When we want to debug our Ratpack application written in Java we can simply use the Debug action on the main application class. When we described the application with the Groovy DSL we must add a Groovy runtime configuration to our project in IntelliJ IDEA to support debugging of the (R|r)atpack.groovy script file.
Since Grails 2.2 by default the run-app command will launch the Grails application in a separate Java Virtual Machine. This is called forked Tomcat execution in Grails. This way the class path of the Grails build system and the application will not intervene and also both processes will have their own memory settings. We can see the settings in grails-app/conf/BuildConfig.groovy where we find the configuration property grails.project.fork.run. When we want to debug our application in an IDE like IntelliJ IDEA we cannot use the Debug command, because this will only allow us to debug the Grails build system. We will not reach breakpoints in our source code. But Grails 2.3 introduces an extra argument for the run-app command: --debug-fork. If we use this extra argument the JVM running the Grails application will stop and listen for a debug session to be attached and then continue. We can configure a Debug configuration in IntelliJ IDEA (or another IDE) to attach to the waiting Grails application and use breakpoints and other debugging tools like we are used to.