How to get a JsonProcessingException using Jackson

Question

Might be a strange question but indeed I would like to achieve a a bit more coverage on my tests and although I coded against a JsonProcessingException I can't create a payload that generates this exception, maybe because Jackson is quite smart and converts everything to a string, and even for bad strings it goes around the JSON specs. My problem is that Jackson is quite good :)

I basically want a payload that when I run this, it break with JsonProcessingException:

String jsonPayload = objectMapper.writeValueAsString(payload); 

I've tried some like:

HashMap<String, String> invalidJSONPayload= new HashMap<>();  invalidJSONPayload.put("021",021); invalidJSONPayload.put("---",021); invalidJSONPayload.put("~",021); 

I'm not fussed with the type, so feel free to suggest another one. An empty object for example, throws JsonMappingException and I already catch that one as well.

Answer 1

I wanted to do the same thing, and eventually accomplished it by using the Mockito "spy" function, which wraps a real object with a mock object. All calls to the mock object get forwarded to the real object, except those you are trying to mock. For example:

ObjectMapper om = Mockito.spy(new ObjectMapper()); Mockito.when( om.writeValueAsString(ErrorObject.class)).thenThrow(new JsonProcessingException("") {}); 

All usages of om will be handled by the underlying ObjectMapper instance until an instance of ErrorObject gets passed in, at which point the JsonProcessingException will be thrown.

The newJsonProcessingException is created as an anonymous class, as it is a protected class and only a sub-class can be instantiated.