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