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February 8, 2013

Groovy Goodness: Combining Annotations with AnnotationCollector

Groovy 2.1 introduces a new annotation to group multiple annotations and define an alias for the group. The new @AnnotationCollector annotation is an AST transformation. If we repeatedly have to add multiple annotations we can now create an alias for the multiple annotations and use it in our code.

We can define the group of annotations as an argument for the @AnnotationCollector annotations. For example in the next sample code we create an annotation to apply the @EqualsAndHashCode and @ToString annotations and use the alias Simple. We pass a list of annotations to the @AnnotationColletor:

@AnnotationCollector([EqualsAndHashCode, ToString])
@interface Simple {}


@Simple
class User {
    String username
    int age
}

def user = new User(username: 'mrhaki', age: 39)
assert user.toString() == 'User(mrhaki, 39)'

// We still have 2 annotations:
assert User.class.annotations.size() == 2


// We can use the attributes from the 
// grouped annotations.
@Simple(excludes = 'street') 
class Address {
    String street, town
}

def address = new Address(street: 'Evergreen Terrace', town: 'Springfield') 
assert address.toString() == 'Address(Springfield)'

Notice we can also use the attributes from the grouped annotations. If an attribute cannot be mapped to the existing attributes of the grouped annotations we get an error. We can write custom code to handle new attributes in a so-called processor class. In the processor we can define how we want to handle the attributes. In the following sample we define a custom attribute dontUse and we use the value for the excludes attribute of the grouped annotations:

import org.codehaus.groovy.transform.*
import org.codehaus.groovy.ast.*
import org.codehaus.groovy.control.*

class SimpleProcessor extends AnnotationCollectorTransform {

    public List<AnnotationNode> visit(AnnotationNode collector, 
                                      AnnotationNode aliasAnnotationUsage, 
                                      AnnotatedNode aliasAnnotated, 
                                      SourceUnit source) {
                                      
        // Get attributes and attribute value for dontUse.
        def attributes = aliasAnnotationUsage.getMembers()
        def dontUse = attributes.get('dontUse')
        attributes.remove('dontUse')
        
        if (dontUse) {
            // Assign value of dontUse to excludes attributes.
            aliasAnnotationUsage.addMember("excludes", dontUse)
        }
        
        super.visit(collector, aliasAnnotationUsage, aliasAnnotated, source)
    }    
    
}

new GroovyShell(this.class.classLoader).evaluate '''
import groovy.transform.*

@AnnotationCollector(value = [EqualsAndHashCode, ToString], processor = 'SimpleProcessor')
@interface Simple {}


@Simple(dontUse = 'age')
class User {
    String username
    int age
}

def user = new User(username: 'mrhaki', age: 39)
assert user.toString() == 'User(mrhaki)'
'''

Another way to define an alias for a group of annotations is to simply define an annotation, add the annotations we want to group and as last annotation we add the @AnnotationCollector. In the next sample we combine the @EqualsAndHashCode and @ToString annotations into the alias Simple:

@EqualsAndHashCode
@ToString
@AnnotationCollector
@interface Simple {}


@Simple
class User {
    String username
}

def user = new User(username: 'mrhaki')
assert user.toString() == 'User(mrhaki)'

More info and samples are also available on the blog of Andres Steingress.

Written with Groovy 2.1.