Master Custom JSON Serialization in Spring Boot 3 with @JsonComponent

1. Introduction

The article explains how to customize JSON serialization and deserialization in Spring Boot using a programming approach, complementing the annotation‑based method previously covered.

2. Practical Example

By annotating a class with @JsonComponent , you can directly register custom JsonSerializer and JsonDeserializer implementations.

<code>@JsonComponent
public class PackJsonComponent {
  // Custom serializer
  public static class Serializer extends JsonSerializer<User> {
    @Override
    public void serialize(User value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
      jgen.writeStartObject();
      jgen.writeStringField("name", value.getName());
      jgen.writeNumberField("age", value.getAge());
      jgen.writeEndObject();
    }
  }

  // Custom deserializer
  public static class Deserializer extends JsonDeserializer<User> {
    @Override
    public User deserialize(JsonParser jsonParser, DeserializationContext ctxt) throws IOException {
      ObjectCodec codec = jsonParser.getCodec();
      JsonNode tree = codec.readTree(jsonParser);
      String name = tree.get("name").textValue();
      int age = tree.get("age").intValue();
      long id = tree.get("id").longValue() + 1000L; // added for demonstration
      return new User(id, name, age);
    }
  }
}
</code>

The following controller demonstrates the usage of the custom components:

<code>@RestController
@RequestMapping("/jackson")
public class JacksonController {
  @GetMapping("/user")
  public User queryUser() {
    return new User(1L, "Zhang San", 10);
  }

  @PostMapping("/save")
  public User saveUser(@RequestBody User user) {
    return user;
  }
}

// User class
public class User {
  private Long id;
  private String name;
  private Integer age;
  // getters and setters
}
</code>

Testing the /user and /save endpoints returns the expected JSON, as shown in the screenshots below.

All beans annotated with @JsonComponent are automatically registered with Jackson because the annotation itself is meta‑annotated with @Component , so standard component‑scanning rules apply.

Spring Boot also provides JsonObjectSerializer and JsonObjectDeserializer base classes for more flexible handling:

<code>@JsonComponent
public class PackJsonComponent {
  public static class Serializer extends JsonObjectSerializer<User> {
    @Override
    protected void serializeObject(User value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
      jgen.writeNumberField("id", value.getId());
      jgen.writeStringField("name", value.getName());
      jgen.writeNumberField("age", value.getAge());
      jgen.writeStringField("nation", "China🇨🇳");
      Map<String, Object> params = new HashMap<>();
      params.put("q", "java");
      params.put("s", "pack");
      jgen.writeObjectField("params", params);
    }
  }

  public static class Deserializer extends JsonObjectDeserializer<User> {
    @Override
    protected User deserializeObject(JsonParser jsonParser, DeserializationContext context, ObjectCodec codec, JsonNode tree) throws IOException {
      long id = nullSafeValue(tree.get("id"), Long.class) + 1L;
      String name = nullSafeValue(tree.get("name"), String.class);
      int age = nullSafeValue(tree.get("age"), Integer.class);
      return new User(id, name, age);
    }
  }
}
</code>

Note that you must not call jgen.writeObject(value) inside a custom serializer, as it will cause an error; instead, write individual fields or other objects as demonstrated.

<code>// Incorrect usage – will throw an exception
jgen.writeObject(value);
</code>

All custom serializers/deserializers only affect the specific generic type they target; other types remain unaffected.