Tomcat Interview Questions and Answers

Question: How can I access members of a custom Realm or Principal?

Answer: When you create a custom subclass of RealmBase or GenericPrincipal and attempt to use those classes in your webapp code, you'll probably have problems with ClassCastException. This is because the instance returned by request.getUserPrincipal() is of a class loaded by the server's classloader, and you are trying to access it through you webapp's classloader. While the classes maybe otherwise exactly the same, different (sibling) classloaders makes them different classes.


This assumes you created a My Principal class, and put in Tomcat's server/classes (or lib) directory, as well as in your webapp's WEB-INF/classes (or lib) directory. Normally, you would put custom realm and principal classes in the server directory because they depend on other classes there.

Principal p = request.getUserPrincipal(); 

String emailAddress = p.getClass().getMethod("getEmailAddress", null).invoke(p, null);

You could put your custom classes in a classloader that is common to both the server and your webapp - e.g., either the "common" or bootstrap classloaders. To do this, however, you would also need to move the classes that your custom classes depend on up to the common classloader, and that seems like a bad idea, because there a many of them and they a core server classes. 

Rather than move the implementing custom classes up, you could define interfaces for your customs classes, and put the interfaces in the common directory. You're code would look like this: 

public interface MyPrincipalInterface extends java.security.Principal { 

public String getEmailAddress();

}

public class MyPrincipal implements MyPrincipalInterface { 

...

public String getEmailAddress() {

return emailAddress;

}

}

public class MyServlet implements Servlet { 

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws

ServletException, IOException {

MyPrincipalInterface p = (MyPrincipalInterface)request.getUserPrincipal();

String emailAddress = p.getEmailAddress();

...

}

Notice that this method gives you pretty much the webapp code you wanted in the first place

You might want to try serializing the response of 'request.getUserPrincipal()' and deserialize it to an instance of [webapp]MyPrincipal.