How to Mock Custom Jackson Deserializer Response in Spring Boot 1.5.9

Question

In a Spring Boot 1.5.9 project, I'm currently trying to use @WebMvcTest to run a test against one of my controllers. The entity being operated on has a @JsonDeserializer() annotation on one of its properties pointing to a custom class. I'm attempting to mock the result of the deserialize() call in a test without invoking the body.

However, when trying to do the following, I'm getting a NullPointerException error on a line within the deserialize() method, which suggests the actual method body is being executed:

@Autowired
private MockMvc mvc;
@MockBean
private MyDeserializer myDeserializer
[...]
@Test
public void myTestMethod() {
  doReturn(myDeserializedValue)
      .when(myDeserializer)
      .deserialize(
          any(JsonParser.class),
          any(DeserializationContext.class)
      );
  this.mvc.perform([...]) // perform mvc call that would invoke myDeserializer
  logger.debug("Call complete"); // never gets to this line
}

I'm assuming the custom deserializer class is being invoked (possibly new ed up) outside of the knowledge of Spring's ApplicationContext.

Is there any way to mock a custom deserializer, or do I need to bump this class up to use the full ApplicationContext via @SpringBootTest and let it fully execute?

Answer 1

If you want Jackson to use a specific deserializer object instance you need to register it via a module on the ObjectMapper instance. See the docs for an example with a serializer; you'll have to modify it slightly for your deserializer.

Otherwise, I assume Jackson will just instantiate a new instance of your class every time and never use your mock (or bean?) at all.