DTO conversions using Jackson

JSON to DTO

This page shows process of converting JSON to a DTO in Java using Lombok to simplify the DTO class.

What is Lombok?

Lombok is a Java library that helps reduce boilerplate code by generating common methods like getters, setters, constructors, toString(), equals(), and hashCode() at compile time using annotations. This helps make DTOs cleaner and easier to maintain.

1. Adding Dependencies for Lombok and Jackson

If you’re using Maven, add the following dependencies to your pom.xml file:

<!-- Jackson for JSON handling -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.0</version> <!-- or the latest version -->
</dependency>

<!-- Lombok for reducing boilerplate code -->
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.20</version> <!-- or the latest version -->
    <scope>provided</scope>
</dependency>

2. Example JSON

Assume we have the following JSON that we want to map to a DTO:

{
  "name": "Alice",
  "age": 30,
  "email": "alice@example.com"
}

3. DTO Class Using Lombok

Using Lombok, we can avoid manually writing getters, setters, constructors, and toString() methods by using appropriate Lombok annotations.

import lombok.Data;

@Data
public class UserDTO {
    private String name;
    private int age;
    private String email;
}

Lombok Annotations Explained

  • @Data: This is a combination of several Lombok annotations:
    • @Getter and @Setter (generates getters and setters for all fields),
    • @ToString (generates a toString() method),
    • @EqualsAndHashCode (generates equals() and hashCode() methods),
    • @RequiredArgsConstructor (generates a constructor for final fields).

So the UserDTO class now automatically has all the necessary methods without having to manually write them.

4. Converting JSON to DTO Using Jackson and Lombok

You can now use Jackson’s ObjectMapper to convert the JSON string to the UserDTO object.

import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonToDtoWithLombokExample {
    public static void main(String[] args) {
        // Example JSON string
        String json = "{ \"name\": \"Alice\", \"age\": 30, \"email\": \"alice@example.com\" }";
        
        // Create ObjectMapper instance
        ObjectMapper objectMapper = new ObjectMapper();

        try {
            // Convert JSON string to UserDTO
            UserDTO user = objectMapper.readValue(json, UserDTO.class);
            
            // Print the UserDTO object
            System.out.println(user);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

5. Explanation

  • ObjectMapper.readValue(String content, Class<T> valueType): This method reads the JSON string and converts it into a Java object of the specified class (UserDTO in this case).
  • Thanks to Lombok, you don’t need to write getters, setters, or the toString() method in the UserDTO class.

Output

When you run this code, it will print the UserDTO object:

UserDTO(name=Alice, age=30, email=alice@example.com)

Customizing Field Names (Optional)

If the field names in the JSON are different from the DTO fields, you can use Jackson’s @JsonProperty annotation to map them. Here’s an example:

import com.fasterxml.jackson.annotation.JsonProperty;
import lombok.Data;

@Data
public class UserDTO {
    @JsonProperty("full_name")
    private String name;

    @JsonProperty("user_age")
    private int age;

    @JsonProperty("user_email")
    private String email;
}

In this case, the JSON would have fields like "full_name", "user_age", and "user_email", but these would map to the name, age, and email fields of the DTO.

Conclusion

Using Lombok reduces boilerplate code in your DTOs, making your code cleaner and easier to maintain. Combined with Jackson, it becomes very easy to convert JSON into DTOs in Java

DTO to JSON

Converting a DTO (Data Transfer Object) to JSON in Java can be done using libraries like Jackson. This process is called serialization, where you convert a Java object into a JSON string.

Let’s walk through an example using Jackson and Lombok.

1. Adding Dependencies

If you’re using Maven, ensure you have the following dependencies for Jackson and Lombok:

<!-- Jackson for JSON serialization -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.0</version> <!-- or the latest version -->
</dependency>

<!-- Lombok for reducing boilerplate code -->
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.20</version> <!-- or the latest version -->
    <scope>provided</scope>
</dependency>

2. DTO Class Using Lombok

Here’s an example of a simple DTO using Lombok’s @Data annotation to avoid writing boilerplate code (like getters, setters, toString(), etc.).

import lombok.Data;

@Data
public class UserDTO {
    private String name;
    private int age;
    private String email;
}

3. Converting DTO to JSON

Now, let’s write the code to create a UserDTO object and convert it to a JSON string using Jackson.

import com.fasterxml.jackson.databind.ObjectMapper;

public class DtoToJsonExample {
    public static void main(String[] args) {
        // Create a new UserDTO object
        UserDTO user = new UserDTO();
        user.setName("Alice");
        user.setAge(30);
        user.setEmail("alice@example.com");

        // Create an ObjectMapper instance
        ObjectMapper objectMapper = new ObjectMapper();

        try {
            // Convert the UserDTO object to a JSON string
            String json = objectMapper.writeValueAsString(user);
            
            // Print the JSON string
            System.out.println(json);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Explanation

  • ObjectMapper.writeValueAsString(Object value): This method is used to convert a Java object (in this case, UserDTO) into a JSON string.
  • The DTO UserDTO object is serialized into JSON format using Jackson.

Output

When you run the program, it will print the following JSON string:

{
  "name": "Alice",
  "age": 30,
  "email": "alice@example.com"
}

Customizing the JSON Output

If you need to customize the field names in the generated JSON (for example, to match an API requirement), you can use Jackson’s @JsonProperty annotation in the DTO class.

import com.fasterxml.jackson.annotation.JsonProperty;
import lombok.Data;

@Data
public class UserDTO {
    @JsonProperty("full_name")
    private String name;

    @JsonProperty("user_age")
    private int age;

    @JsonProperty("user_email")
    private String email;
}

Now, when you convert the DTO to JSON, the output will look like this:

{
  "full_name": "Alice",
  "user_age": 30,
  "user_email": "alice@example.com"
}

Conclusion

Converting a DTO to JSON in Java using Jackson is straightforward. By using Lombok for the DTO and Jackson for serialization, you can easily convert a Java object to a JSON string with minimal boilerplate code.

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3. semester efterår 2024