Checking if session exists problem

[shaf maff]

Hi Guys

I have the following code:

HttpSession checkUserSession = request.getSession(false);
user = (UserSession) checkUserSession.getAttribute( "UserSession" );
if(user == null){ response.sendRedirect("../../Login"); }// Redirect if no session

The above code checks if a session exists. If it doesnt then the page should redirect to the login page. The problem is whenever the session cannot be found I get a 500 java.lang.NullPointerException error. Does anyone know where I am going wrong ?

[Steve Luke]

Originally posted by shaf maff:
Hi Guys

I have the following code:

HttpSession checkUserSession = request.getSession(false);
user = (UserSession) checkUserSession.getAttribute( "UserSession" );
if(user == null){ response.sendRedirect("../../Login"); }// Redirect if no session

The above code checks if a session exists. If it doesnt then the page should redirect to the login page. The problem is whenever the session cannot be found I get a 500 java.lang.NullPointerException error. Does anyone know where I am going wrong ?

What that code does is check if the user has signed in yet by checking if a token was placed in a session. To check if a session exists you need to do this:

//Get Session if it already exists (don't create a new one)
HttpSession checkUserSession = request.getSession(false);
if (checkUserSession == null) {
  //no session yet
  //Do redirect?
} else {
  //There is a session, but is the user logged in?
  user = (UserSession) checkUserSession.getAttribute( "UserSession" );
  if(user == null){ response.sendRedirect("../../Login"); }
}

Or you could force a new session to be made if one doesn't already exist, and check it for the user token (which won't be there if there was no session to start with):

//Get session, making a new one if one doesn't exist
HttpSession checkUserSession = request.getSession(true);
user = (UserSession) checkUserSession.getAttribute( "UserSession" );
if(user == null){ response.sendRedirect("../../Login"); }// Redirect if no session

[shaf maff]

Hi Steve,

I tried your suggestion and I still get the same error when I force tomcat to expire the session. See error followed by the code:

25-Sep-2008 05:43:57 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet GETContent threw exception
java.lang.NullPointerException
at Get.doGet(Get.java:241)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)

LINE 241 Get.java: lettersession.constructLetters(request, user.getId());

code:

//Check Session
HttpSession usersession = request.getSession(false);
if(usersession == null) { response.sendRedirect("../../Login"); }
else { 
user = (UserSession) usersession.getAttribute("UserSession");
if(user == null) {
response.sendRedirect("../../Login");
}
}

[William Brogden]

Your code should include a return after the sendRedirect.
Bill

[shaf maff]

Do you mean a return false after each statment ? I tried that and it gives me a cannot return value from a method whose result type is void.

[Steve Luke]

lettersession.constructLetters(request, user.getId());

Then either lettersession or user is null when you try to use it. Figure out which and prevent it from causing an exception.

Originally posted by shaf maff:
Do you mean a return false after each statment ? I tried that and it gives me a cannot return value from a method whose result type is void.

No, not return false. Just return; The idea is that once you do a sendRedirect you want to make sure that none of the rest of the code in the method (the code after the if/else statements) gets executed.

//Check Session
HttpSession usersession = request.getSession(false);
if(usersession == null) 
{ 
    //session is null, send to login
    response.sendRedirect("../../Login"); 
    return;
}
 
//session not null, get user
user = (UserSession) usersession.getAttribute("UserSession");
if(user == null) 
{
    //user is null, send to login
    response.sendRedirect("../../Login");
    return;
}
 
//user already logged in
//Rest of code here

An alternative is to put all your code inside the if/else clauses so there doesn't need to be artificial returns:

//Check Session
HttpSession usersession = request.getSession(false);
if(usersession == null) 
{ 
    //Session is null, send to login
    response.sendRedirect("../../Login"); 
}
else 
{
    //Session not null, get user 
    user = (UserSession) usersession.getAttribute("UserSession");
    if(user == null) 
    {
        //user is null, send to login
        response.sendRedirect("../../Login");
    }
    else
    {
        //user is already logged in
        //do the rest of your code
    }
}

Which you do depends on style.
[ September 26, 2008: Message edited by: Steve Luke ]