Mocking dependencies in unit tests using JMockIt


Ein weiterer Blogpost aus dem Firmenblog, wieder in Englisch.

Garage & GarageTest

This test case demonstrate the use of JMockIt and its Expectations to resolve dependencies in unit tests. Expectations are a way to easily mock the behavior of an object in unit tests, without creating interfaces, classes and other overhead. Take a look at the full unit test first, and I’ll explain what’s done there later. You can find the interface Garage and the classes CheapGarage and EngineFactory at the end of this blogpost.

Now, back to the method testRepairWithExpectationEngineFactory(). The EngineFactory is a dependency buried down in the CheapGarage class that is responsible for replacing the Engine in a Car. There is a mocked instance of EngineFactory in this unit test that will return a new DieselEngine every time the method instantiate() is called with any kind of argument. This way I can control exactly how the hidden call to a factory behaves. JMockIt has a bunch of other useful features and I recommend it for all unit tests. If you care about your tests and try to avoid testing dependencies instead of your logic, then you should start using JMockIt today. 😀

Car.java

package de.kopis.example;

public class Car {
	protected Engine engine;
	
	public Car(Engine e) {
		engine = e;
	}
	
	public void replaceEngine(Engine e) {
		engine = e;
	}
}

Engine.java

package de.kopis.example;

public interface Engine {}

class GasolineEngine implements Engine {}
class DieselEngine implements Engine {}

Garage/CheapGarage.java

package de.kopis.example;

public interface Garage {
  public Car repairEngine(Car car);
}

abstract class AbstractGarage implements Garage {
  public abstract Car repairEngine(Car car);
}

class CheapGarage extends AbstractGarage {
  public Car repairEngine(Car car) {
    replaceEngineWithNewInstance(car);
    return car;
  }

  private void replaceEngineWithNewInstance(Car car) {
    try {
      Engine newEngine = EngineFactory.instantiate(car.engine);
      car.replaceEngine(newEngine);
    } catch ( InstantiationException e ) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch ( IllegalAccessException e ) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }
}

class EngineFactory {
  public EngineFactory() {}

  public static Engine instantiate(Engine engine) throws InstantiationException, IllegalAccessException {
    Class<? extends Engine> engineClass = engine.getClass();
    Engine newEngine = engineClass.newInstance();
    return newEngine;
  }
}

GarageTest.java

package de.kopis.example;

import static org.junit.Assert.*;

import mockit.Expectations;

import org.junit.Test;

public class GarageTest {
  @Test
  public void testRepair() {
    Garage g = new CheapGarage();
    Engine expectedEngine = new GasolineEngine();
    Car expectedCar = new Car(expectedEngine);
    Car actualCar = g.repairEngine(expectedCar);
    assertSame(expectedCar, actualCar);
    assertNotSame(expectedCar.engine, expectedEngine);
  }

  @Test
  public void testRepairWithExpectationEngineFactory() throws InstantiationException, IllegalAccessException {
    final Engine expectedEngine = new DieselEngine();
    final Engine actualEngine = new GasolineEngine();
    final Garage g = new CheapGarage();
    final Car expectedCar = new Car(actualEngine);
    // here it is still the Engine built into the car
    assertSame(expectedCar.engine, actualEngine);

    new Expectations() {
      EngineFactory mock;
      {
        mock.instantiate(actualEngine); returns(new DieselEngine());
      }
    };

    Car actualCar = g.repairEngine(expectedCar);
    assertSame(expectedCar, actualCar);
    // normally this will be a GasolineEngine, because that was built into the car
    // but because of the mocked EngineFactory, we get a DieselEngine instead
    assertTrue(expectedCar.engine instanceof DieselEngine);
    assertNotSame(expectedCar.engine, expectedEngine);
  }
}

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