Niklas Tech Blog

System under test class example:

import my.special.sclient
...

class App {
  Sclient sclient = getNewSclient();
  
  public String methodA(String input) { 
    return sclient.getValue(input);
  }

  protected Sclient getNewSclient() {
    return new Sclient();
  }
}

Test class example:

import org.mockito.InjectMocks;
import org.mockito.Spy;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

import org.testng.annotations.BeforeMethod;
import org.testng.annotations.Test;
...

class Test {
  @Spy
  @InjectMocks
  private App sut;

  @Mock
  Sclient mockClient;

  @BeforeMethod
  public void setUp() {
    MockitoAnnotations.initMocks(this);
    when(sut.getNewSclient()).thenReturn(mockClient);
    when(mockClient.getValue("bar")).thenReturn("foo");
  }

  @Test
  public void myTest() {
    String value = sut.getValue("bar");
    assertThat(value, is("foo"));
  }
}

So, how does this work? I will try to explain below:

1. First we need to be able to intercept the ‘sclient’ in the App class. This is done by simply creating a ‘getNewSclient()’ function and then mock it in the Test class so that we instead can use a mocked version of it.

2. Next we need to setup our test class. Here we need both a @Spy and @InjectMocks on the App object. We also need a mocked instance of the ‘sclient’ we are going to use

3. Now we need to set upp the desired behaviour of our mocked ‘sclient’. We set it so that if you input “bar” you should get “foo” back

4. Done!

Tested on Java v1.8.0_252, Mockito v3.3.3, TestNG v6.11 and Spring Boot v2.2.4