线程池基础篇

一、什么是线程池？为什么要用线程池？

1、 降低资源的消耗。降低线程创建和销毁的资源消耗；

2、 提高响应速度：线程的创建时间为T1，执行时间T2,销毁时间T3，免去T1和T3的时间

3、 提高线程的可管理性。

二、线程池的创建

ThreadPoolExecutor，jdk所有线程池实现的父类

三、参数介绍

先看构造函数：

/**
     * 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.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

再看看参数的具体解释：

// 阻塞队列
private final BlockingQueue<Runnable> workQueue;

// 互斥锁
private final ReentrantLock mainLock = new ReentrantLock();

// 线程集合。一个Worker对应一个线程。
private final HashSet<Worker> workers = new HashSet<Worker>();

// “终止条件”，与“mainLock”绑定。
private final Condition termination = mainLock.newCondition();

// 线程池中线程数量曾经达到过的最大值。
private int largestPoolSize;

// 已完成任务数量
private long completedTaskCount;

// ThreadFactory对象，用于创建线程。
private volatile ThreadFactory threadFactory;

// 拒绝策略的处理句柄。
private volatile RejectedExecutionHandler handler;

// 保持线程存活时间。
private volatile long keepAliveTime;

private volatile boolean allowCoreThreadTimeOut;

// 核心池大小
private volatile int corePoolSize;

// 最大池大小
private volatile int maximumPoolSize;

参数详解：

1.核心线程 　int  corePoolSize：核心池大小

the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set

线程池在new 出来后是没有线程的，仅当执行任务的时候才会创建线程；

当然也可以调用线程池的prestartAllCoreThreads()方法，让线程池在创建时就创建corePoolSize数目的线程；

核心线程在任务完成时，会new 新的线程，并且不会销毁，除非设置allowCoreThreadTimeOut

public void allowCoreThreadTimeOut(boolean value) {
        if (value && keepAliveTime <= 0)
            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
        if (value != allowCoreThreadTimeOut) {
            allowCoreThreadTimeOut = value;
            if (value)
                interruptIdleWorkers();
        }
    }

2 最大线程数　int  maximuxPoolSize：最大线程池大小

the maximum number of threads to allow in the pool 线程池所允许创建的最大线程数;

3 存活时间　long  keepAliveTime：线程存活时间

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.

当线程池中的线程数量大于核心池大小后，超过核心数量的线程在keepAliveTime的时间内可以等待一个新任务，超过时间就会销毁;

4. TimeUnit  timeUnit：keepAliveTime的单位，

TimeUnit  枚举值有：DAYS、HOURS、MINUTES、SECONDS、MILLISECONDS(毫秒)、MICROSECONDS(微秒)、NANOSECONDS（纳秒);

5.工作队列　BlockingQueue  workQueue：阻塞任务队列

　　执行前的任务，存储在队列中（线程池会先使用核心线程，当核心线程用完才会使用队列）；

　　线程池只会execute Runnable任务，Callable任务也会包装成Runnable任务

　　主要实现类有：

　　1）LinkedBlockingQueue：基于链表的无界（默认构造函数为：最大值Integer.MAX_VALUE容量）阻塞队列，按FIFO(先进先出)的规则存取任务

　　2）ArrayBlockingQueue：基于数组的有界阻塞队列，按FIFO的规则对任务进行存取，必须传入参数来定义队列大小

　　3）DelayedWorkQueue：基于堆的延迟队列，Executors.newScheduledThreadPool(...)中使用了该队列

　　4）PriorityBlockingQueue：具有优先级的阻塞队列

　　5）SynchronousQueue：不存储任务的阻塞队列，每一个存入对应一个取出，串行化队列

　　吞吐量：SynchronousQueue > LinkedBlockingQueue > ArrayBlockingQueue 

6 线程创建工厂　ThreadFactory  threadFactory：线程工厂

用来创建线程，可以通过自定义线程工厂给新创建的线程设置更合理的名字、设置优先级和是否守护线程。可以根据需要自定义线程工厂。

static class DefaultThreadFactory implements ThreadFactory {
        private static final AtomicInteger poolNumber = new AtomicInteger(1);
        private final ThreadGroup group;
        private final AtomicInteger threadNumber = new AtomicInteger(1);
        private final String namePrefix;

        DefaultThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            //线程组
            group = (s != null) ? s.getThreadGroup() :
                                  Thread.currentThread().getThreadGroup();
            //名称前缀，这就是我们打印的线程池中线程名称前缀
            //可以修改的，如果自定义线程工厂
            namePrefix = "pool-" +
                          poolNumber.getAndIncrement() +
                         "-thread-";
        }

        //工厂创建线程
        public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            if (t.isDaemon())
                //非守护线程
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                //常规优先级
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }

7 拒绝策略　RejectedExecutionHandler  handler：拒绝任务的接口处理器

 达到队列和线程池的最大线程数限制，就执行拒绝策略

 拒绝策略有：

　　1）AbortPolicy：拒绝任务并抛出异常，默认的策略

　　2）DiscardPolicy：直接拒绝不抛出异常

　　3）DiscardOldestPolicy：丢弃队列中最远的一个任务（最先进入队列的，FIFO），并执行当前任务；

　　4）CallerRunsPolicy：只用调用者所在的线程来执行任务，不管其他线程的事。

四、我们看线程池的主要方法

1.execute（）方法

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.
         */
        //AtomicInteger存
        int c = ctl.get();
        //工作线程小于corePoolSize
        if (workerCountOf(c) < corePoolSize) {
            //添加一个core线程，此处参数为true，表示添加的线程是core容量下的线程
            if (addWorker(command, true))
                return;
            //刷新数据，乐观锁就是没有锁
            c = ctl.get();
        }
        //判断线程池运行状态，工作队列是否有空间
        if (isRunning(c) && workQueue.offer(command)) {
            //再次检测
            int recheck = ctl.get();
            //线程池已停止，就移除队列
            if (! isRunning(recheck) && remove(command))
                //执行拒绝策略
                reject(command);
            //线程池在运行，有效线程数为0 
            else if (workerCountOf(recheck) == 0)
                //添加一个空线程进线程池，使用非core容量线程
                //仅有一种情况，会走这步，core线程数为0，max线程数>0,队列容量>0
                //创建一个非core容量的线程，线程池会将队列的command执行
                addWorker(null, false);
        }
        //线程池停止了或者队列已满，添加maximumPoolSize容量工作线程，如果失败，执行拒绝策略
        else if (!addWorker(command, false))
            reject(command);
    }

    final void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }

看上面的英文注释，有3步：

1） 检查core线程池数量<corePoolSize数量，是，可以提交任务或新建线程执行任务 。

2）如果corePoolSize线程数量已使用，如果队列容量未满，则加入队列。

3）队列已满，创建maximumPoolSize线程数量执行；如果失败则执行关闭线程池或者拒绝策略。

继续跟踪 private boolean addWorker(Runnable firstTask, boolean core) 

rivate final ReentrantLock mainLock = new ReentrantLock();

    private final HashSet<Worker> workers = new HashSet<Worker>(); 

   /**
     * Checks if a new worker can be added with respect to current
     * pool state and the given bound (either core or maximum). If so,
     * the worker count is adjusted accordingly, and, if possible, a
     * new worker is created and started, running firstTask as its
     * first task. This method returns false if the pool is stopped or
     * eligible to shut down. It also returns false if the thread
     * factory fails to create a thread when asked.  If the thread
     * creation fails, either due to the thread factory returning
     * null, or due to an exception (typically OutOfMemoryError in
     * Thread.start()), we roll back cleanly.
     *
     * @param firstTask the task the new thread should run first (or
     * null if none). Workers are created with an initial first task
     * (in method execute()) to bypass queuing when there are fewer
     * than corePoolSize threads (in which case we always start one),
     * or when the queue is full (in which case we must bypass queue).
     * Initially idle threads are usually created via
     * prestartCoreThread or to replace other dying workers.
     *
     * @param core if true use corePoolSize as bound, else
     * maximumPoolSize. (A boolean indicator is used here rather than a
     * value to ensure reads of fresh values after checking other pool
     * state).
     * @return true if successful
     */
    private boolean addWorker(Runnable firstTask, boolean core) {
        //自旋检查
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            //如果线程池已关闭；线程池正在关闭，提交的任务为null，任务队列不为空，则直接返回失败  
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                //工作线程数达到或超过最大容量，或者添加core线程达到最大容量或者添加max线程达到最大容量，直接失败
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //线程数+1，CAS原子操作，成功后跳出循环
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                //原子操作失败，判断线程池运行状态是否已改变
                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 {
            //所有代码的核心，new Worker，创建了线程，或者复用线程
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                //线程池是操作多线程，加锁
                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());

                    //再检查一次，线程池在运行或正在停止
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        //添加的线程不能是活跃线程
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        //添加线程存储，HashSet存储
                        workers.add(w);
                        //重入锁最大池数量
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    //上面的标记，启动线程
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                //失败处理
                addWorkerFailed(w);
        }
        return workerStarted;
    }

Worker 是啥

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        /**
         * This class will never be serialized, but we provide a
         * serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;

        /** Thread this worker is running in.  Null if factory fails. */
        final Thread thread;
        /** Initial task to run.  Possibly null. */
        Runnable firstTask;
        /** Per-thread task counter */
        volatile long completedTasks;

        /**
         * Creates with given first task and thread from ThreadFactory.
         * @param firstTask the first task (null if none)
         */
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            //此处创建了线程，线程工厂，使用this
            this.thread = getThreadFactory().newThread(this);
        }

        /** Delegates main run loop to outer runWorker  */
        public void run() {
            runWorker(this);
        }

Worker实现AQS和Runnable接口 ，是线程接口实现

追踪 run方法

/**
     * 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;
        //任务线程的锁状态默认为-1（构造函数设置的），此时解锁+1，变为0，是锁打开状态，允许中断。  
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            //如果添加的任务存在或者队列取
            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
                
                //线程池正在停止，当前线程未中断
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    //中断当前线程
                    wt.interrupt();
                try {
                    //准备，空方法，可自定义实现
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        //本质，直接调用run方法
                        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 {
                        //结束，空方法，可自定义实现
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    //记录任务数
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            //worker结束处理
            processWorkerExit(w, completedAbruptly);
        }
    }

跟踪 getTask

private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?
        //自旋锁
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            //检查线程池是否关闭，队列为空
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                //减少工作线程数量
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            //工作线程数超过core或者max，或者队列为空，工作线程存在
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                //减少任务数
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                //超时机制控制队列取元素
                //take        移除并返回队列头部的元素     如果队列为空，则阻塞
                //poll        移除并返问队列头部的元素    如果队列为空，则返回null
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

跟踪 processWorkerExit(w, completedAbruptly); 

/**
     * Performs cleanup and bookkeeping for a dying worker. Called
     * only from worker threads. Unless completedAbruptly is set,
     * assumes that workerCount has already been adjusted to account
     * for exit.  This method removes thread from worker set, and
     * possibly terminates the pool or replaces the worker if either
     * it exited due to user task exception or if fewer than
     * corePoolSize workers are running or queue is non-empty but
     * there are no workers.
     *
     * @param w the worker
     * @param completedAbruptly if the worker died due to user exception
     */
    private void processWorkerExit(Worker w, boolean completedAbruptly) {
        //正常执行，此处设置为法false
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            //任务数增加
            completedTaskCount += w.completedTasks;
            //移除HashSet的线程
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }

