ThreadPoolExecuotor源码参考

Executor -->  ExecutorService  -->  AbstractExecutorService --> ThreadPoolExecuotor


Executor接口，只有一个execute方法，参数为线程任务

ExecutorService接口继承Executor接口，增加了submit、shutdown、invokeAll等方法

AbstractExecutorService抽象类，实现ExecutorService接口，提供了submit、invokeAny、invokeAll默认实现方法， execute、
shutdown、shutdownNow等待没有提供默认的实现


ThreadPoolExecutor

线程池状态值
    ---RUNNING  接受新任务并且处理阻塞队列里的任务
    ---SHUTDOWN  拒绝信任务但是处理阻塞队列里的任务
    ---STOP  拒绝新任务并且抛弃阻塞队列里的任务同时会中断正在处理的任务
    ---TIDYING  所有任务都执行完当前线程池活动线程为0，将调用terminated方法
    ---TERMINATED  终止状态

构造函数
    ---有四个构造函数，其他三个都是调用下面这个构造函数
    public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) {}
    ---参数说明
        corePoolSize  核心线程数
        maximumPoolSize  最大线程数
        keepAliveTime  存活时间
        unit  时间单位
        workQueue  存放线程的队列
        threadFactory  创建线程的工厂
        handler  拒绝策略

提交任务
    ---submit
    public Future<?> submit(Runable task) {
        if(task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }       
    public <T> Future<T> submit(Runnable task, T result) {
        if(task == null) throw new NullPointerExecetion();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }
    public <T> Future<T> submit(Callable<T> task) {
        if(task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }
    --- execute
    public void execute(Runnable command) {
        if(command == null) 
            throw new NullPointerException();
        //获取当前线程池的状态+线程个数
        int c = ctl.get();
        //判断当前线程池数量是否小于corePoolSize，小于则调用addWorker方法创建新线程运行，且传进来的Runnable当做第一个任务执行
        if(workerCountOf(c) < corePoolSize) {
            if(addWorker(command, true))
                return;
            c = ctl.get();    
        }

        //如果线程池处于Running状态，则添加任务到阻塞队列
        if(isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //如果当前线程池状态不是Running则从队列删除任务，并执行拒绝策略
            if(!isRunning(recheck) && remove(command))
                reject(command);
            //如果当前线程池为空，则添加一个线程    
            else if(workercountOf(recheck) == 0) 
                addWorker(null, false);    
        }
        //新增线程失败则执行拒绝策略
        else if(!addWorker(command, false))
            reject(command);
    }

    ---addWorker
    private boolean addWorder(Runnable firstTask, boolean core) {
        retry:
        for(;;) {
            int c = ctl.get();
            int rs = runStateOf(c);
            //检查当前线程池状态是否是shutdown、stop、tidying、terminated且!(当前状态为shutdown、传入的任务为null、队列不为null),条件都成立时返回false
            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();
                if(runStateOf(c) != rs) 
                    continue retry;    
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try{
            // new thread
            w = new Worker(firstTask);
            final Thread t = w.thread();
            if(t != null) {
                //lock
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try{
                    //check thread pool state
                    int rs = runStateOf(ctl.get());
                    if(rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null) ) {
                        //检查线程是否可启动
                        if(t.isAlive())
                            throw new IllegalThreadStateException();
                        workers.add(w)   ;
                         int s = workers.size();
                         if(s > largestPoolSize)
                            largestPoolSize = s;
                         workerAdded = true;    
                    } 
                }finally {
                    mainLock.unlock();
                }
                //判断worker是否添加成功，成功则启动线程，将workerStarted设置为true
                if(workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        }finally{
            if(!workerStarted) 
                addWorkerFailed(w);
        }
        return workerStarted;
    }
    ---runWorker,线程启动时调用了runWorker方法
    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock();
        boolean completedAbruptly = true;
        try {
            //循环获取任务
            while(task != null || (task = getTask()) != null ) {
                w.lock();
                // 当线程池是处于STOP状态或者TIDYING、TERMINATED状态时，设置当前线程处于中断状态
                // 如果不是，当前线程就处于RUNNING或者SHUTDOWN状态，确保当前线程不处于中断状态
                // 重新检查当前线程池的状态是否大于等于STOP状态
                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 Error(x);
                    }finally {
                        afterExecute(taskm thrown);
                    }
                }finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                } 
            }
            completedAbruptly = false;
        }finally {
            processWokerExit(w, completedAbruptly);
        }
    }

    ---getTask
    private Runnable getTask() {
        boolean timeOut = false;
        for(;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            if(rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty() )) {
                decrementWorkerCount();
                return null;
            }
            int wc = workerCountof(c);
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            //（当前线程数是否大于最大线程数或者）
            //且（线程数大于1或者任务队列为空）
            //这里有个问题(timed && timedOut)timedOut = false，好像(timed && timedOut)一直都是false吧
            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;
            }
        }
    }

    ---shutdown 关闭线程池
    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try{
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown();
        }finally{
            mainLock.unlock();
        }
        tryTerminate();
    }