Force SpingBoot to use Gson over Jackson - spring-boot

Question

I am trying to force SpringBoot to use Gson instead of Jackson. I've read most of the articles I've found online and I am still seeing Jackson being used. Here's what I've done
Added
spring:
http: { converters: { preferred-json-mapper: gson } }
mvc: { converters: {preferred-json-mapper: gson } }
in application.yaml
Updated POM
Added gson dependency
Added jackson-databind to exclusion list in spring-boot-starter-web depedency.
Added #EnableAutoConfiguration(exclude = JacksonAutoConfiguration.class) to main class.
Written below #Configuration class:
#Configuration
#Slf4j
public class MyConfig implements WebMvcConfigurer {
#Override
public void extendMessageConverters (List<HttpMessageConverters<?>> converters) {
log.debug("Setting gson converter");
converters.add(new GsonHttpMessageConverter(myCustomGsonInstance()));
}
public Gson myCustomGsonInstance() {
return new Gson();
}
}
When running tests in debug, I can see that Jackson is still listed in the HttpMessageConverters list and Gson is not.
Update:
This behavior is seen while running live and in the below test class.
#AutoConfigureMockMvc
#SpringBootTest(webEnvironment = MOCK)
#ExtendWith(MockitoExtension.class)
public class MyTestClass {
#Autowired
private MyController controller;
private MockMvc mockMvc;
#BeforeEach
public void setUp(){
mockMvc = MockMvcBuilders.standaloneSetup(controller)
// .setMessageConverters(new GsonHttpMessageConverter(myCustomGsonInstance())) // if I add this, the test passes.
.build();
}
#Test
public void happyFlow(){
// given
URI uri = "/test/uri";
HttpHeaders headers = new HttpHeaders();
headers.set(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE);
// when
String responseBody = mockMvc.perform(get(uri).headers(headers)).andReturn().getResponse().getContentAsString();
// then
assertThat(responseBody, wasSerializedByGson());
}
}

Answer 1

It looks like you're using the wrong property for configuring the preferred JSON mapper. You are using spring.http.converters.preferred-json-mapper but the correct property is spring.mvc.converters.preferred-json-mapper. In application.yaml, that would be the following:
spring:
mvc:
converters:
preferred-json-mapper: gson

Answer 2

Spring Boot comes with Gson Auto Configuration support: Source Code
So you have to Autowire the Gson singleton instance to be used by your WebMvcConfigurer in addition to enabling the yaml property:
#Configuration
#Slf4j
public class MyConfig implements WebMvcConfigurer {
#Autowired
private Gson gson;
#Override
public void extendMessageConverters (List<HttpMessageConverters<?>> converters) {
log.debug("Setting gson converter");
converters.add(new GsonHttpMessageConverter(gson));
}
}
And the yaml properties borrowed from Andy Wilkinson:
spring:
mvc:
converters:
preferred-json-mapper: gson
With this setup Spring MVC is using the same Gson instance as the one Autowired in your configuration.
And in your test, it should look like this:
#WebMvcTest(MyController.class)
public class MyTestClass {
#Autowired
private MockMvc mockMvc;
#Autowired
private MyController controller;
#Test
public void happyFlow(){
// given
URI uri = "/test/uri";
HttpHeaders headers = new HttpHeaders();
headers.set(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE);
// when
String responseBody = mockMvc.perform(get(uri).headers(headers)).andReturn().getResponse().getContentAsString();
// then
assertThat(responseBody, wasSerializedByGson());
}
}