable接口，就有了线程的属性。
• 其次，这个work继承了AbstractQueuedSynchronizer就有了锁的功能
• 最后，我们来看一下runWorker这个方法。

runWorker:

//运行当前线程
 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) {
                w.lock();
      
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    //执行前操作
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        //运行当前线程
                        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 {
                       //执行后操作
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

getTask:

private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?
        // 主要功能就是从队列里面获取线程
        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;
            }
            // 获取任务
            try {
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

线程池源码之shutdown

先来搞清楚几个概念：
interrupt():为当前线程打上停止的标记
interrupted():测试当前是否中断。此方法具有清除功能
isInterrupted(): 测试线程是否中断。此方法不会清除

shutdown:停止线程池，优雅停掉，等队列里面的线程执行完再停掉。

public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            //将线程池的状态设置为SHUTDOWN
            advanceRunState(SHUTDOWN);
          //为所有线程打上停止标记
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }
//为所有线程打上停止标记
private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

// 尝试tryTerminate
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
        }
    }

线程池源码之shutdownNow

shutdownNow:比较粗暴的停掉，不会等阻塞队列执行完再停掉。

  public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            //把线程池状态改为STOP
            advanceRunState(STOP);
            interruptWorkers();
            // 把队列的任务取出来
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }
//把队列的任务放入到list里面返回
  private List<Runnable> drainQueue() {
        BlockingQueue<Runnable> q = workQueue;
        ArrayList<Runnable> taskList = new ArrayList<Runnable>();
        q.drainTo(taskList);
        if (!q.isEmpty()) {
            for (Runnable r : q.toArray(new Runnable[0])) {
                if (q.remove(r))
                    taskList.add(r);
            }
        }
        return taskList;
    }
// 打上停止的标记
void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }