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" %>
...
<ul>
    <li>Exception: <c:out value="${requestScope['javax.servlet.error.exception']}" /></li>
    <li>Exception type: <c:out value="${requestScope['javax.servlet.error.exception_type']}" /></li>
    <li>Exception message: <c:out value="${requestScope['javax.servlet.error.message']}" /></li>
    <li>Request URI: <c:out value="${requestScope['javax.servlet.error.request_uri']}" /></li>
    <li>Servlet name: <c:out value="${requestScope['javax.servlet.error.servlet_name']}" /></li>
    <li>Status code: <c:out value="${requestScope['javax.servlet.error.status_code']}" /></li>
</ul>

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" %>
<jsp:useBean id="date" class="java.util.Date" />
...
<ul>
    <li>Timestamp: <fmt:formatDate value="${date}" type="both" dateStyle="long" timeStyle="long" /></li>
    <li>User agent: <c:out value="${header['user-agent']}" /></li>
</ul>

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" %>
...
<pre>${pageContext.out.flush()}${exception.printStackTrace(pageContext.response.writer)}</pre>

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:

<?xml version="1.0" encoding="UTF-8" ?>
<taglib 
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
    version="2.1">

    <display-name>Custom Functions</display-name>    
    <tlib-version>1.0</tlib-version>
    <uri>http://example.com/functions</uri>

    <function>
        <name>printStackTrace</name>
        <function-class>com.example.Functions</function-class>
        <function-signature>java.lang.String printStackTrace(java.lang.Throwable)</function-signature>
    </function>
</taglib>

And can be used as

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

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;
}

Answer 2

yes, In my opinion session is a good place to store exceptions relating to the current request.

Don't forget to clear the exception after you finish handling it.

Also, you can pass instead of exception, an error code from your backing code to your presentation layer, where that can be translated using property files to some kind of meaning full error for the user.