ThreadPoolExecutor的线程池控制策略

ThreadPoolExecutor有三个参数是用来管理内部池的大小，分别是corePoolSize，maximumPoolSize，workQueue

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        for (;;) {
            if (runState != RUNNING) {
                reject(command);
                return;
            }
            if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
                return;
            if (workQueue.offer(command))
                return;
            Runnable r = addIfUnderMaximumPoolSize(command);
            if (r == command)
                return;
            if (r == null) {
                reject(command);
                return;
            }
            // else retry
        }
    }

从源码可以看出，池控制的策略如下：

1. 如果当前池容量小于corePoolSize时，启动新的线程去执行command任务

2. 当前池容量大于corePoolSize，但是小于maximumPoolSize时，将任务加入到workQueue中，直至队列满；这种情况下，任务会由线程池中的空闲线程取出执行

3. 当线程队列workQueue也满了的情况下，若池容量尚未达到maximumPoolSize时，则将处于workQueue的队列头的任务取出，并启动新的线程来执行；此时应用要求执行的command任务既没有放入队列，也没有在线程中执行，而是再次进入这个池控制流程，直至该command任务被处理

下面是JAVA API中对线程队列的一段描述：

• If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.
• If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.
• If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.