ThreadPoolExecutor

1、构造方法

• corePoolSize ： 核心线程数
• maximumPoolSize ： 最大线程数量
• keepAliveTime ： 空闲存活时间
• unit ：时间单位
• workQueue ：阻塞队列，用来存储等待执行的任务。
  • ArrayBlockingQueue：数组有界队列，先进先出
  • DelayQueue ： 无界阻塞队列，延迟期满才能提取元素
  • LinkedBlockingQueue：链式结构，先进先出
  • PriorityBlockingQueue：支持优先级排序的无界队列，可以自定义实现compareTo()方法指定元素排序。不能保证优先级元素的顺序
  • SynchronousQueue ：容纳单元素
  • BlockingDeque ：双端队列，在不能够插入元素的时候，阻塞插入。没有元素抽取的时候，阻塞抽取。
  • LinkedBlockingDeque：是双向链表实现的双向并发阻塞队列。该阻塞队列同时支持FIFO和FILO两种操作方式，即可以从队列的头和尾同时操作(插入/删除)；并且，该阻塞队列是支持线程安全。此外，LinkedBlockingDeque还是可选容量的(防止过度膨胀)，即可以指定队列的容量。如果不指定，默认容量大小等于Integer.MAX_VALUE
• threadFactory：线程工厂，用来创建线程
• handler ：饱和策略
  • CallerRunsPolicy：用调用者所在的线程来执行任务
  • AbortPolicy ：默认策略，直接抛出异常
  • DiscardPolicy ：直接丢弃任务
  • DiscardOldestPolicy：丢弃队列最前的任务，并执行当前任务

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;
    }

2、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.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }