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Sunday, March 10, 2013

Grails Goodness: Use Constructor Argument Based Dependency Injection with resources.groovy

We can define extra Spring beans for our Grails application in grails-app/conf/spring/resources.groovy using a DSL. For example we want to use third-party classes as Spring components. Then we can define the bean in resources.groovy and use dependency injection to use it in Grails controllers, services or other classes. If the class is defined so dependencies need to be injected via constructor arguments we must use a special DSL syntax. Normally we define a bean using the following syntax: beanName(BeanClass). To pass constructor arguments we must add those as method arguments after BeanClass. For example if the constructor has a single argument of type String we can use the following syntax: beanName(BeanClass, '42')

We have an alternative way and that is via a closure we can use for the bean definition. The closure will have one argument which is a BeanDefinition object. The BeanDefinition object has a constructorArgs property that takes a list of constructor arguments. So our previous example would become: beanName(BeanClass) { it.constructorArgs = ['42'] }.

Let's see how this works with an example. First we create a simple Grails service, this service will be injected into a Groovy class we will configure via constructor arguments.

// File: grails-app/services/com/mrhaki/grails/spring/LanguageService.groovy
package com.mrhaki.grails.spring

class LanguageService {

    List<String> findAllLanguages() {
        ['Groovy', 'Java', 'Scala', 'Clojure']
    }

}

Next we create a Groovy class LanguageProvider. This class will use the Grails service and a search pattern to find a language, but those are set via the constructor arguments:

// File: src/groovy/com/mrhaki/grails/spring/LanguageProvider.groovy
package com.mrhaki.spring

class LanguageProvider {

    final def languageService
    final String searchPattern

    LanguageProvider(final def languageService, final String searchPattern) {
        this.languageService = languageService
        this.searchPattern = searchPattern
    }

    List<String> getGr8Languages() {
        final List<String> languages = languageService.findAllLanguages()
        final List<String> gr8Languages = languages.findAll { it =~ searchPattern }
        gr8Languages
    }
}

To configure the LanguageProvider class as Spring bean we add the bean definition to resources.groovy:

// File: grails-app/conf/spring/resources.groovy
import com.mrhaki.spring.LanguageProvider

beans = {

    // Pass constructor arguments: ref('languageService') and '^Gr.*'
    // to LanguageProvider.
    languageProvider(LanguageProvider, ref('languageService'), '^Gr.*')

}

Or we can use the BeanDefinition that is passed to the closure:

// File: grails-app/conf/spring/resources.groovy
import com.mrhaki.spring.LanguageProvider

beans = {

    languageProvider(LanguageProvider) { beanDefinition ->
        // Pass constructor arguments: ref('languageService') and '^Gr.*'
        // to LanguageProvider.
        beanDefinition.constructorArgs = [ref('languageService'), '^Gr.*']
    }
}

Grails 2.2.1 is used to write this blog post.

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