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Thursday, November 17, 2011

Groovy Goodness: Create Our Own Script Class

Groovy is a great language to write DSL implementations. The Groovy syntax allows for example to leave out parenthesis or semi colons, which results in better readable DSL (which is actually Groovy code).

To run a DSL script we can use the GroovyShell class and evaluate the script. By default the script is evaluated with an instance of groovy.lang.Script class. But we can extends this Script class and write our DSL allowed methods, which can then be used by the DSL script. We pass our own Script class to the GroovyShell with an CompilerConfiguration object. The CompilerConfiguration allows us to set a new base script class to be used.

The following sample is a simple DSL to change the state of a Car object. Notice we implicitly access the Car object that is passed to the GroovyShell via a binding. The custom CarScript class can access the car object via the binding and change it's state.

import org.codehaus.groovy.control.CompilerConfiguration

// Simple Car class to save state and distance.
class Car {
    String state
    Long distance = 0
}

// Custom Script with methods that change the Car's state.
// The Car object is passed via the binding.
abstract class CarScript extends Script {
    def start() {
        this.binding.car.state = 'started'
    }

    def stop() {
        this.binding.car.state = 'stopped'
    }

    def drive(distance) {
        this.binding.car.distance += distance
    }
}


// Use custom CarScript.
def compilerConfiguration = new CompilerConfiguration()
compilerConfiguration.scriptBaseClass = CarScript.class.name

// Define Car object here, so we can use it in assertions later on.
def car = new Car()
// Add to script binding (CarScript references this.binding.car).
def binding = new Binding(car: car)

// Configure the GroovyShell.
def shell = new GroovyShell(this.class.classLoader, binding, compilerConfiguration)

// Simple DSL to start, drive and stop the car.
// The methods are defined in the CarScript class.
def carDsl = '''
start()
drive 20
stop()
'''

// Run DSL script.
shell.evaluate carDsl

// Checks to see that Car object has changed.
assert car.distance == 20
assert car.state == 'stopped'

2 comments:

Ronald Borman said...

Although maybe not entirely appropriate in this specific example, but in general you can also specify the script base class on the command line by using a system property:

groovy -Dgroovy.script.base=CarScript mytest

Hubert Klein Ikkink said...

@Ronald Borman: Thank you for comment. This is one of the reason I started blogging: getting great tips from other people.

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