Web app root system property already set to different value 的解决办法
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?
不知道各位怎么样，反正我遇到这个问题呢，主要是新item的web.xml里面多了log4j的listener，希望对遇到相同问题的童鞋有所帮助！！不喜勿喷！
