Using @JsonComponent in Spring Boot

https://www.baeldung.com/spring-boot-jsoncomponent

 

1. Overview

This quick article is focused on how to use the @JsonComponent annotation in Spring Boot.

The annotation allows us to expose an annotated class to be a Jackson serializer and/or deserializer without the need to add it to the ObjectMapper manually.

This is part of the core Spring Boot module, so there are no additional dependencies required in a plain Spring Boot application.

2. Serialization

Let’s start with the following User object containing a favorite color:

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public class User {     private Color favoriteColor;       // standard getters/constructors }

If we serialize this object using Jackson with default settings we get:

{
  "favoriteColor": {
    "red": 0.9411764740943909,
    "green": 0.9725490212440491,
    "blue": 1.0,
    "opacity": 1.0,
    "opaque": true,
    "hue": 208.00000000000003,
    "saturation": 0.05882352590560913,
    "brightness": 1.0
  }
}

We can make the JSON a lot more condensed and readable by just printing the RGB values – for example, to be used in CSS.

To this extent, we just have to create a class that implements JsonSerializer:

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@JsonComponent public class UserJsonSerializer extends JsonSerializer<User> {       @Override     public void serialize(User user, JsonGenerator jsonGenerator,       SerializerProvider serializerProvider) throws IOException,       JsonProcessingException {            jsonGenerator.writeStartObject();         jsonGenerator.writeStringField(           "favoriteColor",           getColorAsWebColor(user.getFavoriteColor()));         jsonGenerator.writeEndObject();     }       private static String getColorAsWebColor(Color color) {         int r = (int) Math.round(color.getRed() * 255.0);         int g = (int) Math.round(color.getGreen() * 255.0);         int b = (int) Math.round(color.getBlue() * 255.0);         return String.format("#%02x%02x%02x", r, g, b);     } }

With this serializer, the resulting JSON has been reduced to:

{"favoriteColor":"#f0f8ff"}

Due to the @JsonComponent annotation, the serializer is registered in the Jackson ObjectMapper in the Spring Boot application. We can test this with the following JUnit test:

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@JsonTest @RunWith(SpringRunner.class) public class UserJsonSerializerTest {       @Autowired     private ObjectMapper objectMapper;       @Test     public void testSerialization() throws JsonProcessingException {         User user = new User(Color.ALICEBLUE);         String json = objectMapper.writeValueAsString(user);            assertEquals("{\"favoriteColor\":\"#f0f8ff\"}", json);     } }

3. Deserialization

Continuing with the same example, we can write a deserializer that will turn the web color String into a JavaFX Color object:

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@JsonComponent public class UserJsonDeserializer extends JsonDeserializer<User> {        @Override     public User deserialize(JsonParser jsonParser,       DeserializationContext deserializationContext) throws IOException,       JsonProcessingException {            TreeNode treeNode = jsonParser.getCodec().readTree(jsonParser);         TextNode favoriteColor           = (TextNode) treeNode.get("favoriteColor");         return new User(Color.web(favoriteColor.asText()));     } }

Let’s test the new deserializer and make sure everything works as expected:

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@JsonTest @RunWith(SpringRunner.class) public class UserJsonDeserializerTest {       @Autowired     private ObjectMapper objectMapper;       @Test     public void testDeserialize() throws IOException {         String json = "{\"favoriteColor\":\"#f0f8ff\"}"         User user = objectMapper.readValue(json, User.class);            assertEquals(Color.ALICEBLUE, user.getFavoriteColor());     } }

4. Serializer and Deserializer in one Class

When desired, we can connect the serializer and the deserializer in one class by using two inner classes and adding the @JsonComponent on the enclosing class:

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@JsonComponent public class UserCombinedSerializer {        public static class UserJsonSerializer       extends JsonSerializer<User> {           @Override         public void serialize(User user, JsonGenerator jsonGenerator,           SerializerProvider serializerProvider) throws IOException,           JsonProcessingException {                jsonGenerator.writeStartObject();             jsonGenerator.writeStringField(               "favoriteColor", getColorAsWebColor(user.getFavoriteColor()));             jsonGenerator.writeEndObject();         }           private static String getColorAsWebColor(Color color) {             int r = (int) Math.round(color.getRed() * 255.0);             int g = (int) Math.round(color.getGreen() * 255.0);             int b = (int) Math.round(color.getBlue() * 255.0);             return String.format("#%02x%02x%02x", r, g, b);         }     }       public static class UserJsonDeserializer       extends JsonDeserializer<User> {            @Override         public User deserialize(JsonParser jsonParser,           DeserializationContext deserializationContext)           throws IOException, JsonProcessingException {                TreeNode treeNode = jsonParser.getCodec().readTree(jsonParser);             TextNode favoriteColor = (TextNode) treeNode.get(               "favoriteColor");             return new User(Color.web(favoriteColor.asText()));         }     } }

5. Conclusion

This quick tutorial showed how to quickly add a Jackson serializer/deserializer in a Spring Boot application by leveraging component scanning with the @JsonComponent annotation.

The code snippets can be found over on GitHub.