How to mock field type Resource of Spring with JUNIT5 and Mockito?

Question

I have a class that reads JSON File from my directory, but I Could't mock the Resource field:

import org.springframework.core.io.Resource;

public class MyClass{

   
  @Value("classpath:file.json")
  private Resource resourceFile;

  public MyDTO getInfoFromJSONFile() {
    try {
        
     //Read file
    } catch (IOException e) {
      throw new MyException("General Error");
    }
  }

This my test:

import org.springframework.core.io.Resource;

@ContextConfiguration({ "classpath:file.json" })
class MyClassTest{

    private MyClass subject;

    @Mock
    private Resource resourceFile;

    @BeforeEach
    void setUp() {
        this.subject = new MyClass();
        MockitoAnnotations.initMocks(this);
     }

    @Test
    @ParameterizedTest
    @ValueSource(strings = {"myParam"})
    void testReadInfoFromJsonFileSuccessfully(String param){
        MyDTO myDTO= subject.getInfoFromJSONFile(providerConfigId);
        Assertions.assertEquals(myDTO.getMyField(), "VALUE");
    }
}

The field resourceFile never is initialized with a mock value, How can I mock this type of field ?

Answer 1

Mockito @Mock does not perform Spring Dependency Injection mechanism. It just creates a mock and assigns it to the resourceFile field inside your test suite.

There are two options:

1. Replace field injection with constructor injection and put the value manually.

public class MyClass{
  private Resource resourceFile;

  public MyClass(@Value("classpath:file.json") Resource resourceFile) {
    this.resourceFile = resourceFile;
  }
}

class MyClassTest{

    private MyClass subject;

    @Mock
    private Resource resourceFile;

    @BeforeEach
    void setUp() {
        MockitoAnnotations.initMocks(this);
        this.subject = new MyClass(resourceFile);
     }
}

2. Apply @SpringBootTest to run the Spring Context and proceed the bean lifecycle.

@SpringBootTest
class MyClassTest{
    @Autowired
    private MyClass subject;
    ...
}

Though in this case, you won't be able to mock Resource. Instead, you should put the file with required content to test/resources.