Converting Grails Applications to Plugins and vice versa

Burt BeckwithJuly 29th, 2013Last Updated: July 29th, 2013

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I was in London last week on vacation with my family and was lucky that there was a London GGUG meetup during my visit. David Dawson discussed modularizing Grails applications by refactoring them into multiple plugins (you can see the video of his talk here). One thing that he mentioned was the idea of plugins that can be run like standalone applications since the structures of Grails applications and plugins are so similar, and I thought it would be interesting to look at the process of converting an application to a plugin or a plugin to an application.

The approaches I’ll discuss here include a lot of bias on my part about how I like to create apps and plugins, so YMMV. I also won’t go into partial conversions since David covered this well in his talk.

So to convert an application to a plugin, the general workflow would be something like

Create the plugin descriptor, FooGrailsPlugin.groovy. The easiest way to do this is to run grails create-plugin pluginname and copy the generated file from there
delete everything from application.properties except the app.grails.version property
if you have jars in the lib directory that are available in a Maven repo, delete them and replace with BuildConfig.groovy dependencies
change any plugin and jar dependencies that are needed for development and testing but not when the plugin is installed to not be exported, by adding export = false
If you need the _Install.groovy, _Uninstall.groovy, or _Upgrade.groovy scripts (you probably don’t) grab those from the dummy plugin from step 1 (but delete any you don’t need, they’re all optional)
delete ApplicationResources.groovy if you aren’t using it and don’t depend on resources plugin
move code from BootStrap.groovy init() to FooGrailsPlugin.doWithApplicationContext and/or FooGrailsPlugin.doWithDynamicMethods and destroy() to FooGrailsPlugin.onShutdown, and delete BootStrap.groovy
add a dependency for the release plugin in BuildConfig.groovy
delete everything but the log4j configuration from Config.groovy
delete UrlMappings.groovy unless you have exported mappings; only keep the added ones
move bean definitions from resources.groovy to FooGrailsPlugin.doWithSpring and delete resources.groovy
delete grails-app/i18n message bundle files unless you added messages; only keep the added ones
delete everything from grails-app/views that you don’t use (in particular error.gsp, index.gsp, and layouts/main.gsp)
delete everything from web-app that you don’t use (including WEB-INF xml and tld files)
now would be a great time to write those tests you’ve been meaning to get to
create one or more test applications to install the plugin into to ensure that it works as a plugin; consider scripting this
write documentation for how to use the plugin; at a minimum a README file, but Grails gdoc files would be much better (run grails doc --init to get started)

Converting a plugin to an application is similar, except for the most part reversed:

Create a dummy application with grails create-app appname to copy missing files from
- Move BootStrap.groovy, Config.groovy, UrlMappings.groovy, and ApplicationResources.groovy into grails-app/conf, merging if needed
- Move resources.groovy into grails-app/conf/spring
- Move message bundle files to grails-app/i18n, merging if needed
- Move missing GSPs to grails-app/views
- Move static resources from web-app
- Move xml and tld files from web-app/WEB-INF
Move code from FooGrailsPlugin.doWithApplicationContext and FooGrailsPlugin.doWithDynamicMethods to BootStrap.groovy init(), and code from FooGrailsPlugin.onShutdown to destroy()
Move bean definitions from FooGrailsPlugin.doWithSpring to resources.groovy
Delete the plugin descriptor
Restore missing properties in application.properties
Delete the _Install.groovy, _Uninstall.groovy, and _Upgrade.groovy scripts
Remove the dependency for the release plugin from BuildConfig.groovy
Now would be a great time to write those tests you’ve been meaning to get to
If you make changes in the plugin’s doWithWebDescriptor, run grails install-templates and add them to src/templates/war/web.xml
If you add custom artifacts, or were supporting development-environment code or config changes in onChange and/or onConfigChange, these aren’t directly doable in an application. Use the pluginator plugin for these, and also for dynamic web.xml changes that require runtime logic