ThreadPoolExecutor线程的创建与销毁分析

线程池类型

• 固定线程池(newFixedThreadPool)
  

指定固定大小线程数，如果执行的任务超过线程数，则添加至阻塞队列中，当线程池线程处理完在从队列中进行获取任务进行处理

   public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {   return new ThreadPoolExecutor(nThreads,nThreads,0L,TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>(),threadFactory);    }

• 单例线程池(newSingleThreadExecutor)
  

指定1个线程数，如果执行的任务超过线程数，则添加至阻塞队列中，当线程池线程处理完在从队列中进行获取任务进行处理

    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {    return new ThreadPoolExecutor(1, 1,0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())    }

• 缓存线程池(newCachedThreadPool)
  

不指定固定线程数，最大线程数为Integer.MAX_VALUE，如果执行的任务超过最大线程数，则添加至阻塞队列中，注意此处用的是SynchronousQueue，表明是如果添加一个任务必须等待处理线程取出，否则不能继续添加任务。

    public static ExecutorService newCachedThreadPool() {        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,                                      60L, TimeUnit.SECONDS,                                      new SynchronousQueue<Runnable>());    }

线程添加

    public void execute(Runnable command) {        if (command == null)            throw new NullPointerException();        int c = ctl.get();        //如果当前线程数小于核心线程数，则创建线程        if (workerCountOf(c) < corePoolSize) {            //如果创建线程失败，则进行下一步处理            if (addWorker(command, true))                return;            c = ctl.get();        }        //判断线程池是否在运行，如果是 在试着添加任务到队列中        if (isRunning(c) && workQueue.offer(command)) {            int recheck = ctl.get();            if (! isRunning(recheck) && remove(command))                reject(command);            else if (workerCountOf(recheck) == 0)                addWorker(null, false);        }//否则进行继续尝试创建新线程，如果现有线程数大于最大线程数则返回false        else if (!addWorker(command, false))            reject(command);    }    //删除了不必要的代码    private boolean addWorker(Runnable firstTask, boolean core) {        retry:        for (;;) {            int c = ctl.get();            int rs = runStateOf(c);            for (;;) {                int wc = workerCountOf(c);                //判断现有线程数是否大于最大线程数                if (wc >= CAPACITY ||                    wc >= (core ? corePoolSize : maximumPoolSize))                    return false;                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            }        }    }

线程启动

//删除多余代码private boolean addWorker(Runnable firstTask, boolean core) {        boolean workerStarted = false;        boolean workerAdded = false;        Worker w = null;        try {            final ReentrantLock mainLock = this.mainLock;            //创建worker，worker本身也是一个Runnable            w = new Worker(firstTask);            final Thread t = w.thread;            if (t != null) {                mainLock.lock();                try {                    // Recheck while holding lock.                    // Back out on ThreadFactory failure or if                    // shut down before lock acquired.                    int c = ctl.get();                    int rs = runStateOf(c);                    if (rs < SHUTDOWN ||                        (rs == SHUTDOWN && firstTask == null)) {                        if (t.isAlive()) // precheck that t is startable                            throw new IllegalThreadStateException();                        //添加work值workers，后面的运行数统计、关闭线程池都是基于此workers                            workers.add(w);                        int s = workers.size();                        if (s > largestPoolSize)                            largestPoolSize = s;                        workerAdded = true;                    }                } finally {                    mainLock.unlock();                }                if (workerAdded) {                    //启动                    t.start();                    workerStarted = true;                }            }        } finally {            if (! workerStarted)                addWorkerFailed(w);        }        return workerStarted;    }

