Convert Java object to JSON

When learning how to write Java-based software, one of the first snags developers hit is how to connect their code with other software. This is usually where JSON comes in. While you might be a wizard with Java, JSON is another animal. Regardless, this blog post explains all you need to get the job done.

What is a Java object? 

A Java object is a combination of data and procedures that work on the available data.

Objects have both states and behaviors. In Java, an object is created using the keyword “new.”

Objects are created from templates known as classes. 

An object is an instance of a class.

For example, our Cat object has:

• States, like color, name, and breed. The state of an object is stored in fields (variables). 
• Behavior, like purring, eating, and sleeping. Methods (functions) display the object’s behavior.

What is a JSON String?

• JSON is an acronym for JavaScript Object Notation. 
• JSON was designed as a data interchange format and has a syntax that is a subset of JavaScript.
• Context that is surrounded by quotes (single or double), loaded from a text file etc., are called JSON strings. For example:
  {“id”:1,”name”:”SiAm”,”color”:”Cream”,”eyecolor”:”Blue”,”breed”:”Siamese”}
• JSON is interoperable, meaning that it’s language/platform independent.     
• JSON format is used for serializing and transmitting structured data over a network connection. It’s used primarily to transmit data between a server and mobile / web application, serving as an alternative to XML. 

Common uses for converting Java Obj to JSON String

The example below demonstrates a client-server scenario where the RESTful Web Service accepts data in XML/JSON: 

• The RESTful web server app is designed using Java. 
• The enduser doesn’t understand the XML / JSON, but that’s not an issue.
• The enduser communicates with a mobile app that might be Android.
• The enduser communicates with a mobile app that might be php.
• The mobile/web app communicates with the RESTful web service via XML / JSON.

When would you want to convert from Java Obj to JSON string?

In our example diagram above, our RESTful web service was designed using Java. 

Since Java objects are only understood by Java applications, we need to convert the Java object to JSON when creating a web service for the Android app. Let’s say the mobile app is a hybrid app where the frontend is handled by Android view and the data transactions are sent through its own web services using JSON. In this instance, we need to send/receive requests from the Android app to/from our database using web services/API using JSON data structure.

• JSON is a simple string format data. JSON is readable format and it’s very easy to read and infer information from it.
• JSON format is simple to use.
• JSON is quite lightweight compared to other formats like XML.
• JSON format can be easily converted into Java objects in an Object-oriented manner.
• JSON is interoperable (program and platform independent).

Convert a Java Object to JSON String: Step-by-step tutorial

The most common way to convert a Java Object to a JSON string is to use an API. The most common APIs for this purpose are Jackson and GSON.

JACKSON API example

This example shows how to use JACKSON API to convert a Java Object into a JSON String.

We can use the ObjectMapper class provided by the Jackson API for our conversion:

• writeValueAsString() is used to convert java obj to JSON
• readValue() is used to convert JSON into java obj

Step 1: Include the JACKSON JAR files into your classpath.

When using MAVEN for dependency management (recommended), you can include the following dependency to download JAR files and any dependency for JACKSON and automatically include it in your project’s classpath.

Add the following dependency to the pom file:

<dependencies>
<dependency>
		<groupId>com.fasterxml.jackson.core</groupId>
		<artifactId>jackson-databind</artifactId>
		<version>2.9.8</version>
</dependency>
</dependencies>

Step 2: Use the Jackson API ObjectMapper class to convert Java Object to a JSON string

ObjectMapper mapper = new ObjectMapper();
try {
  String json = mapper.writeValueAsString(cat);
  System.out.println("ResultingJSONstring = " + json);
  //System.out.println(json);
} catch (JsonProcessingException e) {
   e.printStackTrace();
}

This example uses the following code: 

class useJACKSONapiToConvertJavaOBJtoJSONstring

import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
 public class useJACKSONapiToConvertJavaOBJtoJSONstring {
     public static void main(String[] args) {
         Cat cat = new Cat();
         cat.setId(1L);
         cat.setName("SiAm");
         cat.setColor("Cream");
         cat.setEyecolor("Blue");
         cat.setBreed("Siamese");
         ObjectMapper mapper = new ObjectMapper();
         try {
             String json = mapper.writeValueAsString(cat);
             System.out.println("ResultingJSONstring = " + json);
             //System.out.println(json);
         } catch (JsonProcessingException e) {
             e.printStackTrace();
 }
 class Cat 

