First of all let's define a Groovy Bean (POGO) class, which holds data representing some vehicle information:

package org.groovy.cookbook

import groovy.transform.ToString

@ToString
class Vehicle {

  static enum FuelType { DIESEL, PETROL, GAS, ELECTRIC }
  static enum TransmissionType { MANUAL, AUTOMATIC }

  @ToString
  static class Transmission {
    long gears
    TransmissionType type
  }

  String brand
  String model
  FuelType fuel
  Long releaseYear
  Transmission transmission

}

As you can notice it's nothing, but a set of fields of simple types, enumerations, and a nested class, which again consists of simple type fields. In order for that class to be available for the conversion script, let's place it into a vehicle.groovy file and compile it with the groovyc command:

groovyc vehicle.groovy

The JSON document to which we want to map the class is the one defined in the recipe, Validating JSON messages. Create a vehicle.json file in the same directory as the vehicle.groovy.

To simplify the mapping, we used the same property names in both, JSON message and the Vehicle class.

Create a new Groovy script, convert.groovy, making sure that it's located in the same folder as the JSON file.

Groovy performs some of the data type mapping automatically for us; making the conversion code simpler. For example, jsonVehicle.fuel is actually a String, but it is transformed to FuelType enumeration automatically since the values are matching. Also, jsonVehicle.transmission.gears is a String in our original JSON message (because it is included in the double quotes), but it is still safely converted to a long field in the Transmission object.

Thanks to the @ToString annotation, the Vehicle class shows all its internal data upon printing by the last line of the script.

Writing the conversion code for every possible type you may need, is time consuming. Fortunately, existing third-party libraries come to the rescue. In the following sections, we will show how Groovy Bean conversion can be achieved with the JSON-lib, Jackson and GSON libraries. Please note that for the next examples to work the Vehicle class is imported, therefore, it must be extracted to a Vehicle.groovy class and added to the classpath when running the code.

All of those frameworks produce similar results, and they can be further tweaked for more complex data conversion requirements.