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?

Answer 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 } 

Answer 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.