request.getSession()与getSession(boolean isNew)

HttpRequest对象有两种形式的getSession方法调用：
一个是getSession()，
另一个是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