Tomcat - Redirect to Error Page when ServletContextListener fails

Question

When Tomcat starts it calls my ServletContextListener to obtain a database connection, which I will later use in other servlets with getServletContext(). It is called in my web.xml as:
listener
listener-class org.ppdc.database.DBCPoolingListener /listener-class
/listener> (I removed the < > because they wouldn't display properly in this message.>
If I cannot connect to the database when Tomcat starts up I get a 404 error, because Tomcat cannot start the application.

How can I redirect the user to a custom error page at this point? I tried the following in my web.xml (I have the < > brackets in the original):
(error-page)
(error-code404/error-code)
(location/file_not_found.html/location)
(/error-page)

Any ideas on how to redirect a user to one of my error pages when Tomcat tries to start the application?

Thanks Vic

Answer 1

If your application fails to load, then that's it. Tomcat is not running it and does not serve your error-pages.

So, if you want to handle a half-dead state, you need to start in a half-dead state. Fortunately, the code in your servlets can be spared checks whether the app is half-dead if you install a Filter, that does it before control is transfered to any servlet.

Declare a filter in web.xml:

<filter>
  <filter-name>IsHalfDeadFilter</filter-name>
  <filter-class>my.package.IsHalfDeadFilter</filter-class>
</filter>

<filter-mapping>
  <filter-name>IsHalfDeadFilter</filter-name>
  <url-pattern>*</url-pattern>
</filter-mapping>

Then implement doFilter method to redirect to your error page.

@Override
public void doFilter (
        final ServletRequest request,
        final ServletResponse response,
        final FilterChain chain
    ) throws
        IOException,
        ServletException
{
    if ( isHalfDead )
    {
        // redirect to error page
        return;
    }

    chain.doFilter( request, response );
}

Read more about Filters here