request.getSession(false)和request.getSession(true)区别
getSession(boolean create)意思是返回当前reqeust中的HttpSession ，如果当前reqeust中的HttpSession 为null，当create为true，就创建一个新的Session，否则返回null；简而言之：HttpServletRequest.getSession(ture) 等同于 HttpServletRequest.getSession() HttpServletRequest.getSession(false) 等同于 如果当前Session没有就为null；【问题和bug】：我周围很多同事是这样写的；view plaincopy to clipboardprint?HttpSession session = request.getSession(); // a new session created if no session exists，如果session不存在的话你又创建了一个！ String user_name = session.getAttribute("user_name"); HttpSession session = request.getSession(); // a new session created if no session exists， 如果session不存在的话你又创建了一个！String user_name = session.getAttribute("user_name2");需要注意的地方是request.getSession() 等同于 request.getSession(true)，除非我们确认session一定存在或者sesson不存在时明确有创建session的需要，否则尽量使用request.getSession(false)。在使用request.getSession()函数，通常在action中检查是否有某个变量/标记存放在session中。这个场景中可能出现没有session存在的情况，正常的判断应该是这样：view plaincopy to clipboardprint?HttpSession session = request.getSession(false); if (session != null) { String user_name = session.getAttribute("user_name"); } HttpSession session = request.getSession(false);if (session != null) { String user_name = session.getAttribute("user_name");} 【投机取巧】：如果项目中用到了Spring（其实只要是Java的稍大的项目，Spring是一个很好的选择），对session的操作就方便多了。如果需要在Session中取值，可以用WebUtils工具（org.springframework.web.util.WebUtils）的getSessionAttribute(HttpServletRequest request, String name)方法，看看高手写的源码吧：哈哈。。view plaincopy to clipboardprint?/** * Check the given request for a session attribute of the given name. * Returns null if there is no session or if the session has no such attribute. * Does not create a new session if none has existed before! * @param request current HTTP request * @param name the name of the session attribute * @return the value of the session attribute, or <code>null</code> if not found */ public static Object getSessionAttribute(HttpServletRequest request, String name) { Assert.notNull(request, "Request must not be null"); HttpSession session = request.getSession(false); return (session != null ? session.getAttribute(name) : null); } /** * Check the given request for a session attribute of the given name. * Returns null if there is no session or if the session has no such attribute. * Does not create a new session if none has existed before! * @param request current HTTP request * @param name the name of the session attribute * @return the value of the session attribute, or <code>null</code> if not found */ public static void setSessionAttribute(HttpServletRequest request, String name, Object value) { Assert.notNull(request, "Request must not be null"); if(value != null) { request.getSession().setAttribute(name, value); } else { HttpSession session = request.getSession(false); if(session != null) session.removeAttribute(name); } }注：Assert是Spring工具包中的一个工具，用来判断一些验证操作，本例中用来判断reqeust是否为空，若为空就抛异常。上面的代码又可以简洁一下啦，看吧：view plaincopy to clipboardprint? String user_name = WebUtils.getSessionAttribute(reqeust, "user_name");一般在获得一个属性的值时用request.getSession(false)，设置值时用request.getSession()