java线程池趣味事：这不是线程池

要想写出高性能高并发的应用，自然有许多关键，如io，算法，异步，语言特性，操作系统特性，队列，内存，cpu，分布式，网络，数据结构，高性能组件。

胡说一通先。

回到主题，线程池。如果说多线程是提高系统并发能力利器之一，那么线程池就是让这个利器更容易控制的一种工具。如果我们自己纯粹使用多线程基础特性编写，那么，必然需要相当老道的经验，才能够驾驭复杂的环境。而线程池则不需要，你只需知道如何使用，即可轻松掌控多线程，安全地为你服务。

1. 常见线程池的应用样例

线程池，不说本身很简单，但应用一定是简单的。

线程池有许多的实现，但我们只说 ThreadPoolExecutor 版本，因其应用最广泛，别无其他。当然了，还有一个定时调度线程池 ScheduledThreadPoolExecutor 另说，因其需求场景不同，无法比较。

下面，我就几个应用级别，说明下我们如何快速使用线程池。（走走过场而已，无关其他）

1.1. 初级线程池

初级版本的使用线程池，只需要借助一个工具类即可： Executors . 它提供了许多静态方法，你只需随便选一个就可以使用线程池了。比如：

// 创建固定数量的线程池
Executors.newFixedThreadPool(8);
// 创建无限动态创建的线程池
Executors.newCachedThreadPool();
// 创建定时调度线程池
Executors.newScheduledThreadPool(2);
// 还有个创建单线程的就不说了，都一样

使用上面这些方法创建好的线程池，直接调用其 execute() 或者 submit() 方法，就可以实现多线程编程了。没毛病！

1.2. 中级线程池

我这里所说的中级，实际就是不使用以上超级简单方式使用线程池的方式。即你已经知道了 ThreadPoolExecutor 这个东东了。这不管你的出发点是啥！

// 自定义各线程参数
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(4, 20, 20, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<>());

具体参数解释就不说了，咱们不扫盲。总之，使用这玩意儿，说明你已经开始有点门道了。

1.3. 高级线程池

实际上，这个版本就没法具体说如何做了。

但它可能是，你知道你的线程池应用场景的，你清楚你的硬件运行环境的，你会使用线程池命名的，你会定义你的队列大小的，你会考虑上下文切换的，你会考虑线程安全的，你会考虑锁性能的，你可能会自己造个轮子的。。。

2. 这不是线程池

我们通常理解的线程池，就是能够同时跑多个任务的地方。但有时候线程池不一像线程池，而像一个单线程。来看一个具体的简单的线程池的使用场景：

    // 初始化线程池
    private ExecutorService executor
            = new ThreadPoolExecutor(Runtime.getRuntime().availableProcessors(),
                Runtime.getRuntime().availableProcessors(),
                0L, TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(50),
                new NamedThreadFactory("test-pool"),
                new ThreadPoolExecutor.CallerRunsPolicy());
    // 使用线程池处理任务
    public Integer doTask(String updateIntervalDesc) throws Exception {
        long startTime = System.currentTimeMillis();
        List<TestDto> testList;
        AtomicInteger affectNum = new AtomicInteger(0);
        int pageSize = 1000;
        AtomicInteger pageNo = new AtomicInteger(1);
        Map<String, Object> condGroupLabel = new HashMap<>();
        log.info("start do sth:{}", updateIntervalDesc);
        List<Future<?>> futureList = new ArrayList<>();
        do {
            PageHelper.startPage(pageNo.getAndIncrement(), pageSize);
            List<TestDto> list
                    = testDao.getLabelListNew(condGroupLabel);
            testList = list;
            // 循环向线程池中提交任务
            for (TestDto s : list) {
                Future<?> future = executor.submit(() -> {
                    try {
                        // do sth...
                        affectNum.incrementAndGet();
                    }
                    catch (Throwable e) {
                        log.error("error:{}", pageNo.get(), e);
                    }
                });
                futureList.add(future);
            }
        } while (testList.size() >= pageSize);
        // 等待任务完成
        int i = 0;
        for (Future<?> future : futureList) {
            future.get();
            log.info("done:+{} ", i++);
        }
        log.info("doTask done:{}, num:{}, cost:{}ms",
                updateIntervalDesc, affectNum.get(), System.currentTimeMillis() - startTime);
        return affectNum.get();
    }

