Spring Boot JSON Auto-Configuration — Jackson and ObjectMapper

Direct Answer

When Jackson is on the classpath, Spring Boot can auto-configure a Jackson ObjectMapper and Spring MVC can register a Jackson HttpMessageConverter. That makes JSON a common representation, but it does not force every @RestController return value to application/json. Content negotiation, produces, the return type, and all configured converters determine the response.

Code Example

@Configuration
public class JacksonConfig {

    @Bean
    public ObjectMapper objectMapper() {
        var mapper = new ObjectMapper();
        // Include non-null fields only (don't serialize null values)
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        // Use snake_case for JSON field names instead of camelCase
        mapper.setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
        // Write dates as "2024-01-15" instead of timestamps
        mapper.registerModule(new JavaTimeModule());
        mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        return mapper;
    }
}

@RestController
class UserController {
    @GetMapping("/api/users/{id}")
    public User getUser(@PathVariable Long id) {
        // This User will be serialized using our custom ObjectMapper
        return new User(id, "john_doe", null); // null email won't appear in JSON
    }
}

record User(Long id, String username, String email) {}

Common Mistakes & Pitfalls

• Manually adding Jackson dependency: Freshers add jackson-databind to pom.xml when spring-boot-starter-web already includes it. This can cause version conflicts. Always check what your starter includes first.
• Not understanding auto-configuration override: Creating a Bean named objectMapper but not realizing Spring Boot's default is still active because they misspelled the method or used wrong scope. The custom bean must match the name and return type exactly.
• Using @Autowired ObjectMapper in config: Trying to inject ObjectMapper in the same @Configuration class that creates it causes circular dependencies. Define it as a @Bean method, don't inject it.
• Ignoring application.properties: Spring Boot lets you configure common ObjectMapper settings via properties like spring.jackson.default-property-inclusion=non_null. Freshers write a whole @Bean when a single property would work.