在请求时，如果会话不存在可以告诉会话是否创建一个。
When requested, the session can be told whether or not to initiate one if it doesn't already exist.
第二个方法接受一个布尔参数，这个参数用于判定会话不存在时是否创建新会话。
The second method accepts a boolean argument that determines whether or not a new Session will be created if it does not yet exist.
如果不存在会话对象，SIF将创建一个新的安全上下文，并将它放到security context holder中。
IF the session object does not exist, SIF creates a new security context and puts it in the security context holder.
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