We have seen some features of the @Builder AST transformation in previous and other blog post. We can use another strategy to let the AST transformation generate a class where the class only has a constructor that accepts a builder class. And with @CompileStatic we can even make sure that all required properties must have a value set by the builder before the constructor accepts the argument. We use the builderStrategy annotation parameter and set it to InitializerStrategy:

import groovy.transform.builder.Builder
import groovy.transform.builder.InitializerStrategy

@Builder(builderStrategy = InitializerStrategy)
class Message {
    String from, to, subject, body
}

def message = Message.createInitializer()
        .from('mrhaki@mrhaki.com')
        .subject('Groovy 2.3 is released')

// Returned object is not Message, but
// internal class Message$MessageInitializer
assert !(message instanceof Message)

// Now we can use the initializer in the
// only constructor of Message.
def messageInstance = new Message(message)

assert messageInstance instanceof Message
assert messageInstance.from == 'mrhaki@mrhaki.com'
assert messageInstance.subject == 'Groovy 2.3 is released'

We can customize which properties need to be set with the includes and/or excludes annotation parameter. We can also customize the name of the method that create the builder instance with the parameter builderMethodName:

package com.mrhaki.blog.groovy

import groovy.transform.CompileStatic
import groovy.transform.builder.Builder
import groovy.transform.builder.InitializerStrategy

@Builder(builderStrategy = InitializerStrategy,
        excludes = 'body',
        builderMethodName = 'creator')
class Message {
    String from, to, subject, body
}

// With @CompileStatic the compiler will check
// that all properties defined in the @Builder
// configuration are set. This way we can
// implement required properties for a class
// that need to be set before the object can
// be created.
@CompileStatic
def createMessage() {
    def messageInit = Message.creator()
            .from('mrhaki@mrhaki.com')
            .to('mail@host.nl')
            .subject('Groovy 2.3 is released')

    def message = new Message(messageInit)
    message
}

final Message message = createMessage()

assert message.from == 'mrhaki@mrhaki.com'
assert message.subject == 'Groovy 2.3 is released'

Code written with Groovy 2.3.

Original article