public class Cat {
     private Long id;
     private String name;
     private String color;
     private String eyecolor;
     private String breed;
     public Cat() {
     public Cat(Long id, String name) {
         this.id = id;
         this.name = name;
     // Getters & Setters
     @Override
     public String toString() {
         return "Cat{" +
             "id=" + id +
             ", name='" + name +
             ''' +
             '}';
 public Long getId() { return id; }
 public void setId(Long id) { this.id = id; }
 public String getName() { return name; }
 public void setName(String name) { this.name = name; }
 public String getColor() {  return color; }
 public void setColor(String color) { this.color = color; }
 public String getEyecolor() { return eyecolor;
 public void setEyecolor(String eyecolor) { this.eyecolor = eyecolor; }
 public String getBreed() {  return breed; }
 public void setBreed(String breed) { this.breed = breed; }
 }  

Step 3: RUN useJACKSONapitoConvertJavaOBJtoJSONstring 

ResultingJSONstring = {"id":1,"name":"SiAm","color":"Cream","eyecolor":"Blue","breed":"Siamese"}

GSON API example

Find the best examples of Java code snippets using com.google.gson.

The below example shows how to use GSON API to convert a Java Object into a JSON String. 

Step 1: Include the GSON JAR files into your classpath

When using MAVEN for dependency management (recommended), you can include the following dependency to download JAR files and any dependency for GSON and automatically include it in your project’s classpath.

Add the following dependency to the pom file: 

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

Step 2: Create class UseGSONapitoConvertJavaOBJtoJASONstring

Call the GSON API using: Gson gson = new Gson();

This example uses the following code: 

class UseGSONapitoConvertJavaOBJtoJASONstring

import com.google.gson.Gson;
public class UseGSONapitoConvertJavaOBJtoJASONstring{
  public static void main(String args[]) {
   CatDetails user = new CatDetails("SiAm",
	         "Siamese",
	        "siam.cat@gmail.com",
	         9,
	         2129991234L,
	         "NewCatadonia",
	         true);
    Gson gson = new Gson();
    String json = gson.toJson(user);
    System.out.println(json);
}

Class CatDetails

/**
 * Java Program to map a Java object to JSON String using GSON library.
 */
class CatDetails {
  private String name;
  private String breed;
  private String email;
  private int catlives;
  private long phone;
  private String city;
  private boolean likesMice;
  public CatDetails(String name, String breed, String email, int catlives, long phone,
      String city, boolean likesMice) {
    super();
    this.name = name;
    this.email = email;
    this.catlives = catlives;
    this.phone = phone;
    this.city = city;
    this.likesMice = likesMice;
    this.breed = breed;
//getters & setters
public String getName() {
	return name;
}
public void setName(String name) {
	this.name = name;
}
public String getBreed() {
	return breed;
}
public void setBreed(String breed) {
	this.breed = breed;
}
public String getEmail() {
	return email;
}
public void setEmail(String email) {
	this.email = email;
}
public int getCatlives() {
	return catlives;
}
public void setCatlives(int catlives) {
	this.catlives = catlives;
}
public long getPhone() {
	return phone;
}
public void setPhone(long phone) {
	this.phone = phone;
}
public String getCity() {
	return city;
}
public void setCity(String city) {
	this.city = city;
}
public boolean isLikesMice() {
	return likesMice;
}
public void setLikesMice(boolean likesMice) {
	this.likesMice = likesMice;
}
}

Result

Step 3: RUN UseGSONapitoConvertJavaOBJtoJASONstring

{"name":"SiAm","breed":"Siamese","email":"siam.cat@gmail.com","catlives":9,"phone":2129991234,"city":"NewCatadonia","likesMice":true}

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Conclusion

Converting a Java Obj to a JSON string is simple using JACKSON or GSON API. In our examples, we provided the code to make it easy for you to reproduce in your IDE.

All you need to do is:

1. Create a new project (Maven is recommended).
2. Include the JAR files in your classpath by adding dependencies to the pom file.
3. Create your classes.
4. Use the  JACKSON API: ObjectMapper mapper class:
   call writeValueAsString(ObjToConvert) method by passing the object we want to convert into JSON
   or
   Use GSON API:  class Gson:
   call toJson(ObjToConvert) method by passing the object we want to convert into JSON;

Run to convert your Java Obj to JSON string.

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