old《1.1.7 线程池应用及实现原理剖析》

为什么ThreadPoolExecutor使用的是BlockingQueue？参考不怕难之BlockingQueue及其实现，因为：

入队方法比如有：
offer(E e)：如果队列没满，立即返回true； 如果队列满了，立即返回false–>不阻塞
put(E e)：如果队列满了，一直阻塞，直到队列不满了或者线程被中断–>阻塞
JDK文档对put(E e)的描述：Inserts the specified element into this queue, waiting if necessary for space to become available. Queue类中无此方法。

出队方法比如有：
poll()：如果没有元素，直接返回null；如果有元素，出队
take()：如果队列空了，一直阻塞，直到队列不为空或者线程被中断–>阻塞
JDK文档对take()的描述：Retrieves and removes the head of this queue, waiting if necessary until an element becomes available. Queue类中无此方法。

Executors.newCachedThreadPool()所使用的SynchronousQueue是BlockingQueue的一种特殊实现，JDK文档对它有如下描述：
A blocking queue in which each insertoperation must wait for a corresponding remove operation by anotherthread, and vice versa. A synchronous queue does not have anyinternal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is onlypresent when you try to remove it; you cannot insert an element(using any method) unless another thread is trying to remove it;you cannot iterate as there is nothing to iterate.

为什么Executors.newCachedThreadPool()使用SynchronousQueue：