Using Mockito to mock a local variable of a method

Question

I have a class A that needs to the tested. The following is the definition of A:

public class A {
    public void methodOne(int argument) {
        //some operations
        methodTwo(int argument);
        //some operations
    }

    private void methodTwo(int argument) {
        DateTime dateTime = new DateTime();
        //use dateTime to perform some operations
    }
}

And based on the dateTime value some data is to be manipulated, retrieved from the database. For this database, the values are persisted via a JSON file.

This complicates things. What I need is to set the dateTime to some specific date while it is being tested. Is there a way I can mock a local variable's value using mockito?

Solution 1

You cannot mock a local variable. What you could do, however, is extract its creation to a protected method and spy it:

public class A {
  public void methodOne(int argument) {
    //some operations
    methodTwo(int argument);
    //some operations
  }

  private void methodTwo(int argument) {
    DateTime dateTime = createDateTime();
    //use dateTime to perform some operations
  }

  protected DateTime createDateTime() {
    return new DateTime();
  }
}

public class ATest {
  @Test
  public void testMethodOne() {
    DateTime dt = new DateTime (/* some known parameters... */);
    A a = Mockito.spy(new A());
    doReturn(dt).when(a).createDateTime();
    int arg = 0; // Or some meaningful value...
    a.methodOne(arg);
    // assert the result
}

Solution 2

The best way to deal with such a problem is to use an injected Clock service, used to get new instances of DateTime. That way, your test can inject a mock Clock, which returns a specific DateTime instead of the current time.

Note that the new Java 8 time API defines such a Clock class, specifically for that purpose.

Comments

[user657592]

I don't wish to write a protected createDateTime() method. Is there no other way to go about it?

[user657592]

This requires a jdk1.8, right? I am currently on 1.7 and am not planning to upgrade anytime soon.

[JB Nizet]

No. You just need to define your own Clock class or interface, and call clock.newDateTime() instead of new DateTime() to get the current time.

[Alex]

Actually you have to use Mockito.doReturn(dt).when(a).createDateTime() Please see stackoverflow.com/questions/11620103/…

[Mureinik]

This is of course correct @Alex, thanks for noticing! Edited and fixed.

[WesternGun]

Cannot spy() a class when it does not have default constructor. Want to avoid it.

[Mureinik]

@WesternGun in the above snippet, you supply an instance of A you want to spy. It has nothing to do with a default constructor - you can construct this instance however you wish.

[WesternGun]

OK I meant @Spy of mockito.