JacksonTester

public class ExampleObjectJsonTests {

    private JacksonTester<ExampleObject> json;

    @Before
    public void setup() {
        ObjectMapper objectMapper = new ObjectMapper();
        JacksonTester.initFields(this, objectMapper);
    }

    @Test
    public void testWriteJson() throws IOException {
        ExampleObject object = //...
        assertThat(json.write(object)).isEqualToJson("expected.json");
    }

}