[[features.json]]
== JSON
Spring Boot provides integration with three JSON mapping libraries:

- Gson
- Jackson
- JSON-B

Jackson is the preferred and default library.



[[features.json.jackson]]
=== Jackson
Auto-configuration for Jackson is provided and Jackson is part of `spring-boot-starter-json`.
When Jackson is on the classpath an `ObjectMapper` bean is automatically configured.
Several configuration properties are provided for <<howto#howto.spring-mvc.customize-jackson-objectmapper,customizing the configuration of the `ObjectMapper`>>.



[[features.json.jackson.custom-serializers-and-deserializers]]
==== Custom Serializers and Deserializers
If you use Jackson to serialize and deserialize JSON data, you might want to write your own `JsonSerializer` and `JsonDeserializer` classes.
Custom serializers are usually https://github.com/FasterXML/jackson-docs/wiki/JacksonHowToCustomSerializers[registered with Jackson through a module], but Spring Boot provides an alternative `@JsonComponent` annotation that makes it easier to directly register Spring Beans.

You can use the `@JsonComponent` annotation directly on `JsonSerializer`, `JsonDeserializer` or `KeyDeserializer` implementations.
You can also use it on classes that contain serializers/deserializers as inner classes, as shown in the following example:

include::code:MyJsonComponent[]

All `@JsonComponent` beans in the `ApplicationContext` are automatically registered with Jackson.
Because `@JsonComponent` is meta-annotated with `@Component`, the usual component-scanning rules apply.

Spring Boot also provides {spring-boot-module-code}/jackson/JsonObjectSerializer.java[`JsonObjectSerializer`] and {spring-boot-module-code}/jackson/JsonObjectDeserializer.java[`JsonObjectDeserializer`] base classes that provide useful alternatives to the standard Jackson versions when serializing objects.
See {spring-boot-module-api}/jackson/JsonObjectSerializer.html[`JsonObjectSerializer`] and {spring-boot-module-api}/jackson/JsonObjectDeserializer.html[`JsonObjectDeserializer`] in the Javadoc for details.

The example above can be rewritten to use `JsonObjectSerializer`/`JsonObjectDeserializer` as follows:

include::code:object/MyJsonComponent[]



[[features.json.jackson.mixins]]
==== Mixins
Jackson has support for mixins that can be used to mix additional annotations into those already declared on a target class.
Spring Boot's Jackson auto-configuration will scan your application's packages for classes annotated with `@JsonMixin` and register them with the auto-configured `ObjectMapper`.
The registration is performed by Spring Boot's `JsonMixinModule`.



[[features.json.gson]]
=== Gson
Auto-configuration for Gson is provided.
When Gson is on the classpath a `Gson` bean is automatically configured.
Several `+spring.gson.*+` configuration properties are provided for customizing the configuration.
To take more control, one or more `GsonBuilderCustomizer` beans can be used.



[[features.json.json-b]]
=== JSON-B
Auto-configuration for JSON-B is provided.
When the JSON-B API and an implementation are on the classpath a `Jsonb` bean will be automatically configured.
The preferred JSON-B implementation is Apache Johnzon for which dependency management is provided.