java: how to mock Calendar.getInstance()?

Question

In my code I have something like this:

private void doSomething() {
   Calendar today = Calendar.getInstance();
   ....
}

How can I "mock" it in my junit test to return a specific date?

Answer 1

You can mock it using PowerMock in combination with Mockito:

On top of your class:

@RunWith(PowerMockRunner.class)
@PrepareForTest({ClassThatCallsTheCalendar.class})

The key to success is that you have to put the class where you use Calendar in PrepareForTest instead of Calendar itself because it is a system class. (I personally had to search a lot before I found this)

Then the mocking itself:

mockStatic(Calendar.class);
when(Calendar.getInstance()).thenReturn(calendar);

Answer 2

As far as I see it you have three sensible options:

1. Inject the Calendar instance in whatever method/class you set that day in.

   private void method(final Calendar cal) { Date today = cal.getTime(); }

2. Use JodaTime instead of Calendar. This is less an option and more a case of a suggestion as JodaTime will make your life a lot easier. You will still need to inject this time in to the method.

   DateTime dt = new DateTime();

   Date jdkDate = dt.toDate();

3. Wrap Calendar inside some interface that allows you to fetch the time. You then just mock that interface and get it to return a constant Date.

   Date today = calendarInterfaceInstance.getCurrentDate()

Answer 3

Don't mock it - instead introduce a method you can mock that gets dates. Something like this:

interface Utility {

    Date getDate();
}

Utilities implements Utility {


    public Date getDate() {

        return Calendar.getInstance().getTime();
    }

}

Then you can inject this into your class or just use a helper class with a bunch of static methods with a load method for the interface:

public class AppUtil {

    private static Utility util = new Utilities();

    public static void load(Utility newUtil) {

         this.util = newUtil;
    }

    public static Date getDate() {

        return util.getDate();
    }

}

Then in your application code:

private void doSomething() {
   Date today = AppUtil.getDate();
   ....
}

You can then just load a mock interface in your test methods.

@Test
public void shouldDoSomethingUseful() {
     Utility mockUtility = // .. create mock here
     AppUtil.load(mockUtility);

     // .. set up your expectations

     // exercise the functionality
     classUnderTest.doSomethingViaAPI();

     // ... maybe assert something 

}

See also Should you only mock types you own? and Test smell - everything is mocked

Answer 4

Using Mockito and PowerMockito:

Calendar endOfMarch = Calendar.getInstance();
endOfMarch.set(2011, Calendar.MARCH, 27);
PowerMockito.mockStatic(Calendar.class);
Mockito.when(Calendar.getInstance()).thenReturn(endOfMarch);

Refer to the link for the complete code.

Answer 5

Write a class called DateHelper with a method getCalendar that returns Calendar.getInstance(). Refactor the class that you're testing so that it has a member variable of type DateHelper, and a constructor that injects that member variable. Use that constructor in your test, to inject a mock of DateHelper, in which getCalendar has been stubbed to return some known date.

Answer 6

You can mockit using JMockit. Here you can see how you can do it: Mock Java Calendar - JMockit vs Mockito.