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:
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:
@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:
@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:
@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:
@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:
@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.