Servlets-Session属性变为NULL

Question

I am new to servlets and jsp. In my program the flow is as below:

loginpage.html -> controller (servlet - here I created session like this)

HttpSession session = request.getSession(true);
session.setAttribute("uid", uname);
System.out.println("session:"+session.getAttribute("uid"));// shows the value of uid

-> create_user.html -> conroller ( now it shows uid value as null)-> view_customers.jsp (need uid value here but its null ).

How to avoid session attribute from becoming null? Thanks in advance!

Answer 1

How to avoid session attribute from becoming null?

You can monitor the attribute state (added/removed/replaced) in session scope using HttpSessionAttributeListener .

Sample code:

public class MySessionAttributeListener implements HttpSessionAttributeListener {

    public MySessionAttributeListener() {
    }

    public void attributeAdded(HttpSessionBindingEvent sessionBindingEvent) {

        // Get the session
        HttpSession session = sessionBindingEvent.getSession();

        // Log some information
        System.out.println("[SessionAttr] " + new java.util.Date()
                + " Attribute added, session " + session + ": " + sessionBindingEvent.getName()
                + "=" + sessionBindingEvent.getValue());
    }

    public void attributeRemoved(HttpSessionBindingEvent sessionBindingEvent) {

        // Get the session
        HttpSession session = sessionBindingEvent.getSession();

        // Log some information
        System.out.println("[SessionAttr] " + new java.util.Date()
                + " Attribute removed, session " + session + ": "
                + sessionBindingEvent.getName());
    }

    public void attributeReplaced(HttpSessionBindingEvent sessionBindingEvent) {

        // Get the session
        HttpSession session = sessionBindingEvent.getSession();

        // Log some information
        System.out.println("[SessionAttr] " + new java.util.Date()
                + " Attribute replaced, session " + session + ": "
                + sessionBindingEvent.getName() + "=" + sessionBindingEvent.getValue());
    }
}

web.xml: (add below line in web.xml)

<listener>
    <listener-class>com.x.y.z.MySessionAttributeListener</listener-class>
</listener>