getSession(true)和getSession(),有什么区别

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.

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
帮助上是这么说的，
但实际用起来好像 getSession(true)有时会自动创建一个新的session，而把原来的session替换掉