Recently I’ve been heads down writing a Grails Solr plugin for using Solr to index and search Grails domain objects, content and other documents. I share more on that in the next several day and weeks.  While doing so I implement annotations for the first time. In this example, I create an annotation type that lets you annotate a class property to specify a custom Solr index field name.  In usage it looks like this:

import org.grails.plugins.solr.Solr
class aDomain {
  @Solr(field="thedomainstitle")
  String title
}

The imported class above is actually an interface that defines the annotation:

package org.grails.plugins.solr
import java.lang.annotation.*

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Solr {
  String field();
}

This is fairly self explanatory. The name of the annotation is “Solr” and there is one propery field with a value of String. Almost identical to the java equivalent.

Now to access the annotation value for the field, you simply need the get a reference to the annotation and call the method on it defined above: field(). So with a bit of context below, I define an indexSolr() method for each domain class, loop through each property and check if there is an annotation of Solr defined for the field, if so I set the value of field() to the docKey which is used as the Solr field name for the property.

application.domainClasses.each { dc ->
  dc.metaClass.indexSolr < < { ->
    //.......
    application.getArtefact(DomainClassArtefactHandler.TYPE, delegate.class.name).getProperties().each {
      //.......
      def theField = delegate.class.declaredFields.find{ field -> field.name == it.name}

      if(theField.isAnnotationPresent(Solr)) {
        docKey = theField.getAnnotation(Solr).field()
      }
      //........
    }
    //........
  }
}

Powerful, yet simple.