HttpRequest对象的getSession()方法

HttpRequest对象有两种形式的getSession方法调用：
1、getSession()，
2、getSession(boolean isNew)

这样的，前者会检测当前时候有session存在，如果不存在则创建一个，如果存在就返回当前的。
getSession()相当于getSession(true)，

参数为true时，若存在会话则返回该会话，否则新建一个会话。
参数为false时，若存在会话则返回该会话，否则应该返回一个NULL

这是文档：

getSession
public HttpSession getSession(boolean create)Returns the current HttpSession associated with this request or, if if there is no current session and create is true, returns a new session.
If create is false and the request has no valid HttpSession, this method returns null.

To make sure the session is properly maintained, you must call this method before the response is committed. If the container is using cookies to maintain session integrity and is asked to create a new session when the response is committed, an IllegalStateException is thrown.

Parameters:
true - to create a new session for this request if necessary; false to return null if there's no current session
Returns:
the HttpSession associated with this request or null if create is false and the request has no valid session