主要业务就是，从数据库中取出许多任务，放入线程池中运行。因为任务又涉及到db等的io操作，所以使用多线程处理，非常合理。

然而，有一种情况的出现，也许会打破这个平衡：那就是当单个任务能够快速执行完成时，而且快到刚上一任务提交完成，还没等下一次提交时，就任务就已被执行完成。这时，你就可能会看到一个神奇的现象，即一直只有一个线程在运行任务。这不是线程池该干的事，更像是单线程任务在跑。

然后，我们可能开始怀疑：某个线程被阻塞了？线程调度不公平了？队列选择不正确了？触发jdk bug了？线程池未完全利用的线程了？等等。。。

然而结果并非如此，纠其原因只是当我们向线程池提交任务时，实际上只是向线程池的队列中添加了任务。即上面显示的 ArrayBlockingQueue 添加了任务，而线程池中的各worker负责从队列中获取任务进行执行。而当任务数很少时，自然只有一部分worker会处理执行中了。至于为什么一直是同一个线程在执行，则可能是由于jvm的调度机制导致。事实上，是受制于 ArrayBlockingQueue.poll() 的公平性。而这个poll()的实现原理，则是由 wait/notify 机制的公平性决定的。

如下，是线程池的worker工作原理：

    // java.util.concurrent.ThreadPoolExecutor#runWorker
    /**
     * Main worker run loop.  Repeatedly gets tasks from queue and
     * executes them, while coping with a number of issues:
     *
     * 1. We may start out with an initial task, in which case we
     * don't need to get the first one. Otherwise, as long as pool is
     * running, we get tasks from getTask. If it returns null then the
     * worker exits due to changed pool state or configuration
     * parameters.  Other exits result from exception throws in
     * external code, in which case completedAbruptly holds, which
     * usually leads processWorkerExit to replace this thread.
     *
     * 2. Before running any task, the lock is acquired to prevent
     * other pool interrupts while the task is executing, and then we
     * ensure that unless pool is stopping, this thread does not have
     * its interrupt set.
     *
     * 3. Each task run is preceded by a call to beforeExecute, which
     * might throw an exception, in which case we cause thread to die
     * (breaking loop with completedAbruptly true) without processing
     * the task.
     *
     * 4. Assuming beforeExecute completes normally, we run the task,
     * gathering any of its thrown exceptions to send to afterExecute.
     * We separately handle RuntimeException, Error (both of which the
     * specs guarantee that we trap) and arbitrary Throwables.
     * Because we cannot rethrow Throwables within Runnable.run, we
     * wrap them within Errors on the way out (to the thread's
     * UncaughtExceptionHandler).  Any thrown exception also
     * conservatively causes thread to die.
     *
     * 5. After task.run completes, we call afterExecute, which may
     * also throw an exception, which will also cause thread to
     * die. According to JLS Sec 14.20, this exception is the one that
     * will be in effect even if task.run throws.
     *
     * The net effect of the exception mechanics is that afterExecute
     * and the thread's UncaughtExceptionHandler have as accurate
     * information as we can provide about any problems encountered by
     * user code.
     *
     * @param w the worker
     */
    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            // worker 不停地向队列中获取任务，然后执行
            // 其中获取任务的过程，可能被中断，也可能不会，受到线程池伸缩配置的影响
            while (task != null || (task = getTask()) != null) {
                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
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        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 {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }
    /**
     * Performs blocking or timed wait for a task, depending on
     * current configuration settings, or returns null if this worker
     * must exit because of any of:
     * 1. There are more than maximumPoolSize workers (due to
     *    a call to setMaximumPoolSize).
     * 2. The pool is stopped.
     * 3. The pool is shutdown and the queue is empty.
     * 4. This worker timed out waiting for a task, and timed-out
     *    workers are subject to termination (that is,
     *    {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
     *    both before and after the timed wait, and if the queue is
     *    non-empty, this worker is not the last thread in the pool.
     *
     * @return task, or null if the worker must exit, in which case
     *         workerCount is decremented
     */
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        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;
            }

