Groovy Singleton and testing issue (with Spock)

So basically you want to test that a singleton is not a singleton. This strikes me as rather odd. But anyway, I am regarding this question rather as a puzzle which I am going to solve for its own sake because it is a nice challenge. (Don't do this at home, kids!)

Groovy singleton:

package de.scrum_master.stackoverflow @Singleton class Highlander { def count = 0 def fight() { println "There can be only one!" count++ doSomething() } def doSomething() { println "Doing something" } } 

Singleton helper class:

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()) } } 

Spock test:

This test shows how to

• re-instantiate a Groovy singleton before feature method execution
• use a Stub() for a Groovy singleton
• use a Mock() for a Groovy singleton
• use a Spy() for a Groovy singleton (needs Objenesis)

package de.scrum_master.stackoverflow import org.junit.Rule import org.junit.rules.TestName import spock.lang.Specification import spock.lang.Unroll class HighlanderTest extends Specification { def singletonTool = new GroovySingletonTool<Highlander>(Highlander) @Rule TestName gebReportingSpecTestName def setup() { println "\n--- $gebReportingSpecTestName.methodName ---" } @Unroll def "Highlander fight no. #fightNo"() { given: singletonTool.reinitialiseSingleton() def highlander = Highlander.instance when: highlander.fight() then: highlander.count == 1 where: fightNo << [1, 2, 3] } @Unroll def "Highlander stub fight no. #fightNo"() { given: Highlander highlanderStub = Stub() { fight() >> { println "I am a stub" } } singletonTool.setSingleton(highlanderStub) def highlander = Highlander.instance when: highlander.fight() then: highlander == highlanderStub where: fightNo << [1, 2, 3] } @Unroll def "Highlander mock fight no. #fightNo"() { given: Highlander highlanderMock = Mock() { fight() >> { println "I am just mocking you" } } singletonTool.setSingleton(highlanderMock) def highlander = Highlander.instance when: highlander.fight() then: highlander == highlanderMock 0 * highlander.doSomething() where: fightNo << [1, 2, 3] } @Unroll def "Highlander spy fight no. #fightNo"() { given: // Unset not necessary because Objenesis creates object without constructor call // singletonTool.unsetSingleton() Highlander highlanderSpy = Spy(useObjenesis: true) // Spy's member is not initialised by Objenesis highlanderSpy.count = 0 singletonTool.setSingleton(highlanderSpy) def highlander = Highlander.instance when: highlander.fight() then: highlander == highlanderSpy highlander.count == 1 1 * highlander.doSomething() >> { println "I spy" } where: fightNo << [1, 2, 3] } } 

Console log:

--- Highlander fight no. 1 --- There can be only one! Doing something --- Highlander fight no. 2 --- There can be only one! Doing something --- Highlander fight no. 3 --- There can be only one! Doing something --- Highlander stub fight no. 1 --- I am a stub --- Highlander stub fight no. 2 --- I am a stub --- Highlander stub fight no. 3 --- I am a stub --- Highlander mock fight no. 1 --- I am just mocking you --- Highlander mock fight no. 2 --- I am just mocking you --- Highlander mock fight no. 3 --- I am just mocking you --- Highlander spy fight no. 1 --- There can be only one! I spy --- Highlander spy fight no. 2 --- There can be only one! I spy --- Highlander spy fight no. 3 --- There can be only one! I spy