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Entity to DTO conversion in Java using Jackson

It's very common to have the DTO class for a given entity in any application. When persisting data, we use entity objects and when we need to provide the data to end user/application we use DTO class. Due to this we may need to have similar properties on DTO class as we have in our Entity class and to share the data we populate DTO objects using entity objects. To do this we may need to call getter on entity and then setter on DTO for the same data which increases number of code line. Also if number of DTOs are high then we need to write lot of code to just get and set the values or vice-versa.
To overcome this problem we are going to use Jackson API and will see how to do it with minimal code only.

Maven dependency

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.9.9</version>
        </dependency>

Entity class

Below is our entity class.
@Entity
@Table(name = "EMPLOYEE")
public class Employee{
    @Id @GeneratedValue
    @Column(name = "id")
    private long id;
    
    @Column(name = "name")
    private String name;
    
    @Column(name = "age")
    private Integer age;
    
    public Employee() {}
    public Employee(String name, Integer age) {
        super();
        this.name = name;
        this.age = age;
    }
    //getter methods
    //setter methods
}

DTO Class

Below is our DTO class which contains @JsonIgnoreProperties annotation to ignore the Employee instance properties during conversion which are not available in EmpDto class. Also EmptDto can have extra fields which are not available in entity class. I have not create the property for "age" in DTO which is there in entity class to showcase this scenario. Also it has "dept" property which is not there in entity class.
@JsonIgnoreProperties(ignoreUnknown = true)
class EmpDto {
    private String name;
    private long id;
    private String dept = "IT";

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    //getter methods
    //setter methods
    public String toString() {
        return "ID: " + id + ", Dept: " + dept + ", Name: " + name;
    }
}

Conversion code

Below generic method is created using Jackson API which can be used for any type of class conversion.
public  T convertObjToXXX(Object o, TypeReference ref) {
    ObjectMapper mapper = new ObjectMapper();
    return mapper.convertValue(o, ref);
}

Entity class conversion and execution

Now we will create the object of Employee class (Entity) and convert it to multiple types like Map, Properties and DTO class (can be any POJO class).
//Create entity object to be converted
Employee emp = new Employee("Emp-1", 30);
emp = empRepository.save(emp);//saving to populate the id field

//Convert to Map<String, String> class
Map<String, String> map = convertObjToXXX(emp, new TypeReference<Map<String, String>>(){});

System.out.println("Convert to Map<String, String> :");
System.out.println(map);

//Convert to Properties class
Properties props = convertObjToXXX(emp, new TypeReference<Properties>(){});
System.out.println("\nConvert to Properties :");
System.out.println(props);

//Convert to DTO class
EmpDto dto = convertObjToXXX(emp, new TypeReference<EmpDto>(){});
System.out.println("\nConvert to DTO :");
System.out.println(dto);

Output:

Below is the output of execution.
Convert to Map :
{id=1, name=Emp-1, age=30}

Convert to Properties :
{age=30, name=Emp-1, id=1}

Convert to DTO :
ID: 1, Dept: IT, Name: Emp-1

You may check my another post where I have explained how to use annotation processor to generate the DTO classes using Entity class at compile time. Link is given below.
https://www.thetechnojournals.com/2019/12/annotation-processor-to-generate-dto.html

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