Date serialization & deserialization – POJO to JSON (GSON & example)

• Given the user defined object or POJO, having Date field, we would like to serialize the POJO to JSON.
• During serialization, we would like save Date as timestamp.
• We will use the Google gson library to achieve the POJO to JSON conversion and vice versa.
• We will create Person class and we will perform the following operations with Person class
  1. Convert Person Object to JSON string
     • Serialize the date as timestamp
  2. Convert the JSON string to corresponding person object
     • DeSerialize the timestamp as Date

GSON maven dependency:

<dependency>
 <groupId>com.google.code.gson</groupId>
 <artifactId>gson</artifactId>
 <version>2.5</version>
</dependency>

Program – Date to / from timestamp conversion (POJO <->JSON)

1.) Person Class:

• The person class containing date data member.
• We have overloaded toString method to print person information.

package org.learn.gson;
import java.util.Date;

public class Person {
 public String firstName;
 public String lastName;
 public Date date;
 public String contact;
 public Person(String firstName, String lastName,
    Date date, String contact) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.date = date;
  this.contact = contact;
 }
 public String toString() {
     return "[" + firstName + " " + lastName +
         " \"" + date.toString() + "\" " +contact +"]";
 }
}

2.) Date2Json Class:

Date2Json is responsible for performing following operations.

• Convert person object to JSON (serialization).
  • We have written custom DateSerializer class, to serialize date as timestamp.
• Convert JSON to person object (Deserialization).
  • We have written custom DateSerializer class, to deserialize timestamp as Date.

package org.learn.gson;
import java.lang.reflect.Type;
import java.util.Date;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import com.google.gson.reflect.TypeToken;

class DateDeserializer implements JsonDeserializer {
  
 public Date deserialize(JsonElement json, Type typeOfT,
         JsonDeserializationContext context)
   throws JsonParseException {
  return json == null ? null : new Date(json.getAsLong());
 }
}

class DateSerializer implements JsonSerializer { 
 public JsonElement serialize(Date date, Type typeOfSrc,
         JsonSerializationContext context) {
  return date == null ? null : new JsonPrimitive(date.getTime());
 }
}

public class Date2Json {
 public static void main(String[] args) {
  GsonBuilder gson = new GsonBuilder();
  gson.registerTypeAdapter(Date.class, new DateDeserializer());
  gson.registerTypeAdapter(Date.class, new DateSerializer());
  
  Gson objGson = gson.setPrettyPrinting().create();
  Person objPerson = new Person("Mike","harvey",new Date(),"00186");
  // Convert Person object to json
  String json = objGson.toJson(objPerson);
  System.out.println("1. Convert Person object to Json");
  System.out.println(json);

  // Convert to json to person object
  Type listType = new TypeToken() {}.getType();
  System.out.println("2. Convert JSON to person object");
  Person objFromJson = objGson.fromJson(json, listType);
  System.out.println(objFromJson);
 }
}

Download Sample Code – GSON Object to JSON

Output – Date serialization & deserialization (POJO <-> JSON)

1. Convert Person object to Json
{
"firstName": "Mike",
"lastName": "harvey",
"date": 1454426955623,
"contact": "00186"
}
2. Convert JSON to person object
[Mike harvey "Tue Feb 02 20:59:15 IST 2016" 00186]