Internationalizing Domain Classes and Enums

It's common to provide a toString implementation on a domain object that may well end up being used in the view. However, this doesn't allow for internationalization in the view. A good solution that I've used a number of times is to have classes implement Spring's MessageSourceResolvable interface. Consider this domain class that represents an image file:

grails-app/domain/Image.groovy

class Image {
    String name
    String path
    User uploadedBy
    org.joda.time.DateTime dateCreated

    static transients = ['file']
    File getFile() {
        new File(ConfigurationHolder.config.image.base.dir, path)
    }

    String toString() {
        "$name uploaded by $uploadedBy.username on $dateCreated - ${file.size()} bytes"
    }
}

The toString implementation is all well and good if we're targeting an English-speaking audience but with some simple changes we can make it i18n compliant:

grails-app/domain/Image.groovy

class Image implements org.springframework.context.MessageSourceResolvable {

    // properties as above

    static transients = ["codes", "arguments", "defaultMessage"]

    Object[] getArguments() {
        [name, uploadedBy.username, dateCreated.toDate(), file.size()] as Object[]
    }

    String[] getCodes() {
        ['image.info'] as String[]
    }

    String getDefaultMessage() {
        "$name uploaded by $uploadedBy.username on $dateCreated - ${file.size()} bytes"
    }
}

In our message properties file we can add:

grails-app/i18n/messages.properties

image.info={0} uploaded by {1} on {2,date} - ${3,number,integer} bytes

In the view we can display our object like this:

<g:message error="${imageInstance}"/>

Yes, that is the error attribute we're passing to the message tag! Grails intends the attribute to be used for outputting validation errors but the underlying mechanism is the same - Spring's ObjectError implements MessageSourceResolvable and that's how Grails' message tag resolves the displayed error message. Rather than passing separate code, args and default attributes to the tag we can pass the single MessageSourceResolvable instance and its implementation will take care of supplying those values.

Note: I added a message attribute to the message tag to avoid the confusion caused by using error. This is in Grails from version 1.2-M1 onwards.

We can now add translations of our object description. For example:

grails-app/i18n/messages_af.properties

image.info={0} opgelaai deur {1} op {2,date} - ${3,number,integer} grepe

It's worth noting that the format of the dateCreated property will be automatically determined by the request locale so the value will be formatted correctly for the user.

I've found this technique can also be very useful on enum classes. For example:

src/groovy/com/mycompany/Season.groovy

package com.mycompany

enum Season implements org.springframework.context.MessageSourceResolvable {
    SPRING, SUMMER, AUTUMN, WINTER

    Object[] getArguments() { [] as Object[] }

    String[] getCodes() {
        ["${getClass().name}.${name()}"] as String[]
    }

    String getDefaultMessage() { name() }
}

grails-app/i18n/messages.properties

com.mycompany.Season.SPRING=Spring
com.mycompany.Season.SUMMER=Summer
com.mycompany.Season.AUTUMN=Autumn
com.mycompany.Season.WINTER=Winter

grails-app/i18n/messages_en_US.properties

com.mycompany.Season.AUTUMN=Fall