Global Exception Handling for Struts and Tiles Applications

Update: The solution explained in this post has been tested only on Weblogic 8.1 and 9.2. On Tomcat 5.x this will not work and please post a comment with the results if you happen to test this on any other server or find a work around for the Tomcat issue.

I was looking at implementing a global error page for unhandled exceptions in a Struts / Tiles application. Normally this was achieved by adding a section in struts-config.xml with a custom exception handler to log the exceptions and the url to the global error page.

This would forward to the error page if an unhandled exception was thrown from the Struts action but will not do so if the exception occurred in one of the tiles JSPs. An exception in a tile would result in a partly rendered page but this was fine in previous app as there were hardly any logic in JSP files and it was very rare to have an exception at the JSP level in production.

In the new app though this wasn’t the case as there were lot of logic that went in to JSPs in the form of custom tags etc… that could potentially throw all sorts of exceptions at run time and I wanted to handle all these exceptions in a unified manner.

New Solution

I abandoned the global-exceptions approach in struts-config altogether and added a <error-page> section to the web.xml file which instructed the app server to forward the request to the specified URL on all unhandled exceptions.

<error-page>
<exception-type>java.lang.Throwable</exception-type>
<location>/error </location>
</error-page>

Then added a servlet and a servlet mapping to the /error URL

<servlet>
<servlet-name>exceptionHandler</servlet-name>
<servlet-class>com.test.ExceptionHandlerServlet</servlet-class>
<init-param>
<param-name>errorPageURL</param-name>
<param-value>error.html</param-value>
</init-param>
<load-on-startup>3</load-on-startup>
</servlet>


<servlet-mapping>
<servlet-name>exceptionHandler</servlet-name>
<url-pattern>/error</url-pattern>
</servlet-mapping>

The ExceptionHandlerServlet would log the exception and redirect the browser to the URL defined in the servlet configuration (error.html).

ExceptionHandlerServlet Code:

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { try { logger.debug("Handling exception in ErrorHandlerServlet"); Throwable exception = null; // Check if struts has placed an exception object in request Object obj = request.getAttribute(Globals.EXCEPTION_KEY); if (obj == null) { // Since no struts exception is found, // check if a JSP exception is available in request. obj = request.getAttribute("javax.servlet.jsp.jspException"); } if ((obj != null) && (obj instanceof Throwable)) { exception = (Throwable) obj; } if (logger.isDebugEnabled()) { logger.debug("Request URI: " + request.getAttribute("javax.servlet.forward.request_uri")); } // request uri containing the original URL value will be available // only on servers implementing servlet 2.4 spec String requestURI = (String) request.getAttribute("javax.servlet.forward.request_uri"); logger.error("Exception while handling request: " + requestURI, exception); response.sendRedirect(errorPageURL); } catch (Exception e) { // Throwing exceptions from this method can result in request // going in to an infinite loop forwarding to the error servlet recursively. e.printStackTrace(); } }

Now on all unhandled exceptions the servlet will log the exception. Yet on some occasions where the exception was thrown in one of the tile JSPs the browser would display a partially rendered page and would not redirect to the error page.

This was due to tiles flushing content to the response buffer before the whole page was rendered. In such a situation the browser will happily display what it received and the redirect to the error page will have no effect.

In order to prevent or minimize the flushing of response buffer:

1. Set flush attribute to false in all tiles insert tags
2. Increase the response buffer size to minimize auto flushing

This was achieved by modifying the tiles layout jsp page.

<!-- sets the response buffer size to 64kb -->
<%@ page buffer="64kb"%>
<%@ taglib uri="/WEB-INF/struts-tiles.tld" prefix="tiles"%>
…
<tiles:insert attribute="header" flush="false" />
<tiles:insert attribute="leftnav" flush="false" />
<tiles:insert attribute="body" flush="false" />
<tiles:insert attribute="footer" flush="false" />

In the above code segment all html formatting was removed for clarity. The response buffer was set to 64 kilobytes and this value should be decide based on the average page size of the application and the performance impact of the increased memory usage.