This strategy is used with the Builder AST transform to create a builder helper class for the fluent creation of instances of a specified class. It can be used at the class, static method or constructor levels. You use it as follows:

import groovy.transform.builder.*

@Builder
class Person {
    String firstName
    String lastName
    int age
}
def person = Person.builder().firstName("Robert").lastName("Lewandowski").age(21).build()
assert person.firstName == "Robert"
assert person.lastName == "Lewandowski"
assert person.age == 21

The prefix annotation attribute can be used to create setters with a different naming convention. The default is the empty string but you could change that to "set" as follows:

@groovy.transform.builder.Builder(prefix='set')
class Person {
    String firstName
    String lastName
    int age
}
def p2 = Person.builder().setFirstName("Robert").setLastName("Lewandowski").setAge(21).build()

or using a prefix of 'with' would result in usage like this:

def p3 = Person.builder().withFirstName("Robert").withLastName("Lewandowski").withAge(21).build()

You can also use the @Builder annotation in combination with this strategy on one or more constructor or static method instead of or in addition to using it at the class level. An example with a constructor follows:

import groovy.transform.ToString
import groovy.transform.builder.Builder

@ToString
class Person {
    String first, last
    int born

    @Builder
    Person(String roleName) {
        if (roleName == 'Jack Sparrow') {
            first = 'Johnny'; last = 'Depp'; born = 1963
        }
    }
}
assert Person.builder().roleName("Jack Sparrow").build().toString() == 'Person(Johnny, Depp, 1963)'

In this case, the parameter(s) for the constructor or static method become the properties available in the builder. For the case of a static method, the return type of the static method becomes the class of the instance being created. For static factory methods, this is normally the class containing the static method but in general it can be any class.

import groovy.transform.builder.*
import groovy.transform.*

@ToString
@Builder
class Person {
    String first, last
    int born

    Person(){} // required to retain no-arg constructor

    @Builder(builderClassName='MovieBuilder', builderMethodName='byRoleBuilder')
    Person(String roleName) {
        if (roleName == 'Jack Sparrow') {
            this.first = 'Johnny'; this.last = 'Depp'; this.born = 1963
        }
    }

    @Builder(builderClassName='SplitBuilder', builderMethodName='splitBuilder')
    static Person split(String name, int year) {
        def parts = name.split(' ')
        new Person(first: parts[0], last: parts[1], born: year)
    }
}

assert Person.splitBuilder().name("Johnny Depp").year(1963).build().toString() == 'Person(Johnny, Depp, 1963)'
assert Person.byRoleBuilder().roleName("Jack Sparrow").build().toString() == 'Person(Johnny, Depp, 1963)'
assert Person.builder().first("Johnny").last('Depp').born(1963).build().toString() == 'Person(Johnny, Depp, 1963)'

The 'forClass' annotation attribute for the @Builder transform isn't applicable for this strategy. The 'useSetters' annotation attribute for the @Builder transform is ignored by this strategy which always uses setters.