线程池ThreadPoolExecutor分析

线程池。线程池是什么，说究竟，线程池是处理多线程的一种形式，管理线程的创建，任务的运行，避免了无限创建新的线程带来的资源消耗，可以提高应用的性能。非常多相关操作都是离不开的线程池的，比方android应用中网络请求的封装。这篇博客要解决的问题是：

1.线程池的工作原理及过程。

要分析线程池的工作原理及过程，还是要从它的源代码实现入手，首先是线程是构造方法，何谓构造方法。构造方法就是对成员变量进行初始化，在这里，我们能够看到它的构造方法：

/**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    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;
    }

第一个參数，corePoolSize核心线程的数量；maximumPoolSize最大线程数量。keepAliveTime和 TimeUnit非核心线程闲置时间，超过这个设置时间将会被终止，TimeUnit含有多种静态成员变量作为单位，如seconds；BlockingQueue任务堵塞队列，当核心线程创建数量达到最大值时，任务首先会加入到堵塞队列。等待运行;ThreadFactory 线程构造工厂。经常使用的有DefaultThreadFactory，我们也能够重写它的newThread方法，实现这个类；RejectedExecutionHandler,当任务被拒绝加入时。将会交给这个类处理。好了，构造的过程。就是这么简单，就是初始化一些成员变量。

分析的时候。从关键点着手，这里分析是从execute()方法入手的：

 /**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current {@code RejectedExecutionHandler}.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     *         {@code RejectedExecutionHandler}, if the task
     *         cannot be accepted for execution
     * @throws NullPointerException if {@code command} is null
     */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        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);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

上面execute的方法，传入的是一个Runnable，这种方法就是我们须要运行的任务。以下我们分析execute是怎么运行这个任务的，也就是execute运行的过程：

1.假设线程池中线程的数量小于核心线程数目，则启动一个新的线程处理这个任务。

2.假设核心线程处于非空暇状态，则将任务插入到堵塞队列中，当有线程空暇时，会自己主动取出任务运行。

3.假设堵塞队列任务已经满了。而且当前线程小于最大线程数目，则启动新的线程。运行任务。假设超过了最大线程数，则拒绝接受新的任务。

上面是整个代码的过程，如今我们队代码进行分析

 if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;

workerCountOf(c)表示的是当前线程的数量。假设这个数量小于corePoolSize，则调用addWorker(command,true)方法，假设addWorker运行成功，返回true，表示任务运行完毕。以下我们看addWorker方法：

 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;
                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
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                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 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) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

addWorker方法比較长，我们分为两块，第一块的for循环，第二块try...。第一块，for循环。主要是推断当前能否够运行任务，假设能够，则进行以下的try,通过的条件。能够分析出来。是当前线程小于核心线程。或者当前堵塞队列已满，线程数小于最大线程数量。满足这两个中的条件，才干往下走。       try里面通过当前的參数，新建了一个Worker，这个Worker实现了Runable接口，Worker里面通过ThreadFactory构建了一个Thread来运行这个任务，代码的后面，调用了t.start()，实际上，调用的是，Worker实现Runable的run方法。run方法调用的又是runWorker(),我们看runWorker方法

 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 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);
        }
    }

首先我们要知道，整个runWorker方法是在Thread的线程中运行的。runWorker中，while循环中，加锁，第一次task不为空，运行这个firstTask,当task.run()运行完，解锁，然后调用getTask（）。getTask是从堵塞队列中取出任务来运行，因此，这里，我们得出结论。当线程完毕一个任务时。会从堵塞队列里取出任务来运行。        while循环的条件是，task 不为空。或者getTask不为空，假设task为空。而且getTask也为空，就跳出循环，可是，先请看getTask()方法，

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 {
                Runnable r = timed ?

workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

这种方法的主要作用是从队列中取出一个任务运行，然而。我们细致分析。假设allowCoreThreadTimeOut为true（也就是说同意核心线程超时），或者当前线程是非核心线程，那么timed=true,这时候。进入if语句推断，wc>maximuPoolsize,这个一般不成立。直接看timed&&timeOut,timed=true,timeOut为false,直接进入try,最后timeOut=true,当后面循环时，假设队列的任务为空，就会运行到compareAndDecrementWorkerCount(c)降低一个线程数量,然后返回null，线程也就终止运行了；可是假设allowCoreThreadTimeOut=false,就会直接调用workQueue.take(),直接取出一个Runnable。假设runnable为空，就将一直循环，线程堵塞在这里，也就是核心线程处于空暇状态。

这里得出新的结论。假设核心线程没有同意超时。那么它将一直处于空暇状态，不会被回收。

我们再次回到execute方法。第二部分

 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);
        }

假设当前线程池处于执行状态，而且这个任务能插入到队列中（由于这里线程数目已经时等于核心线程数了。因此插入到队列中，等待执行）。第二次推断。假设当前线程池不处于执行状态。那么移除这个任务，调用reject方法处理；假设当前线程池的线程数量为0，addWorker(),传入null和false，也就是说，不加入新任务，而且时启动一个非核心线程，这个核心线程会通过getTask()方法取出任务执行。这个和上面分析的过程一样。

第三部分

 else if (!addWorker(command, false))
            reject(command);

先启动一个非核心线程运行任务。假设非核心线程达到了最大线程数，才会拒绝运行任务。

最后的结论就是：

1.假设线程池中线程的数量小于核心线程数目，则启动一个新的线程处理这个任务。

2.假设核心线程处于非空暇状态，则将任务插入到堵塞队列中。当有线程空暇时，会自己主动取出任务运行。

3.假设堵塞队列任务已经满了，而且当前线程小于最大线程数目，则启动新的线程。运行任务，假设超过了最大线程数，则拒绝接受新的任务。

假设核心线程没有设置为同意超时。那么核心线程会一直存在，即时等待运行任务。