How to grab uncaught exceptions in a Java servlet web application

Question

Is there a standard way to catch uncaught exceptions that happen inside of a java servlet container like tomcat or Jetty? We run a lot of servlets that come from libraries so we cannot easily put our on try/catch code. It would also be nice to in as generic of a way as possible catch and log all uncaught exceptions in our web application (which runs in Jetty) to our bug tracker via the API provided.

Please not I need to log the exceptions only, whether a a redirect is issues to a custom error page will not help me. We do everything via GWT-RPC so the user would never see an error page.

Answer 1

I think a custom filter actually works best.

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    try {
        chain.doFilter(request, response);
    } catch (Throwable e) {
        doCustomErrorLogging(e);
        if (e instanceof IOException) {
            throw (IOException) e;
        } else if (e instanceof ServletException) {
            throw (ServletException) e;
        } else if (e instanceof RuntimeException) {
            throw (RuntimeException) e;
        } else {
            //This should never be hit
            throw new RuntimeException("Unexpected Exception", e);
        }
    }
}

Answer 2

In web.xml (the deployment descriptor) you can use the <error-page> element to specify error pages by exception type or HTTP response status code. For example:

<error-page>
    <error-code>404</error-code>
    <location>/error/404.html</location>
</error-page>
<error-page>
    <exception-type>com.example.PebkacException</exception-type>
    <location>/error/UserError.html</location>
</error-page>

For a NetBeans-centric description, mosey on over to Configuring Web Applications: Mapping Errors to Error Screens (The Java EE 6 Tutorial) (or see the Java EE 5 Tutorial's version).