Springboot Json Serialization and Deserialization

---

📌 JSON Serialization & Deserialization in Spring Boot

---

1. Core Concept

• Serialization → Java Object ➝ JSON String (Response to Client).
• Deserialization → JSON String ➝ Java Object (Request from Client).
• Spring Boot uses Jackson library by default (via MappingJackson2HttpMessageConverter).

---

2. How It Happens in Spring Boot

🔹 Serialization (Response Flow)

1. Controller returns a Java object.
2. Spring MVC finds a suitable HttpMessageConverter.
3. For application/json, it uses MappingJackson2HttpMessageConverter.
4. This internally uses Jackson’s ObjectMapper to convert object ➝ JSON.
5. JSON is written to the HTTP Response Body.

🔹 Deserialization (Request Flow)

1. Client sends JSON request body.
2. Spring MVC detects @RequestBody on method parameter.
3. MappingJackson2HttpMessageConverter is selected for JSON.
4. Jackson ObjectMapper maps JSON ➝ Java object.
5. The controller method gets the populated Java object.

---

3. Important Classes Involved

• ObjectMapper → Core Jackson class for serialization/deserialization.
• MappingJackson2HttpMessageConverter → Bridge between Spring MVC and Jackson.
• HttpMessageConverter → Strategy interface to convert objects to/from HTTP body.

---

4. Key Annotations (Jackson)

• @JsonProperty("fieldName") → Rename JSON property.
• @JsonIgnore → Skip field from JSON.
• @JsonInclude(Include.NON_NULL) → Exclude null fields.
• @JsonFormat(pattern="yyyy-MM-dd") → Format dates.
• @JsonCreator / @JsonValue → Custom object creation.

---

5. Customization Options

1. Configure ObjectMapper Globally

   @Bean
   public ObjectMapper objectMapper() {
       return new ObjectMapper()
               .setSerializationInclusion(JsonInclude.Include.NON_NULL)
               .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
   }

2. Custom Serializer/Deserializer

   public class UserSerializer extends JsonSerializer<User> {
       @Override
       public void serialize(User user, JsonGenerator gen, SerializerProvider serializers) throws IOException {
           gen.writeStartObject();
           gen.writeStringField("username", user.getName());
           gen.writeEndObject();
       }
   }

   Usage:

   @JsonSerialize(using = UserSerializer.class)
   private User user;

---

6. Interview-Level Insights

• Default Library: Jackson (but you can plug in Gson, JSON-B, etc.).

• Spring Boot Auto-Configuration: Auto-registers Jackson’s ObjectMapper.

• Converter Priority: If multiple converters exist, Spring chooses based on Content-Type & Accept headers.

• Common Interview Qs:

  1. How does Spring Boot convert objects to JSON internally? → Through MappingJackson2HttpMessageConverter using ObjectMapper.
  2. How to ignore null values in JSON response? → Use @JsonInclude(Include.NON_NULL) or configure ObjectMapper.
  3. What happens if a JSON field doesn’t match Java field? → By default, Jackson ignores unknown fields (FAIL_ON_UNKNOWN_PROPERTIES = false). Can be configured.
  4. Can you replace Jackson? → Yes, by excluding Jackson dependency & adding Gson or JSON-B.
  5. How does Spring know which converter to use? → By checking the Content-Type of the request & Accept header of the response.

---

One-Liner Summary for Interviews: In Spring Boot, JSON serialization (Java ➝ JSON) and deserialization (JSON ➝ Java) are handled automatically by MappingJackson2HttpMessageConverter, which delegates to Jackson’s ObjectMapper. Behavior can be customized via Jackson annotations or by configuring the ObjectMapper bean.

---