When working with Java and JSON, Jackson is often the go-to library. It provides a powerful and flexible way to convert Java objects to JSON and vice versa. But what really makes Jackson shine are its annotations, which allow fine-grained control over serialization and deserialization.
In this post, we’ll explore the most useful Jackson annotations with real-world examples to help you master JSON mapping in Java.
🔧 Setup
Add Jackson to your pom.xml:
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.15.0</version>
</dependency>
1. @JsonProperty: Rename Fields
public class User {
@JsonProperty("full_name")
private String name;
}
Used to map JSON property names to Java fields when they don't match.
2. @JsonIgnore: Exclude Fields
public class User {
private String name;
@JsonIgnore
private String password;
}
Prevents sensitive data from being serialized.
3. @JsonInclude: Conditional Serialization
@JsonInclude(JsonInclude.Include.NON_NULL)
public class Product {
private String name;
private String description;
}
Avoids sending nulls in JSON payload.
4. @JsonFormat: Format Dates or Numbers
public class Event {
@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd HH:mm")
private LocalDateTime dateTime;
}
Perfect for readable date/time serialization.
5. @JsonIgnoreProperties: Ignore Unmapped JSON Fields
@JsonIgnoreProperties(ignoreUnknown = true)
public class Customer {
private String id;
private String name;
}
Avoids errors from unexpected JSON fields.
6. @JsonCreator + @JsonProperty: Constructor-Based Deserialization
public class Account {
private final String id;
private final String type;
@JsonCreator
public Account(@JsonProperty("id") String id, @JsonProperty("type") String type) {
this.id = id;
this.type = type;
}
}
Used when there's no default constructor.
7. @JsonValue: Custom Enum Serialization
public enum Status {
ACTIVE("active"),
INACTIVE("inactive");
private final String value;
Status(String value) {
this.value = value;
}
@JsonValue
public String getValue() {
return value;
}
}
Control how enums are represented.
8. @JsonAlias: Support Multiple JSON Field Names
public class UserDTO {
@JsonAlias({"fullName", "user_name"})
private String name;
}
Makes deserialization more flexible.
9. @JsonAnySetter: Dynamic JSON Properties to Map
public class Config {
private Map<String, String> settings = new HashMap<>();
@JsonAnySetter
public void addSetting(String key, String value) {
settings.put(key, value);
}
}
Great for handling dynamic key-value pairs.
10. @JsonAnyGetter: Dynamic Map to JSON
public class Config {
private Map<String, String> settings;
@JsonAnyGetter
public Map<String, String> getSettings() {
return settings;
}
}
Outputs dynamic fields from a map.
11. @JsonUnwrapped: Flatten Nested Object
public class Address {
private String city;
private String zip;
}
public class Person {
private String name;
@JsonUnwrapped
private Address address;
}
Useful when you don’t want a nested structure in the output.
12. @JsonDeserialize / @JsonSerialize: Custom (De)Serializers
public class Item {
@JsonSerialize(using = CustomDateSerializer.class)
private Date created;
}
Full control over how specific fields are (de)serialized.
13. @JsonRootName: Wrap with Root Object
Jackson can wrap JSON inside a root name using @JsonRootName. This is useful in SOAP-like or legacy APIs.
@JsonRootName(value = "user")
public class User {
private String id;
private String name;
}
Enable root wrapping:
ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
User user = new User();
user.setId("1");
user.setName("Alice");
String json = mapper.writeValueAsString(user);
Output:
{
"user": {
"id": "1",
"name": "Alice"
}
}
To enable deserialization:
mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
💡 Final Thoughts
Jackson’s annotation model is rich and adaptable to almost any real-world JSON structure you’ll face. Whether you’re building REST APIs, processing external JSON feeds, or persisting data, mastering these annotations will make your job much easier.
Have questions or a tricky mapping scenario? Drop it in the comments! 👇
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