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

Answer 1

I found this in Jackson Github issue; but it solved my problem and I am able to throw the JsonProcessingException.

 @Test
public void forceJsonParseException() {
try {
    Object mockItem = mock(Object.class);
    when(mockItem.toString()).thenReturn(mockItem.getClass().getName());
    new ObjectMapper().writeValueAsString(mockItem);
    fail("did not throw JsonProcessingException");
} catch (JsonProcessingException e) {
    //pass
}
}

Now how I used this in my code

Need to test this method

public String geResponse(MyObject myObject) {

try {
  return objectMapper.writeValueAsString(myObject);
} catch (JsonProcessingException e) {
  log.error("Service response JsonParsing error {} ", e.getMessage());
  return "Validation Service response JsonParsing error {} "+ e.getMessage();
 }
}

This how I test the JsonProcessingException

@SneakyThrows
@Test
public void testGetValidationResponseNegative() {
    MyObject mockItem = mock(MyObject.class);
    when(mockItem.toString()).thenReturn(mockItem.getClass().getName());
    String vr = geResponse(mockItem);
    assertTrue(!vr.isEmpty());
}

I hope this helps!!!

Answer 2

Trying to mock using mock(ObjectMapper.class) will invariably result in Checked exception is invalid for this method! as it is not possible to throw checked exception (JsonProcessingException extends IOException). Creating a self referencing value object like other answers suggested could be too convoluted for many cases and looks like a hack.

The easiest way I found is to extend ObjectMapper and then use that in your test method. You should pass the subclass to SUT

@Test
public void buildJsonSwallowsJsonProcessingException() {

    class MyObjectMapper extends ObjectMapper {
        @Override
        public String writeValueAsString(Object value)
                throws com.fasterxml.jackson.core.JsonProcessingException {
            throw new com.fasterxml.jackson.core.JsonProcessingException("Forced error") {};
        }
    }

    ObjectMapper objectMapper = new MyObjectMapper();

    SUTBean sutbean = new SUTBean(objectMapper);

    sutbean.testMethod();

    assertTrue(expected, actual);
}