线程池源码分析

[diaosichengxuyuan]

1.线程池原理图

2.创建

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();
        //核心线程数
        this.corePoolSize = corePoolSize;
        //最大线程数
        this.maximumPoolSize = maximumPoolSize;
        //任务队列
        this.workQueue = workQueue;
        //最大线程数-核心线程数=非核心线程数，此处是非核心线程空闲时
        //存活时间
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        //线程工厂
        this.threadFactory = threadFactory;
        //拒绝策略，DiscardPolicy/AbortPolicy等
        this.handler = handler;
    }

3.execute

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        //如果工作线程数小于核心线程数，创建核心线程并执行任务
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //如果线程池正在运行，将任务添加到阻塞队列
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //如果线程池不再运行，执行删除任务，然后执行拒绝策略
            if (! isRunning(recheck) && remove(command))
                reject(command);
            //如果工作线程的数量为0，启动非核心线程
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //启动非核心线程并执行任务，如果失败，执行拒绝策略
        else if (!addWorker(command, false))
            reject(command);
    }
    
    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //增加工作线程数量
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            //创建一个Worker对象，先看下Worker的分析
            w = new Worker(firstTask);
            //取出Worker中的线程
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    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();
                }
                if (workerAdded) {
                    //启动线程
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (!workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }
    
    //Worker对象继承自AQS，可以方便执行一些锁的逻辑
    //同时Worker实现了Runnable接口，可以将自身作为一个Thread任务执行
    //同时Worker持有了一个Thread，可以执行启动，而firstTask就是我们
    //提交到线程池的任务
    private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        final Thread thread;
        Runnable firstTask;
    
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }
        
        //线程start后会回调到这个方法
        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) {
                }
            }
        }    
    }
    
    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock();
        boolean completedAbruptly = true;
        try {
            //看下这个地方的逻辑，一个Worker进来时，如果firstTask不
            //为null，则先执行firstTask；同时w.firstTask被置成
            //null，这样下次循环就会调用getTask()从阻塞队列中获取任
            //务，如果获取不到，就一直循环
            while (task != null || (task = getTask()) != null) {
                //线程执行任务时是加锁的
                w.lock();
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    //前置拦截器
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        //执行runnable
                        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 {
                        //后置拦截器
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

4.shutdown

    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            //设置状态是SHUTDOWN，如果线程池状态一旦设为SHUTDOWN，则在线程池中会出
            //现两种现象：
            //1.你不能再往线程池中添加任何任务，否则会抛
            //RejectedExecutionException异常(详细请看ThreadPoolExecutor的
            //addIfUnderCorePoolSize方法)。
            //2.工作线程Worker获得池队列中的任务时(详细看Worker中的getTask()方
            //法)的处理逻辑也发生了变化：如果线程池为RUNNING状态，并且池队列中没任
            //务时，它会一直等待，直到你提交任务到池队列中，然后取出任务，返回。但
            //是，一旦你执行了shutDown()方法,线程池状态为SHUTDOWN状态，它将不再等
            //待了，直接返回null。如果返回null，则工作线程没有要执行的任务，直接退
            //出(详细看Worker中run()方法)。
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }
    
    //遍历线程池所有线程，进行中断，过程中加锁，防止被重复shutdown
    //只有w.tryLock()成功的任务，才会中断，上面分析过，线程池执行
    //任务的线程是加锁锁定的，所以此时执行任务的线程不会被中断
    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();
        }
    }

5.shutdownNow()

  public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            //设置状态为STOP
            advanceRunState(STOP);
            //将所有线程(无论是否执行任务)立即中断
            interruptWorkers();
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }
    
    private void interruptWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers)
                w.interruptIfStarted();
        } finally {
            mainLock.unlock();
        }
    }
    
    void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }

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