How to Properly Handle Exceptions in a JSP/Servlet App?

Question

How do you properly handle errors encountered in a servlet? Right now, the app that I inherited (uses only plain JSP/Servlet) has a superclass called Controller which extends HttpServlet and which all other servlets extend from. In that Controller class is a try and catch block like the following:

try {
    // execute doPost or doGet here
} catch (Exception e) {
    // show a generic error page
}

Is this the proper way of doing it? It seems clunky and doesn't seem to always work. I'm only an intern so I don't have a lot of experience with this. Any advice? I'm trying to make the app for robust..

Answer 1

The standard thing to do is have your Servlet's doXxx() method (eg. doGet(), doPost(), etc.) throw a ServletException and allow the container to catch and handle it. You can specify a custom error page to be shown in WEB-INF/web.xml using the <error-page> tag:

<error-page>
    <error-code>500</error-code>
    <location>/error.jsp</location>
</error-page>

If you end up catching an Exception you can't elegantly handle, just wrap it in a ServletException like this:

try {
    // code that throws an Exception
} catch (Exception e) {
    throw new ServletException(e);
}

Answer 2

Or you can intercept all your exceptions using a servlet:

<!-- WEB-INF/web.xml -->
<servlet>
    <servlet-name>ErrorServlet</servlet-name>
    <servlet-class>com.domain.MyErrorServlet</servlet-class>
</servlet>    
<servlet-mapping>
<servlet-name>ErrorServlet</servlet-name>
    <url-pattern>/error</url-pattern>
</servlet-mapping>
<error-page>
    <exception-type>java.lang.Throwable</exception-type>
    <location>/error</location>
</error-page>

Then in the servlet you can handle the exception like this

public class MyErrorServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response){
       Throwable throwable = (Throwable) request.getAttribute("javax.servlet.error.exception");
       // You can log the exception, send to email, etc
    }
}