            try {
                // 可能调用超时方法，也可能调用阻塞方法
                // 固定线程池的情况下，调用阻塞 take() 方法
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

即线程池worker持续向队列获取任务，执行即可。而队列任务的获取，则由两个读写锁决定：

    // java.util.concurrent.ArrayBlockingQueue#take
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        // 此处锁，保证执行线程安全性
        lock.lockInterruptibly();
        try {
            while (count == 0)
                // 此处释放锁等待，再次唤醒时，要求必须重新持有锁
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    // 
    /**
     * Inserts the specified element at the tail of this queue, waiting
     * for space to become available if the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }
    /**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void enqueue(E x) {
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;
        count++;
        // 通知取等线程，唤醒
        notEmpty.signal();
    }

所以，具体谁取到任务，就是要看谁抢到了锁。而这，可能又涉及到jvm的高效调度策略啥的了吧。（虽然不确定，但感觉像） 至少，任务运行的表象是，所有任务被某个线程一直抢到。

3. 回归线程池

线程池的目的，在于处理一些异步的任务，或者并发的执行多个无关联的任务。在于让系统减负。而当任务的提交消耗，大于了任务的执行消耗，那就没必要使用多线程了，或者说这是错误的用法了。我们应该线程池做更重的活，而不是轻量级的。如上问题，执行性能必然很差。但我们稍做转变，也许就不一样了。

    // 初始化线程池
    private ExecutorService executor
            = new ThreadPoolExecutor(Runtime.getRuntime().availableProcessors(),
                Runtime.getRuntime().availableProcessors(),
                0L, TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(50),
                new NamedThreadFactory("test-pool"),
                new ThreadPoolExecutor.CallerRunsPolicy());
    // 使用线程池处理任务
    public Integer doTask(String updateIntervalDesc) throws Exception {
        long startTime = System.currentTimeMillis();
        List<TestDto> testList;
        AtomicInteger affectNum = new AtomicInteger(0);
        int pageSize = 1000;
        AtomicInteger pageNo = new AtomicInteger(1);
        Map<String, Object> condGroupLabel = new HashMap<>();
        log.info("start do sth:{}", updateIntervalDesc);
        List<Future<?>> futureList = new ArrayList<>();
        do {
            PageHelper.startPage(pageNo.getAndIncrement(), pageSize);
            List<TestDto> list
                    = testDao.getLabelListNew(condGroupLabel);
            testList = list;
            // 一批任务只向线程池中提交任务
            Future<?> future = executor.submit(() -> {
                for (TestDto s : list) {
                    try {
                        // do sth...
                        affectNum.incrementAndGet();
                    }
                    catch (Throwable e) {
                        log.error("error:{}", pageNo.get(), e);
                    }
                }
            });
            futureList.add(future);
        } while (testList.size() >= pageSize);
        // 等待任务完成
        int i = 0;
        for (Future<?> future : futureList) {
            future.get();
            log.info("done:+{} ", i++);
        }
        log.info("doTask done:{}, num:{}, cost:{}ms",
                updateIntervalDesc, affectNum.get(), System.currentTimeMillis() - startTime);
        return affectNum.get();
    }

即，让每个线程执行的任务足够重，以至于完全忽略提交的消耗。这样才能够发挥多线程的作用。