Grails Tip of the Day: Always use packages

First of all I must say I'm lazy and I don't like overhead... That's also the reason why I don't use packages for Grails domain classes. Most Grails applications I've build so far consist of no more then 20 domain classes, so I had never the need to separate them in packages. In fact I like to have them in the top folder; no overhead of subfolders.

As Grails 1.1 was released I decided to restart Grails on Sakila. I recreated the domain classes including a "Category" domain class. I generated the default Controller and Views without problems and started the application. When browsing to the list page of the Category domain class I got (GRAILS-4233):

Welcome to Grails 1.1 - http://grails.org/
Licensed under Apache Standard License 2.0
Grails home is set to: D:\Grails\grails-1.1

Base Directory: D:\Grails\projects\grails-on-sakila
Running script D:\Grails\grails-1.1\scripts\RunApp.groovy
Environment set to development
  [groovyc] Compiling 2 source files to D:\Users\moverdijk\.grails\1.1\projects\grails-on-sakila\classes
  [groovyc] org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed, D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 40: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 40, column 32.
  [groovyc]            def categoryInstance = new Category()
  [groovyc]                                   ^
  [groovyc] D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 46: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 46, column 32.
  [groovyc]            def categoryInstance = new Category(params)
  [groovyc]                                   ^
  [groovyc] D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 40: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 40, column 32.
  [groovyc]            def categoryInstance = new Category()
  [groovyc]                                   ^
  [groovyc] D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 46: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 46, column 32.
  [groovyc]            def categoryInstance = new Category(params)
  [groovyc]                                   ^
  [groovyc] D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 40: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 40, column 32.
  [groovyc]            def categoryInstance = new Category()
  [groovyc]                                   ^
  [groovyc] D:\Grails\projects\grails-on-sakila\grails-app\controllers\CategoryController.groovy: 46: You cannot create an instance from the abstract interface 'groovy.lang.Category'.
  [groovyc]  @ line 46, column 32.
  [groovyc]            def categoryInstance = new Category(params)
  [groovyc]                                   ^
  [groovyc] 
  [groovyc] 6 errors
Compilation error: Compilation Failed

First thing I thought I made some mistake but this wasn't the case. The problem is that Groovy resolves the Category class to groovy.lang.Category and not to my Grails domain class. This means if you have domain classes with the same name as a class in groovy.lang and don't use packages you will get into troubles.

So from now on I will create/put my Grails artifacts in a package, which is a good practice anyway. When running the Grails command line scripts to create artifacts you can specify the package so Grails automatically creates the package structure: e.g.:

grails create-domain-class org.company.Book

PS: I you want to read more about Groovy Categories have a look at the Groovy User Guide.