How to implement custom serializer using @JsonSerialize annotation in Java?

The @JsonSerialize annotation is used to declare custom serializer during the serialization of a field. We can implement a custom serializer by extending the StdSeralizer class. and need to override the serialize() method of StdSerializer class.

Syntax

@Target(value={ANNOTATION_TYPE,METHOD,FIELD,TYPE,PARAMETER})
@Retention(value=RUNTIME)
public @interface JsonSerialize

In the below program, we can implement a custom serializer using @JsonSerialize annotation

Example

import java.io.*;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.annotation.*;
import com.fasterxml.jackson.databind.ser.std.*;
public class JsonSerializeAnnotationTest {
   public static void main (String[] args) throws JsonProcessingException, IOException {
      Employee emp = new Employee(115, "Adithya", new String[] {"Java", "Python", "Scala"});
      ObjectMapper mapper = new ObjectMapper();
      String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(emp);
      System.out.println(jsonString);
   }
}
// CustomSerializer class
class CustomSerializer extends StdSerializer {
   public CustomSerializer(Class t) {
      super(t);
   }
   public CustomSerializer() {
      this(Employee.class);
   }
   @Override
   public void serialize(Employee emp, JsonGenerator jgen, SerializerProvider sp) throws IOException, JsonGenerationException {
      StringBuilder sb = new StringBuilder();
      jgen.writeStartObject();
      jgen.writeNumberField("id", emp.getId());
      jgen.writeStringField("name", emp.getName());
      for(String s: emp.getLanguages()) {
         sb.append(s).append(";");
      }
      jgen.writeStringField("languages", sb.toString());
      jgen.writeEndObject();
   }
}
// Employee class
@JsonSerialize(using=CustomSerializer.class)
class Employee {
   private int id;
   private String name;
   private String[] languages;
   public Employee(int id, String name, String[] languages) {
      this.id = id;
      this.name = name;
      this.languages = languages;
   }
   public int getId() {
      return this.id;
   }
   public String getName() {
      return this.name;
   }
   public String[] getLanguages() {
      return this.languages;
   }
   @Override
   public String toString() {
      StringBuilder sb = new StringBuilder("ID: ").append(this.id).append("\nName: ").append(this.name).append("\nLanguages:");
      for(String s: this.languages) {
         sb.append(" ").append(s);
      }
      return sb.toString();
   }
}

Output

{
   "id" : 115,
   "name" : "Adithya",
   "languages" : "Java;Python;Scala;"
}