Mocking a singleton in Groovy

Question

In Groovy / Spock I can mock a class by doing:

def theClass = Mock(TheClass.class)

and then wire that instance into the class under unit test. What about if I want to mock a class that has been annotated as a @Singleon

Answer 1

Here is a little tool class you can use:

package de.scrum_master.stackoverflow

import java.lang.reflect.Field
import java.lang.reflect.Modifier

class GroovySingletonTool<T> {
  private Class<T> clazz

  GroovySingletonTool(Class<T> clazz) {
    this.clazz = clazz
  }

  void setSingleton(T instance) {
    // Make 'instance' field non-final
    Field field = clazz.getDeclaredField("instance")
    field.modifiers &= ~Modifier.FINAL
    // Only works if singleton instance was unset before
    field.set(clazz.instance, instance)
  }

  void unsetSingleton() {
    setSingleton(null)
  }

  void reinitialiseSingleton() {
    // Unset singleton instance, otherwise subsequent constructor call will fail
    unsetSingleton()
    setSingleton(clazz.newInstance())
  }
}

Just call setSingleton(Mock(TheClass)). For more info see this answer, I do not want to repeat the whole sample code here.

Feel free to ask follow-up questions if there is anything you do not understand.

Answer 2

You can use global mocks

def publisher = new Publisher()
publisher << new RealSubscriber() << new RealSubscriber()

def anySubscriber = GroovyMock(RealSubscriber, global: true)

when:
publisher.publish("message")

then:
2 * anySubscriber.receive("message")