Java 线程池的介绍以及工作原理

在什么情况下使用线程池？

1.单个任务处理的时间比较短
2.将需处理的任务的数量大

使用线程池的好处:

1. 降低资源消耗：　　　　　　通过重复利用已创建的线程降低线程创建和销毁造成的消耗。
2. 提高响应速度：　　　　　　当任务到达时，任务可以不需要等到线程创建就能立即执行。
3. 提高线程的可管理性：　　　线程是稀缺资源，如果无限制的创建。不仅仅会降低系统的稳定性，使用线程池可以统一分配，调优和监控。但是要做到合理的利用线程池。必须对于其实现原理了如指掌。

一个线程池包括以下四个基本组成部分：
1、线程池管理器（ThreadPool）：用于创建并管理线程池，包括 创建线程池，销毁线程池，添加新任务；
2、工作线程（PoolWorker）：线程池中线程，在没有任务时处于等待状态，可以循环的执行任务；
3、任务接口（Task）：每个任务必须实现的接口，以供工作线程调度任务的执行，它主要规定了任务的入口，任务执行完后的收尾工作，任务的执行状态等；
4、任务队列（taskQueue）：用于存放没有处理的任务。提供一种缓冲机制。

在JDK1.6中研究ThreadPoolExecutor类:

    volatile int runState;
    static final int RUNNING    = 0;
    static final int SHUTDOWN   = 1;
    static final int STOP       = 2;
    static final int TERMINATED = 3;

runState表示当前线程池的状态，它是一个volatile变量用来保证线程之间的可见性；

当创建线程池后，初始时，线程池处于RUNNING状态；

如果调用了shutdown()方法，则线程池处于SHUTDOWN状态，此时线程池不能够接受新的任务，它会等待所有任务执行完毕；

如果调用了shutdownNow()方法，则线程池处于STOP状态，此时线程池不能接受新的任务，并且会去尝试终止正在执行的任务；

当线程池处于SHUTDOWN或STOP状态，并且所有工作线程已经销毁，任务缓存队列已经清空或执行结束后，线程池被设置为TERMINATED状态。

execute方法：

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
            else if (!addIfUnderMaximumPoolSize(command))
                reject(command); // is shutdown or saturated
        }
    }

addIfUnderCorePoolSize方法检查如果当前线程池的大小小于配置的核心线程数，说明还可以创建新线程，则启动新的线程执行这个任务。

   private boolean addIfUnderCorePoolSize(Runnable firstTask) {
        Thread t = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < corePoolSize && runState == RUNNING)
                t = addThread(firstTask);
        } finally {
            mainLock.unlock();
        }
        return t != null;
    }

addThread:

  private Thread addThread(Runnable firstTask) {
        Worker w = new Worker(firstTask);
        Thread t = threadFactory.newThread(w);
        boolean workerStarted = false;
        if (t != null) {
            if (t.isAlive()) // precheck that t is startable
                throw new IllegalThreadStateException();
            w.thread = t;
            workers.add(w);
            int nt = ++poolSize;
            if (nt > largestPoolSize)
                largestPoolSize = nt;
            try {
                t.start();
                workerStarted = true;
            }
            finally {
                if (!workerStarted)
                    workers.remove(w);
            }
        }
        return t;
    }

Worker，在ThreadPoolExecutor中的内部类

  private final class Worker implements Runnable {
        /**
         * The runLock is acquired and released surrounding each task
         * execution. It mainly protects against interrupts that are
         * intended to cancel the worker thread from instead
         * interrupting the task being run.
         */
        private final ReentrantLock runLock = new ReentrantLock();

        /**
         * Initial task to run before entering run loop. Possibly null.
         */
        private Runnable firstTask;

        /**
         * Per thread completed task counter; accumulated
         * into completedTaskCount upon termination.
         */
        volatile long completedTasks;

        /**
         * Thread this worker is running in.  Acts as a final field,
         * but cannot be set until thread is created.
         */
        Thread thread;

        /**
         * Records that the thread assigned to this worker has actually
         * executed our run() method. Such threads are the only ones
         * that will be interrupted.
         */
        volatile boolean hasRun = false;

        Worker(Runnable firstTask) {
            this.firstTask = firstTask;
        }

        boolean isActive() {
            return runLock.isLocked();
        }

        /**
         * Interrupts thread if not running a task.
         */
        void interruptIfIdle() {
            final ReentrantLock runLock = this.runLock;
            if (runLock.tryLock()) {
                try {
                    if (hasRun && thread != Thread.currentThread())
                        thread.interrupt();
                } finally {
                    runLock.unlock();
                }
            }
        }

        /**
         * Interrupts thread even if running a task.
         */
        void interruptNow() {
            if (hasRun)
                thread.interrupt();
        }

        /**
         * 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.
                 */
                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 {
                    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;
                while (task != null || (task = getTask()) != null) {
                    runTask(task);
                    task = null;
                }
            } finally {
                workerDone(this);
            }
        }
    }

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