Wednesday, August 18, 2010

Tomcat and the web.xml's webapprootkey

I n my company, I have to deal with many Spring based applications deployed on a Tomcat cluster. For each one, we set a property 'webapprootkey' in the web.xml file to avoid errors on startup. Today, I decided to dig about that (... erratum... I decided to tell Google to dig about that). Thus, I found this excellent post.

Hi all,

when deploying two applications build from the riot skeleton within the same Tomcat servlet container, you get an IllegalStateException: Web app root system property already set to different value: 'webapp.root' = [/Users/joe/Workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/webapps/webapp-A/] instead of [/Users/joe/Workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/webapps/webapp-B/] - Choose unique values for the 'webAppRootKey' context-param in your web.xml files!

I will try to explain where this comes from and how to circumvent it, but first the quick fix for the impatient reader: Place a context parameter named 'webAppRootKey' in every project's web.xml and assign a value to it, that is unique for every of your projects like the project name itself.

The webAppRootKey context parameter is introduced by Spring. Along with the WebAppRootListener it allows exposing the web applications root directory as a system property. The value of the context parameter 'webAppRootKey' names the system property to use. If the context parameter 'webAppRootKey' is not set in the application's web.xml, Spring chooses the default value 'app.root'. While some servlet containers like Resin do isolate each web application's system properties, others like Tomcat do not. And that's what the former mentioned IllegalStateException is telling us: The system property 'app.root' already contains the root directory of the first web application when Spring tries to assign the root directoty of the second application to it.

Ok, that's the background information. A deeper look into the web.xml tells us, that there ist no WebAppRootListener configured. Why does this initialisation take place anyway? The stack trace from the exception reveals the culprit: The Log4jConfigListener also tries to set the webAppRootKey, because this is an interesting mechanism for the Spring/Log4j integration. It allows log and config file locations relative to the web applications root directory. The Log4jConfigListener supports three init parameters at the servlet context level: 'log4jConifgLocation', 'log4jRefreshInterval' and 'log4jExposeWebAppRoot'. See JavaDocs for more informations.

But, none of these parameters are set in the riot project skeleton's web.xml and none of the Log4jWebConfigureres features are used by the riot project skeleton. As long as you do stay with default log4j setup, the Log4jConfigListener is superflous.

At the end there are three possible solutions for the initial problem:

(1) Provide any of your applications with a unique 'webAppRootKey'.
(2) Set the servlet context parameter 'log4jExposeWebAppRoot' to 'false'. This eliminates the use of log file locations relative to the web application's root directory but still allows a log4j config location outside the classpath.
(3) Remove the 'Log4jConfigListener' from your application's web.xml.

What do you think is the best solution and should be incorporated into the riot skeleton project?


Alf Werder

Technische Leitung
Head of Engineering

Display Javascript console logs with jsfiddle

Jsfiddle is mainly made to test javascript code in a web context (aka with HTML and CSS). But, if you just want to use is to test backend c...