IllegalArgumentException: No serializer found for class...

 

Testing in Java can sometimes introduce unexpected challenges, especially when dealing with serialization and libraries like Jackson’s ObjectMapper in conjunction with Mockito for mocking dependencies. A common issue many developers encounter is the IllegalArgumentException: No serializer found for class... when they try to automate testing of methods involving object serialization. This blog post delves into solving this problem by improving the testing setup, ensuring that ObjectMapper is properly mocked, and integrating it within test cases effectively.

The Problem at Hand

While writing test cases for a method that uses ObjectMapper to convert values between different object types, developers might face the following error:

java.lang.IllegalArgumentException: No serializer found for class...

This occurs particularly when the ObjectMapper instance is not correctly configured for use with Mockito, leading to failures in recognizing the class properties for serialization.

Sample Scenario and Faulty Method

Consider the following method in a class named Handler which converts an Object to a specified ResponseType using ObjectMapper:

public class Handler {
    public ResponseType handleRequest(Object object) {
        ObjectMapper mapper = new ObjectMapper();
        return mapper.convertValue(object, ResponseType.class);
    }
}

In attempting to test this method, a developer might write:

@Mock
Object object;

@Mock
ResponseType response;

@Test
public void handleRequest() {
    ObjectMapper mapper = new ObjectMapper();
    when(mapper.convertValue(object, ResponseType.class)).thenReturn(response);
    new Handler().handleRequest(object);
}

Here, the handleRequest method is supposed to be tested, but it directly instantiates ObjectMapper, which isn’t linked to the mock used in the test, leading to issues and ineffective tests.

Correcting the Method

To ensure effective testing, we need to refactor both the application code and the test to allow dependency injection of ObjectMapper. Here’s how you can refactor the Handler class:

public class Handler {
    private final ObjectMapper objectMapper;

    public Handler(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    public ResponseType handleRequest(Object object) {
        return objectMapper.convertValue(object, ResponseType.class);
    }
}

For the test case, instead of directly instantiating ObjectMapper, you should mock it and pass it to the Handler:

@Mock
private ObjectMapper mapper;

@Mock
private Object object;

@Mock
private ResponseType response;

@Before
public void setUp() {
    MockitoAnnotations.initMocks(this);
}

@Test
public void handleRequestTest() {
    when(mapper.convertValue(object, ResponseType.class)).thenReturn(response);
    Handler handler = new Handler(mapper);
    ResponseType result = handler.handleRequest(object);
    assertSame(response, result);
}

Key Takeaways

1. Dependency Injection: Modify your classes to accept dependencies. This makes it easier to pass mock objects during testing.
2. Proper Mocking: Ensure that all external dependencies like ObjectMapper are mocked and controlled within your tests.
3. Mockito Annotations: Use annotations like @Mock and @Before to initialize mocks and clean up your test code.

By following these steps, you can effectively test methods that utilize ObjectMapper without running into the ‘no serializer found’ error, ensuring your tests are both robust and reliable.