线程池

TimeUnit.DAYS;               //天
TimeUnit.HOURS;             //小时
TimeUnit.MINUTES;           //分钟
TimeUnit.SECONDS;           //秒
TimeUnit.MILLISECONDS;      //毫秒
TimeUnit.MICROSECONDS;      //微妙
TimeUnit.NANOSECONDS;       //纳秒

ArrayBlockingQueue：//是一个基于数组结构的有界阻塞队列，创建时必须指定大小，此队列按 FIFO（先进先出）原则对元素进行排序
LinkedBlockingQueue：//一个基于链表结构的无界阻塞队列，此队列按FIFO （先进先出） 排序元素，吞吐量通常要高于ArrayBlockingQueue，默认大小为Integer_Max_Value，静态工厂方法Executors.newFixedThreadPool()使用了这个队列
SynchronousQueue;//一个不存储元素的阻塞队列。直接新建线程来执行任务，每个插入操作必须等到另一个线程调用移除操作，否则插入操作一直处于阻塞状态，吞吐量通常要高于LinkedBlockingQueue，静态工厂方法Executors.newCachedThreadPool使用了这个队列
PriorityBlockingQueue：//一个具有优先级的无界阻塞队列。

ThreadPoolExecutor.AbortPolicy()
//丢弃任务并抛出java.util.concurrent.RejectedExecutionException异常
ThreadPoolExecutor.CallerRunsPolicy()
//重试添加当前的任务，在 execute 方法的调用线程中运行被拒绝的任务
ThreadPoolExecutor.DiscardOldestPolicy()
//抛弃队列最前面的任务，然后重新尝试执行任务（重复此过程）
ThreadPoolExecutor.DiscardPolicy()
//抛弃任务且不抛异常

public void execute(Runnable command) {
    if (command ==null)
            thrownew NullPointerException();
            intc = ctl.get();

    　　//1 当前运行的线程数量小于核心线程数量，直接将任务加入worker启动运行。
        if (workerCountOf(c) <corePoolSize) {
            if (addWorker(command,true))
                return;
            c =ctl.get();
        } 
        
        //2 运行线程数量大于核心线程数量时，上面的if分支针对大于corePoolSize，并且缓存队列加入任务操作成功的情况。
          运行中并且将任务加入缓冲队列成功，正常来说这样已经完成了处理逻辑。
          但是为了保险起见，增加了状态出现异常的确认判断，如果状态出现异常会继续remove操作，如果执行true，则按照拒绝处理策略驳回任务；
        if (isRunning(c) &&workQueue.offer(command)) {
            intrecheck = ctl.get();
            if (!isRunning(recheck) && remove(command))
                reject(command);
            elseif (workerCountOf(recheck) == 0)
                addWorker(null,false);
        }

         //3 这里针对运行线程数量超过了corePoolSize，并且缓存队列也已经放满的情况。
           注意第二个参数是false，可以在下面addWorker方法看到，就是针对线程池最大线程数量maximumPoolSize的判断。
           elseif (!addWorker(command,false))
            reject(command);
        }

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
            }
        }
//以上是线程数量的校验与更新逻辑
        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;
    }

public void run() {
            runWorker(this);
        }
final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        //task就是Woker构造函数入参指定的任务，即用户提交的任务
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); 
        boolean completedAbruptly = true;
        try {
            while (task != null || (task = getTask()) != null) {
                w.lock();
                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);
        }
    }

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

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