What is the good approach to forward the exception from servlets to a jsp page?

Question

I know I can put something in the web.xml like this

  
   java.lang.Throwable  
   

        

Answer 1

The information about the exception is already available by several request attributes. You can find the names of all those attributes in the RequestDispatcher javadoc:

• ERROR_EXCEPTION - javax.servlet.error.exeption
• ERROR_EXCEPTION_TYPE - javax.servlet.error.exception_type
• ERROR_MESSAGE - javax.servlet.error.message
• ERROR_REQUEST_URI - javax.servlet.error.request_uri
• ERROR_SERVLET_NAME - javax.servlet.error.servlet_name
• ERROR_STATUS_CODE - javax.servlet.error.status_code

So, in a nutshell, this JSP example should display all the possible exception detail:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
...

    
Exception: 
    
Exception type: 
    
Exception message: 
    
Request URI: 
    
Servlet name: 
    
Status code: 

Additionally, you could also show this useful information:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>

...

    
Timestamp: 
    
User agent: 

The concrete Exception instance itself is in the JSP only available as ${exception} when you mark the page as an error page:

<%@ page isErrorPage="true" %>
...
${exception}

Only if you're using EL 2.2 or newer, then you can print its stacktrace as below:

<%@ page isErrorPage="true" %>
...
${pageContext.out.flush()}${exception.printStackTrace(pageContext.response.writer)}

Or if you're not on EL 2.2 yet, then create a custom EL function for that:

public final class Functions {

    private Functions() {}

    public static String printStackTrace(Throwable exception) {
        StringWriter stringWriter = new StringWriter();
        exception.printStackTrace(new PrintWriter(stringWriter, true));
        return stringWriter.toString();
    }

}

Which is registered in /WEB-INF/functions.tld:

    Custom Functions    
    1.0
    http://example.com/functions

    
        printStackTrace
        com.example.Functions
        java.lang.String printStackTrace(java.lang.Throwable)
    

And can be used as

<%@ taglib prefix="my" uri="http://example.com/functions" %>
...
${my:printStackTrace(exception)}

As to the logging of the exception, easiest place would be a filter which is mapped on an URL pattern of /* and does basically the following:

try {
    chain.doFilter(request, response);
} catch (ServletException e) {
    log(e.getRootCause());
    throw e;
} catch (IOException e) { // If necessary? Usually not thrown by business code.
    log(e);
    throw e;
}