This last April I did a presentation at the Twin Cities Code Camp on Microsoft Silverlight and SOA with a Grails server. I ended up writing a simple Grails application that used several web services to communicate to an in-browser Silverlight application. I specifically wanted to show a Silverlight application interacting with non Microsoft Technologies. I developed the Grails application on Linux on a different physical machine than what I used to develop the Silverlight application.
One of the Issues I ran into was that I was unable to make requests to the XML Web Services in the Grails application. It puzzled me for a minute until a quick Google search turned up a simple issue: I needed a crossdomain.xml policy file (or the clientaccesspolicy.xml). Flash/Flex users run into this all the time and thus most of what you will find is Flash centric. What is the crossdomain.xml file? Well its a way of restricting the domains that can access services. Its basically a white-listing of domains that are allowed to access the services. The browser and in-browser applications are supposed to respect the crossdomain.xml, and sometimes the Services (server-side) may protect themselves. You can think of it as a robots.txt for Web services.
Great, I knew what the problem was, now how do I fix it? I tried a few things, deploying to tomcat, but that didn’t work for me while I was actively developing the application. Once I understood a little more about Grails and Jetty, I realized that I could just modify the Jetty server that launched when invoking grails run-app. I simply had to add another context to Jetty, and bingo it worked. Here is what I did:
I found Grails’ RunApp.groovy script (the one that gets invoked on grails run-app) which was located at $GRAILS_HOME\scripts\RunApp.groovy. (%GRAILS_HOME%\scripts\RunApp.groovy for you Windows folk 
). I had to simply create another context much like the Grails application context was being created. Here is a stripped down example of what RunApp.groovy looked like. (modifications in Bold)
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
/**
* Gant script that executes Grails using an embedded Jetty server
*
* @author Graeme Rocher
*
* @since 0.4
*/
…
grailsContext = null
rootContext = null
…
target( configureHttpServer : "Returns a jetty server configured with an HTTP connector") {
…
setupWebContext()
setupRootWebContext()
server.setHandler( webContext )
server.addHandler( rootContext )
…
}
target( setupRootWebContext: "Sets up the Secondary Root Context"){
rootContext = new WebAppContext("${basedir}/web-app-root","/")
}
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
Most of the magic is in the rootContext = new WebAppContext("${basedir}/web-app-root","/") line. Notice that I had to create a new folder ‘web-app-root’ which resided alongside web-app (I think I used web-app for a while too). So this context responds to everything in the "/" domain which is the root of the site. Once I put my crossdomain.xml file in that folder, I could access http://localhost:8080/crossdomain.xml and the services were then accesible via Silverlight — Yay!
I’m sure there are better ways of doing this, but this is what I did to get the job done. Thanks to JT Dev for his most recent post, which reminded me that I was going to blog about this. I basically did Solution #2 in his blog post on creating multiple jetty contexts. Where was this post back in March? Thanks JT for tipping me off to the Static Resources Plugin!
