GStringImpl cannot be cast to java.lang.String

Posted by Erik Pragt on Aug 29, 2011 in Grails, Groovy | 1 comment

Last week, we got a nasty error in our application:

java.lang.ClassCastException: org.codehaus.groovy.runtime.GStringImpl cannot be cast to java.lang.String

The stacktrace was not helpful at all, and showed that the error was caused by a NullPointerException in Melody, which was a false message. After some searching around, we found the following code to be the culprit:

render(view: 'index', model: ["${command.class.simpleName}": command)

What happens here, is that a GString is used as a map key for rendering the model in the view. It’s never a good idea to use GStrings as a Map key, since the hashCode of GStrings are not guaranteed to be stable. A better approach is to use a String instead, which can be done using the following code:

render(view: 'index', model: [(command.class.simpleName): command)

The above is our current fix, and we changed our CodeNarc GString as Map key to a prio 1 rule. Whenever a prio 1 error occurs, our build will fail, preventing future errors like this.