In addition, avoid returning objects that are null or in an invalid state.
此外，避免返回空对象或者状态无效的对象。
If the file does not exist (as determined by the file open error), write the "null" state to the file.
如果该文件不存在(由文件打开错误确定)，则把“null”状态写入文件。
The second parameter (state) can either be null or an object containing application-specific information.
public void TimerCallback(objectstate);第二个参数（state）可以为空或者是包含程序规范信息的对象。
Any thread can tell which state the queue is in by comparing tail.next to null, which is the key to enabling threads to help other threads "finish" their operation.
任何线程通过比较tail . next是否为null，就可以判断出队列的状态，这是让线程可以帮助其他线程“完成”操作的关键。
In the quiescent state, the next field of the link node pointed to by the tail is always null; in the intermediate state, it is always non-null.
在静止状态时，尾指针指向的链接节点的next字段总为null，而在中间状态时，这个字段为非null。
If this function returns NULL, no exception has been raised and the caller should assume no current thread state is available.
如果这个函数返回NULL，没有抛出异常，调用者会假定当前线程状态无效。
If this function returns NULL, no exception has been raised and the caller should assume no current thread state is available.
如果这个函数返回NULL，没有抛出异常，调用者会假定当前线程状态无效。
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