        //尝试停止线程池
        tryTerminate();

        int c = ctl.get();
        //如果没有停止线程池
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                //core线程数量为0或者队列为空，默认1
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                //工作线程比core线程多，直接返回
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            //当前线程运行结束，增加空线程，容量maximumPoolSize
            addWorker(null, false);
        }
    }

addWorker 失败处理

/**
     * Rolls back the worker thread creation.
     * - removes worker from workers, if present
     * - decrements worker count
     * - rechecks for termination, in case the existence of this
     *   worker was holding up termination
     */
    private void addWorkerFailed(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null)
                //工作线程移除任务HashSet
                workers.remove(w);
            //工作线程数量-1
            decrementWorkerCount();
            //尝试停止线程池
            tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

tryTerminate

/**
     * Transitions to TERMINATED state if either (SHUTDOWN and pool
     * and queue empty) or (STOP and pool empty).  If otherwise
     * eligible to terminate but workerCount is nonzero, interrupts an
     * idle worker to ensure that shutdown signals propagate. This
     * method must be called following any action that might make
     * termination possible -- reducing worker count or removing tasks
     * from the queue during shutdown. The method is non-private to
     * allow access from ScheduledThreadPoolExecutor.
     */
    final void tryTerminate() {
        for (;;) {
            int c = ctl.get();
            //线程池在运行或者状态在变化中，或者正在停止但队列有任务，直接返回
            if (isRunning(c) ||
                runStateAtLeast(c, TIDYING) ||
                (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
                return;
            //有任务工作线程
            if (workerCountOf(c) != 0) { // Eligible to terminate
                //中断所有线程
                interruptIdleWorkers(ONLY_ONE);
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                //线程池状态改变
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        //结束线程池，空方法，does nothing
                        terminated();
                    } finally {
                        //设置线程池状态，结束
                        ctl.set(ctlOf(TERMINATED, 0));
                        //唤醒所有wait线程
                        termination.signalAll();
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

    private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Method invoked when the Executor has terminated.  Default
     * implementation does nothing. Note: To properly nest multiple
     * overridings, subclasses should generally invoke
     * {@code super.terminated} within this method.
     */
    protected void terminated() { }

大量的CAS和自旋锁；位运算算法和重入锁AQS。

看看销毁线程池方法

public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            //检查权限
            checkShutdownAccess();
            //CAS 更新线程池状态
            advanceRunState(SHUTDOWN);
            //中断所有线程
            interruptIdleWorkers();
            //关闭，此处是do nothing
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        //尝试结束，上面代码已分析
        tryTerminate();
    }

2. submit方法

/**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

执行图解：

五、预定义的线程池

FixedThreadPool

创建固定线程数量的，适用于负载较重的服务器，使用了无界队列

SingleThreadExecutor

创建单个线程，需要顺序保证执行任务，不会有多个线程活动，使用了无界队列

CachedThreadPool

会根据需要来创建新线程的，执行很多短期异步任务的程序，使用了SynchronousQueue

WorkStealingPool（JDK7以后） 

基于ForkJoinPool实现

ScheduledThreadPoolExecutor 

需要定期执行周期任务，Timer不建议使用了。

newSingleThreadScheduledExecutor：只包含一个线程，只需要单个线程执行周期任务，保证顺序的执行各个任务

newScheduledThreadPool 可以包含多个线程的，线程执行周期任务，适度控制后台线程数量的时候

方法说明：

schedule：只执行一次，任务还可以延时执行

scheduleAtFixedRate：提交固定时间间隔的任务

scheduleWithFixedDelay：提交固定延时间隔执行的任务

package com.youyou.test.demo01;

import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Demo02 {
    static class TestThreadPool implements Runnable{
        @Override
        public void run(){
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        //指定3个长度的工作队列
        LinkedBlockingDeque<Runnable> workQueue = new LinkedBlockingDeque<>(3);
        //指定线程池参数：核心线程数，线程池最大线程数量，活跃时间，工作队列
        ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(4,7,90,TimeUnit.SECONDS,workQueue);
        for (int i = 0; i < 15; i++) {
            poolExecutor.execute(new Thread(new TestThreadPool(),"线程:".concat(i+"")));
            System.out.println("线程池中活跃线程数"+poolExecutor.getActiveCount());
            if(workQueue.size()>0){
                System.out.println("被阻塞的线程数为："+workQueue.size());
            }
        }
    }
}