ThreadPoolExecutor原理及使用

大家先从ThreadPoolExecutor的总体流程入手：

针对ThreadPoolExecutor代码，我们来看下execute方法：

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
	//poolSize大于等于corePoolSize时不增加线程，反之新初始化线程
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
	    //线程执行状态外为执行，同时可以添加到队列中
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
	    //poolSize大于等于corePoolSize时，新初始化线程
            else if (!addIfUnderMaximumPoolSize(command))
		//无法添加初始化执行线程，怎么执行reject操作（调用RejectedExecutionHandler）
                reject(command); // is shutdown or saturated
        }
    }

我们再看下真正的线程执行者（Worker）：

	private final class Worker implements Runnable {
	/**
         * Runs a single task between before/after methods.
         */
        private void runTask(Runnable task) {
            final ReentrantLock runLock = this.runLock;
            runLock.lock();
            try {
                /*
                 * If pool is stopping ensure thread is interrupted;
                 * if not, ensure thread is not interrupted. This requires
                 * a double-check of state in case the interrupt was
                 * cleared concurrently with a shutdownNow -- if so,
                 * the interrupt is re-enabled.
                 */
		 //当线程池的执行状态为关闭等，则执行当前线程的interrupt()操作
                if ((runState >= STOP ||
                    (Thread.interrupted() && runState >= STOP)) &&
                    hasRun)
                    thread.interrupt();
                /*
                 * Track execution state to ensure that afterExecute
                 * is called only if task completed or threw
                 * exception. Otherwise, the caught runtime exception
                 * will have been thrown by afterExecute itself, in
                 * which case we don't want to call it again.
                 */
                boolean ran = false;
                beforeExecute(thread, task);
                try {
		    //任务执行
                    task.run();
                    ran = true;
                    afterExecute(task, null);
                    ++completedTasks;
                } catch (RuntimeException ex) {
                    if (!ran)
                        afterExecute(task, ex);
                    throw ex;
                }
            } finally {
                runLock.unlock();
            }
        }

        /**
         * Main run loop
         */
        public void run() {
            try {
                hasRun = true;
                Runnable task = firstTask;
                firstTask = null;
		//判断是否存在需要执行的任务
                while (task != null || (task = getTask()) != null) {
                    runTask(task);
                    task = null;
                }
            } finally {
		//如果没有，则将工作线程移除，当poolSize为0是则尝试关闭线程池
                workerDone(this);
            }
        }
    }

    /* Utilities for worker thread control */

    /**
     * Gets the next task for a worker thread to run.  The general
     * approach is similar to execute() in that worker threads trying
     * to get a task to run do so on the basis of prevailing state
     * accessed outside of locks.  This may cause them to choose the
     * "wrong" action, such as trying to exit because no tasks
     * appear to be available, or entering a take when the pool is in
     * the process of being shut down.  These potential problems are
     * countered by (1) rechecking pool state (in workerCanExit)
     * before giving up, and (2) interrupting other workers upon
     * shutdown, so they can recheck state. All other user-based state
     * changes (to allowCoreThreadTimeOut etc) are OK even when
     * performed asynchronously wrt getTask.
     *
     * @return the task
     */
    Runnable getTask() {
        for (;;) {
            try {
                int state = runState;
                if (state > SHUTDOWN)
                    return null;
                Runnable r;
                if (state == SHUTDOWN)  // Help drain queue
                    r = workQueue.poll();
		//当线程池大于corePoolSize，同时，存在执行超时时间，则等待相应时间，拿出队列中的线程
                else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
                    r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
                else
		//阻塞等待队列中可以取到新线程
                    r = workQueue.take();
                if (r != null)
                    return r;
		//判断线程池运行状态，如果大于corePoolSize,或者线程队列为空，也或者线程池为终止的工作线程可以销毁
                if (workerCanExit()) {
                    if (runState >= SHUTDOWN) // Wake up others
                        interruptIdleWorkers();
                    return null;
                }
                // Else retry
            } catch (InterruptedException ie) {
                // On interruption, re-check runState
            }
        }
    }

     /**
     * Performs bookkeeping for an exiting worker thread.
     * @param w the worker
     */
     //记录执行任务数量，将工作线程移除，当poolSize为0是则尝试关闭线程池
    void workerDone(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
            if (--poolSize == 0)
                tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

通过上述代码，总结下四个关键字的用法

• corePoolSize 核心线程数量

线程保有量,线程池总永久保存执行线程的数量

• maximumPoolSize 最大线程数量

最大线程量，线程最多不能超过此属性设置的数量，当大于线程保有量后，会新启动线程来满足线程执行。

• 线程存活时间

获取队列中任务的超时时间，当阈值时间内无法获取线程，则会销毁处理线程，前提是线程数量在corePoolSize 以上

• 执行队列

执行队列是针对任务的缓存，任务在提交至线程池时，都会压入到执行队列中。所以这里大家最好设置下队列的上限，防止溢出

ThreadPoolExecuter的几种实现

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

• CachedThreadPool 执行线程不固定，

好处：可以把新增任务全部缓存在一起，

     坏处：只能用在短时间完成的任务（占用时间较长的操作可以导致线程数无限增大，系统资源耗尽）

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

• 单线程线程池

       好处：针对单cpu，单线程避免系统资源的抢夺

       坏处：多cpu多线程时，不能完全利用cpu资源

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

• 固定长度线程池

        好处：线程数量固定，不会存在线程重复初始化

        坏处：没有对队列大小进行限制，线程初始化后，再也不能回收线程资源