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November 17, 2011

Groovy Goodness: Create Simple Builders with Closures

In Groovy we can use pre-defined builders like the JsonBuilder or MarkupBuilder to create data or text structures. It is very easy to create our own builder simply with closures. A node in the builder is simply a method and we can use a closure as the argument of the method to create a new level in the builder hierarchy.

We can use pre-defined method names in our builder syntax, but can also use dynamic or unkown method names by implementing the methodMissing method. The same goes for properties we can implement with real property methods or by implementing the propertyMissing method.

In our example we create a new builder to create a Reservation object for a travel flight. In the builder we can define a list of passengers, the destination airport, the departing airport and if the flight is a two-way flight.

// Builder syntax to create a reservation with passengers,
// departing and destination airport and make it a 2-way flight.
def reservation = new ReservationBuilder().make {
    passengers {
        name 'mrhaki'
        name 'Hubert A. Klein Ikkink'
    from 'Schiphol, Amsterdam'
    to 'Kastrup, Copenhagen'

assert reservation.flight.from == new Airport(name: 'Schiphol', city: 'Amsterdam')
assert == new Airport(name: 'Kastrup', city: 'Copenhagen')
assert reservation.passengers.size() == 2
assert reservation.passengers == [new Person(name: 'mrhaki'), new Person(name: 'Hubert A. Klein Ikkink')]
assert reservation.retourFlight

// ----------------------------------------------
// Builder implementation and supporting classes.
// ----------------------------------------------
import groovy.transform.*

class Reservation {
    Flight flight = new Flight()
    List<Person> passengers = []
    Boolean retourFlight = false

class Person { String name }

class Airport { String name, city }

class Flight { Airport from, to }

// The actual builder for reservations.
class ReservationBuilder {
    // Reservation to make and fill with property values.
    Reservation reservation

    private Boolean passengersMode = false

    Reservation make(Closure definition) {
        reservation = new Reservation()

        runClosure definition


    void passengers(Closure names) {
        passengersMode = true

        runClosure names

        passengersMode = false

    void name(String personName) {
        if (passengersMode) {
            reservation.passengers << new Person(name: personName)
        } else {
            throw new IllegalStateException("name() only allowed in passengers context.")

    def methodMissing(String name, arguments) {
        // to and from method calls will set flight properties
        // with Airport objects.
        if (name in ['to', 'from']) {
            def airport = arguments[0].split(',')
            def airPortname = airport[0].trim()
            def city = airport[1].trim()
            reservation.flight."$name" = new Airport(name: airPortname, city: city)

    def propertyMissing(String name) {
        // Property access of retourFlight sets reservation
        // property retourFlight to true.
        if (name == 'retourFlight') {
            reservation.retourFlight = true

    private runClosure(Closure runClosure) {
        // Create clone of closure for threading access.
        Closure runClone = runClosure.clone()

        // Set delegate of closure to this builder.
        runClone.delegate = this

        // And only use this builder as the closure delegate.
        runClone.resolveStrategy = Closure.DELEGATE_ONLY

        // Run closure code.