java - working - Use Mockito to mock some methods but not others




mockito when not working (4)

Is there any way, using Mockito, to mock some methods in a class, but not others?

For example, in this (admittedly contrived) Stock class I want to mock the getPrice() and getQuantity() return values (as shown in the test snippet below) but I want the getValue() to perform the multiplication as coded in the Stock class

public class Stock {
  private final double price;
  private final int quantity;

  Stock(double price, int quantity) {
    this.price = price;
    this.quantity = quantity;
  }

  public double getPrice() {
    return price;
  }

  public int getQuantity() {
    return quantity;
  }
  public double getValue() {
    return getPrice() * getQuantity();
  }

  @Test
  public void getValueTest() {
    Stock stock = mock(Stock.class);
    when(stock.getPrice()).thenReturn(100.00);
    when(stock.getQuantity()).thenReturn(200);
    double value = stock.getValue();
    // Unfortunately the following assert fails, because the mock Stock getValue() method does not perform the Stock.getValue() calculation code.
    assertEquals("Stock value not correct", 100.00*200, value, .00001);
}

According to docs :

Foo mock = mock(Foo.class, CALLS_REAL_METHODS);

// this calls the real implementation of Foo.getSomething()
value = mock.getSomething();

when(mock.getSomething()).thenReturn(fakeValue);

// now fakeValue is returned
value = mock.getSomething();

Partial mocking of a class is also supported via Spy in mockito

List list = new LinkedList();
List spy = spy(list);

//optionally, you can stub out some methods:
when(spy.size()).thenReturn(100);

//using the spy calls real methods
spy.add("one");
spy.add("two");

//size() method was stubbed - 100 is printed
System.out.println(spy.size());

Check the 1.10.19 and 2.7.22 docs for detailed explanation.


The accepted answer is not correct according to the question.

The call to Stock stock = mock(Stock.class); calls org.mockito.Mockito.mock(Class<T>) which looks like this:

 public static <T> T mock(Class<T> classToMock) {
    return mock(classToMock, withSettings().defaultAnswer(RETURNS_DEFAULTS));
}

The docs of the value RETURNS_DEFAULTS tell:

/**
 * The default <code>Answer</code> of every mock <b>if</b> the mock was not stubbed.
 * Typically it just returns some empty value. 
 * <p>
 * {@link Answer} can be used to define the return values of unstubbed invocations. 
 * <p>
 * This implementation first tries the global configuration. 
 * If there is no global configuration then it uses {@link ReturnsEmptyValues} (returns zeros, empty collections, nulls, etc.)
 */

What you want is org.mockito.Mockito.CALLS_REAL_METHODS according to the docs:

/**
 * Optional <code>Answer</code> to be used with {@link Mockito#mock(Class, Answer)}
 * <p>
 * {@link Answer} can be used to define the return values of unstubbed invocations.
 * <p>
 * This implementation can be helpful when working with legacy code.
 * When this implementation is used, unstubbed methods will delegate to the real implementation.
 * This is a way to create a partial mock object that calls real methods by default.
 * <p>
 * As usual you are going to read <b>the partial mock warning</b>:
 * Object oriented programming is more less tackling complexity by dividing the complexity into separate, specific, SRPy objects.
 * How does partial mock fit into this paradigm? Well, it just doesn't... 
 * Partial mock usually means that the complexity has been moved to a different method on the same object.
 * In most cases, this is not the way you want to design your application.
 * <p>
 * However, there are rare cases when partial mocks come handy: 
 * dealing with code you cannot change easily (3rd party interfaces, interim refactoring of legacy code etc.)
 * However, I wouldn't use partial mocks for new, test-driven & well-designed code.
 * <p>
 * Example:
 * <pre class="code"><code class="java">
 * Foo mock = mock(Foo.class, CALLS_REAL_METHODS);
 *
 * // this calls the real implementation of Foo.getSomething()
 * value = mock.getSomething();
 *
 * when(mock.getSomething()).thenReturn(fakeValue);
 *
 * // now fakeValue is returned
 * value = mock.getSomething();
 * </code></pre>
 */

Thus your code should look like:

import org.junit.Test;
import static org.mockito.Mockito.*;
import static org.junit.Assert.*;

public class StockTest {

    public class Stock {
        private final double price;
        private final int quantity;

        Stock(double price, int quantity) {
            this.price = price;
            this.quantity = quantity;
        }

        public double getPrice() {
            return price;
        }

        public int getQuantity() {
            return quantity;
        }

        public double getValue() {
            return getPrice() * getQuantity();
        }
    }

    @Test
    public void getValueTest() {
        Stock stock = mock(Stock.class, withSettings().defaultAnswer(CALLS_REAL_METHODS));
        when(stock.getPrice()).thenReturn(100.00);
        when(stock.getQuantity()).thenReturn(200);
        double value = stock.getValue();

        assertEquals("Stock value not correct", 100.00 * 200, value, .00001);
    }
}

To directly answer your question, yes, you can mock some methods without mocking others. This is called a partial mock. See the Mockito documentation on partial mocks for more information.

For your example, you can do something like the following, in your test:

Stock stock = mock(Stock.class);
when(stock.getPrice()).thenReturn(100.00);    // Mock implementation
when(stock.getQuantity()).thenReturn(200);    // Mock implementation
when(stock.getValue()).thenCallRealMethod();  // Real implementation

In that case, each method implementation is mocked, unless specify thenCallRealMethod() in the when(..) clause.

There is also a possibility the other way arround with spy instead of mock:

Stock stock = spy(Stock.class);
when(stock.getPrice()).thenReturn(100.00);    // Mock implementation
when(stock.getQuantity()).thenReturn(200);    // Mock implementation
// All other method call will use the real implementations

In that case, all method implementation are the real one, except if you have defined a mocked behaviour with when(..).

There is one important pitfall when you use when(Object) with spy like in the previous example. The real method will be called (because stock.getPrice() is evaluated before when(..) at runtime). This can be a problem if your method contains logic that should not be called. You can write the previous example like this:

Stock stock = spy(Stock.class);
doReturn(100.00).when(stock).getPrice();    // Mock implementation
doReturn(200).when(stock).getQuantity();    // Mock implementation
// All other method call will use the real implementations

However, with your example, I believe it will still fail, since the implementation of getValue() relies on quantity and price, rather than getQuantity() and getPrice(), which is what you've mocked.

What it really seems like you want is just:

@Test
public void getValueTest() {
    Stock stock = new Stock(100.00, 200);
    double value = stock.getValue();
    assertEquals("Stock value not correct", 100.00*200, value, .00001);
}




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