经典的读者写者问题。设计读写线程，实现对同一文件的读和写的同步。

1.一个线程在写时，其他线程不允许写。

2.一个线程在写时，其他线程不允许读。

3.一个线程在读时，其他线程不允许写。

4.一个线程在读时，其他线程允许读。

-readers to write the classic problem. Design literacy threads, to achieve the same document simultaneously read and write. 1. A thread in writing, the other threads allowed to write. 2. A thread in writing, the time does not allow other threads. 3. A thread in reading, writing does not allow other threads. 4. A thread in Reading, Reading allow other threads.