再看看源码java.util.concurrent.ThreadPoolExecutor.java

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();

        int c = ctl.get();
        //判断当前活跃线程数是否小于corePoolSize
        if (workerCountOf(c) < corePoolSize) {
            //如果小于，则调用addWorker创建线程执行任务
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //如果大于等于corePoolSize，则将任务添加到workQueue队列
        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);
        }
       //如果放入workQueue队列失败，则创建非核心线程执行任务
        else if (!addWorker(command, false))
            reject(command);
    }