Sometimes default serialization/deserialization used by GSON is not sufficient enough and we need a custom behavior to be applied while serializing/deserializing objects to/from JSON. This can easily be done by writing custom JsonSerializer & JsonDeserializer implementation and hooking them with GsonBuilder.

We will write a custom implementation to handle a date property in our example pojo.

---

Step 1: Include GSON dependency in pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>

 <groupId>com.websystique.json</groupId>
 <artifactId>GsonCustomSerializationExample</artifactId>
 <version>1.0.0</version>
 <packaging>jar</packaging>

 <name>GsonCustomSerializationExample</name>

 <dependencies>
  <dependency>
   <groupId>com.google.code.gson</groupId>
   <artifactId>gson</artifactId>
   <version>2.3.1</version>
  </dependency>
 </dependencies>
</project>

Step 2: Sample POJO

Below pojo contains a date property named ‘promoDate’. We will write our custom implementation to provide the date for this property in a specific format(‘dd/MM/yyyy’).

package com.websystique.json.gson.model;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class Car {

 private String name;
 
 private int model;
 
 private double price;
 
 private Date promoDate;
 
 private List<String> colors = new ArrayList<String>();

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public int getModel() {
  return model;
 }

 public void setModel(int model) {
  this.model = model;
 }

 public double getPrice() {
  return price;
 }

 public void setPrice(double price) {
  this.price = price;
 }

 public List<String> getColors() {
  return colors;
 }

 public void setColors(List<String> colors) {
  this.colors = colors;
 }

 public Date getPromoDate() {
  return promoDate;
 }

 public void setPromoDate(Date promoDate) {
  this.promoDate = promoDate;
 }

 @Override
 public String toString() {
  return "Car [name=" + name + ", model=" + model + ", price=" + price
    + ", promoDate=" + promoDate + ", colors=" + colors + "]";
 }
 
 
 
}

Step 3: Write Custom Date Serializer to convert Java date to JSON string in particular format

package com.websystique.json.gson.model;

import java.lang.reflect.Type;
import java.text.SimpleDateFormat;
import java.util.Date;

import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

public class CustomDateSerializer implements JsonSerializer<Date> {

 private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");

 public JsonElement serialize(Date date, Type typeOfSrc, JsonSerializationContext context) {
  return new JsonPrimitive(dateFormat.format(date));
 }
}

Step 4: Write Custom Date Deserializer to convert date from JSON string to Java date

package com.websystique.json.gson.model;

import java.lang.reflect.Type;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;

public class CustomDateDeserializer implements JsonDeserializer<Date> {

 private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");

 public Date deserialize(JsonElement dateStr, Type typeOfSrc, JsonDeserializationContext context) {
  try {
   return dateFormat.parse(dateStr.getAsString());
  } catch (ParseException e) {
   e.printStackTrace();
  }
  return null;
 }
}

Step 5: Register custom Serializer/Deserializer with GsonBuilder, Run

package com.websystique.json.gson;

import java.util.Date;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.websystique.json.gson.model.Car;
import com.websystique.json.gson.model.CustomDateDeserializer;
import com.websystique.json.gson.model.CustomDateSerializer;

public class GsonCustomSerializationDemo {

 public static void main(String args[]){

  GsonBuilder gsonBuilder = new GsonBuilder().setPrettyPrinting();
  /* Register custom serializers with GsonBuilder */  gsonBuilder.registerTypeAdapter(Date.class, new CustomDateSerializer());
  gsonBuilder.registerTypeAdapter(Date.class, new CustomDateDeserializer());
  Gson gson = gsonBuilder.create();  
  
  Car car = new Car();
  car.setName("AUDI");
  car.setModel(2014);
  car.setPrice(30000);
  car.setPromoDate( new Date());
  
  car.getColors().add("GRAY");
  car.getColors().add("BLACK");
  car.getColors().add("WHITE");
  
  String jsonString = gson.toJson(car); // Convert car object to JSON string
  System.out.println(jsonString);
  
  Car car2 = gson.fromJson(jsonString, Car.class);// Convert JSON String to car object
  System.out.println(car2);

 }
}

Following is the output. You can see that promoDate follows the format mentioned via custom implementation.

{
  "name": "AUDI",
  "model": 2014,
  "price": 30000.0,
  "promoDate": "05/06/2015",
  "colors": [
    "GRAY",
    "BLACK",
    "WHITE"
  ]
}
Car [name=AUDI, model=2014, price=30000.0, promoDate=Fri Jun 05 00:00:00 CEST 2015, colors=[GRAY, BLACK, WHITE]]

That’s it. In the next post we will learn about Gson ExclusionStrategy used to exclude some fields from serializaion/deserialization.

References

• Google Gson official Reference
• Json Official Reference