I am writing this post to record all the things I have done in order to migrate an existing Grails 2.2.x app to 2.3.x.

The reasons for me to upgrade are:

• Project progress is at an early stage where is not very risky to do that. We also have a considerable amount of automatic tests. You have also need to take into consideration that the more major versions you skip, the harder it gets to migrate to the latest version.
• Switched dependency resolution engine from Ivy to Aether. I have had problems with Ivy resolving transitive dependencies and snapshots of my own plugin dependencies.
• "Forked Execution" and the "Test Runner Daemon" should speed up the tests execution and therefore speed up
  the tests development. This is one of the mayor caveats I have found in Grails so far. We'll see if they have fixed it with this two new features.

First of all I have read the following posts about this subject, which I think are essential to perform the task at hand:

• Upgrading from Grails 2.2 - Reference Documentation
• Migrating to Grails 2.3
• Upgrading to Grails 2.3 – Gotchas

I decided to follow a different approach by specifying what files need to be changed, why and how:

Files I had to alter:

application.properties

You need to upgrade the grails version number:

app.grails.version=2.3.7

BuildConfig.groovy

• The groupId should be now in the Config.groovy file. Se next file changes

• Enable forked execution (host app and plugins):

grails.project.fork = [
    // configure settings for compilation JVM, note that if you alter the Groovy version forked compilation is required
    //  compile: [maxMemory: 256, minMemory: 64, debug: false, maxPerm: 256, daemon:true],

    // configure settings for the test-app JVM, uses the daemon by default
    test: [maxMemory: 768, minMemory: 64, debug: false, maxPerm: 256, daemon:true],
    // configure settings for the run-app JVM
    run: [maxMemory: 768, minMemory: 64, debug: false, maxPerm: 256, forkReserve:false],
    // configure settings for the run-war JVM
    war: [maxMemory: 768, minMemory: 64, debug: false, maxPerm: 256, forkReserve:false],
    // configure settings for the Console UI JVM
    console: [maxMemory: 768, minMemory: 64, debug: false, maxPerm: 256]
]

• Explicitely set Maven Aether as the default dependency resolution engine (host app and plugins):

grails.project.dependency.resolver = "maven" // or ivy

• Upgrade the following plugin dependencies if you are planning to upgrade a Grails host app:

// plugins for the build system only
build ":tomcat:7.0.52.1"

// plugins for the compile step
compile ":scaffolding:2.0.3"

// plugins needed at runtime but not for compilation
runtime ":hibernate4:4.3.4.1" // or ":hibernate:3.6.10.9"
runtime ":database-migration:1.3.8"
runtime ":resources:1.2.7"
// Uncomment these (or add new ones) to enable additional resources capabilities
//runtime ":zipped-resources:1.0.1"
//runtime ":cached-resources:1.1"
//runtime ":yui-minify-resources:0.1.5"

• If as the snipped above, you want to use Hibernate 4 instead of Hibernate 3, you will have to make sure that your plugin dependencies exclude the Hibernate 3 dependency. This is due to the fact that they are using different artifactId ("hibernate4" instead of "hibernate"). You can exclude dependencies in plugins like this:

compile(':plugin-artifactId:version') {
    excludes "hibernate"
}

• Remove Spock dependencies as they already come built-in with Grails 2.3 (host app and plugins).

• Upgrade the following plugin dependencies if you are planning to upgrade a Grails plugin:

build(":release:3.0.1",
    ":rest-client-builder:1.0.3") {
    export = false
}

Note that you might need to upgrade some other plugins.

Config.groovy

• Automatic reloading is not enabled by default, so if you want it back add the following line:

grails.reload.enabled = true

• And then the change mentioned in teh previous section that says that the groupId should be specified here now:

grails.project.groupId = "whatever" // change this to alter the default package name and Maven publishing destination

• There are quite a few other little changes. So I suggest creating a new empty Grails app from scratch using Grails 2.3.x and compare the resulting Config.groovy file with your own.

All Integration Tests that extend GroovyTestCase

All these tests need to remove that inheritance and annotate the class with this Mixin:

@TestMixin(IntegrationTestMixin)

~/.grails/settings.groovy

If like me, you have your repositories and credentials declared globally, then your settings.xml file should go from this:

...
grails.project.ivy.authentication = {
    repositories {
        mavenRepo "whatever url"

    credentials {
        realm = "Sonatype Nexus Repository Manager"
        host = "whatever"
        username = "whatever"
        password = "whatever"
    }
}
...

to this:

...
grails.project.ivy.authentication = {
    repositories {
    	mavenLocal()
        mavenRepo("whatever url") {
          if (owner.grailsVersion ==~ /2\.3\..*/) {
              updatePolicy "always"
              auth([
                  username: 'whatever',
                  password: 'whatever'
              ])
          }
    	}
    }

    credentials {
        realm = "Sonatype Nexus Repository Manager"
        host = "whatever"
        username = "whatever"
        password = "whatever"
    }
}
...

The possible values for "updatePolicy" are:

• never – Never check for new snapshots.
• always – Always check for new snapshots.
• daily – Check once a day for new snapshots (the default).
• interval:x – Check once every x minutes for new snapshots.

I will keep updating this post if I encounter another migration issue.

I hope you find it useful. Cheers.

• Update 1: Class reloading has to be enabled inside the Config.groovy file not the BuildConfig.groovy file.

• Update 2: Disabled forked execution because it breaks debugging in IntelliJ IDEA

• Update 3: Added possible values for the updatePolicy maven repository settings parameter