线程执行任务

当启动线程后，会调用runWorker进行任务的执行，如果当前任务执行完成，则从阻塞队列中进行获取

final void runWorker(Worker w) {        Thread wt = Thread.currentThread();        Runnable task = w.firstTask;        w.firstTask = null;        w.unlock(); // allow interrupts        boolean completedAbruptly = true;        try {            //先判断当前任务是否不为空，如果不为空则进行当前任务，否则调动getTask()方法，从队列中获取任务            while (task != null || (task = getTask()) != null) {                //进行标记表示正在有任务执行，如果这会进行调用shutdown()方法，会判断标记，否则进行中断线程。                w.lock();                // If pool is stopping, ensure thread is interrupted;                // if not, ensure thread is not interrupted.  This                // requires a recheck in second case to deal with                // shutdownNow race while clearing interrupt                //如果调用shutdownNow()方法设置stop标记，并且当前线程还未中断，则当前线程自己中断，否则在进行一次判断                if ((runStateAtLeast(ctl.get(), STOP) ||                     (Thread.interrupted() &&                      runStateAtLeast(ctl.get(), STOP))) &&                    !wt.isInterrupted())                    wt.interrupt();                try {                   //前处理                    beforeExecute(wt, task);                    Throwable thrown = null;                    try {                       //执行任务                        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 {                       //后处理                        afterExecute(task, thrown);                    }                } finally {                    task = null;                    //记录执行完任务数量                    w.completedTasks++;                    //取消正在执行标记                    w.unlock();                }            }            completedAbruptly = false;        } finally {            //线程退出，这步只有调用shutdown（）、shutdownNow（）才会执行，否则会在getTask()任务阻塞。            processWorkerExit(w, completedAbruptly);        }    }

线程池伸缩

private Runnable getTask() {        boolean timedOut = false; // Did the last poll() time out?        retry:        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;            }            boolean timed;      // Are workers subject to culling?            for (;;) {                int wc = workerCountOf(c);                timed = allowCoreThreadTimeOut || wc > corePoolSize;                if (wc <= maximumPoolSize && ! (timedOut && timed))                    break;                //如果当前线程数大于核心线程数或者设置核心线程数容许超时，那么对现有线程池的线程进行回收                if (compareAndDecrementWorkerCount(c))                    return null;                c = ctl.get();  // Re-read ctl                if (runStateOf(c) != rs)                    continue retry;                // else CAS failed due to workerCount change; retry inner loop            }            try {                Runnable r = timed ?                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :                    workQueue.take();                if (r != null)                    return r;                timedOut = true;            } catch (InterruptedException retry) {                timedOut = false;            }        }    }

线程池销毁

停止线程池

等正在进行任务执行完，进行停止

    public void shutdown() {        final ReentrantLock mainLock = this.mainLock;        mainLock.lock();        try {            checkShutdownAccess();            //设置状态            advanceRunState(SHUTDOWN);            //遍历所有未执行任务的线程，对其设置中断            interruptIdleWorkers();            onShutdown(); // hook for ScheduledThreadPoolExecutor        } finally {            mainLock.unlock();        }        //在此判断所有任务都已退出，如果退出则设置标记 “TERMINATED”        tryTerminate();    }

立即停止线程池

不用等待正在进行任务执行完，立即停止

public List<Runnable> shutdownNow() {        List<Runnable> tasks;        final ReentrantLock mainLock = this.mainLock;        mainLock.lock();        try {            checkShutdownAccess();             //设置状态STOP状态            advanceRunState(STOP);            ////遍历所有任务的线程(包括正在执行任务的线程)，对其设置中断            interruptWorkers();            //返回等待执行的任务列表            tasks = drainQueue();        } finally {            mainLock.unlock();        }         //在此判断所有任务都已退出，如果退出则设置标记 “TERMINATED”        tryTerminate();        return tasks;    }     private List<Runnable> drainQueue() {        BlockingQueue<Runnable> q = workQueue;        List<Runnable> taskList = new ArrayList<Runnable>();        //尝试把队列中任务移动taskList        q.drainTo(taskList);        //这一步判断主要针对DelayQueue，因为drainTo只返回到期的任务，所以需要单独处理        if (!q.isEmpty()) {            for (Runnable r : q.toArray(new Runnable[0])) {                if (q.remove(r))                    taskList.add(r);            }        }        return taskList;    }