Mocking new instances with Powermock

Using Powermock it is possible to mock a new instance to avoid unnecessary depth to your unit test. This is relatively unusual in an IoC environment (inversion of control eg spring) because you'd inject the PropertyLoader and therefore mocking would be easy.  However, particularly in legacy code this can be found.

If a class has this method you many not want to traverse into the PropertyLoader.

public class MyClass {

    /**
     * Method to enrich the object with a property applicable for today.
     *
     * @param myObj The object to be enriched
     */
    public void enrichWithProperties(MyObject myObj) {
        final PropertyLoader propertyLoader = new PropertyLoader();
        myObj.setProperty(propertyLoader.getTodaysProperty());
    }
}

It could be that new PropertyLoader() has a whole series of dependencies that you don't want to end up mocking as this will make the unit test really unclear.  Instead you can use Powermock to return a mocked PropertyLoader instance to greatly simply things.



import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
import static org.powermock.api.mockito.PowerMockito.whenNew;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

/**
 * Test class for the .....
 */
@RunWith(PowerMockRunner.class)
@PrepareForTest(MyClass.class)
public class MyClassTest {

    /** The class under test. */
    private MyClass classUnderTest = new MyClass();

    @Test
    public void testEnrichWithProperties() {

        // Arrange
        final String todaysValue = "todaysValue";
        final MyObject myObj = new MyObject();
        final PropertyLoader mockedPropertyLoader = mock(PropertyLoader.class);
        whenNew(PropertyLoader.class).withAnyArguments().thenReturn(mockedPropertyLoader);
        when(mockedPropertyLoader.getTodaysProperty()).thenReturn(todaysValue);

        // Act
        classUnderTest.enrichWithProperties(myObj);

        // Assert
        assertEquals(todaysValue, myObj.getProperty());
    }

}


Note: Unlike the mocking statics, you have to PrepareForTest the class which instantiates the mocked class not the mocked class itself. So above MyClass is prepared not PropertyLoader.