In this post, we will learn how to create a JSON object from Java object using JSON-P library.

Check out complete JSON-P tutorial at Java JSON Processing Tutorial.

The code example is available at my Github repository.

Check out complete JSON-P tutorial at Java JSON Processing Tutorial.
Check out the Jackson JSON tutorial at https://www.javaguides.net/p/java-jackson-json-tutorial-with-examples.html.
Check out google GSON tutorial at Google GSON Tutorial.

JSON-P

JSON Processing (JSON-P) is a Java API to process (for e.g. parse, generate, transform and query) JSON messages. It produces and consumes JSON text in a streaming fashion (similar to StAX API for XML) and allows to build a Java object model for JSON text using API classes (similar to DOM API for XML).

Read more about JSON-P at official documentation - https://javaee.github.io/jsonp.

Add Dependencies

JSON-P is the reference implementation for Java JSON Processing API. We can use this in maven project by adding the following dependencies:

<dependency>
    <groupId>javax.json</groupId>
    <artifactId>javax.json-api</artifactId>
    <version>1.1</version>
</dependency>

<dependency>
    <groupId>org.glassfish</groupId>
    <artifactId>javax.json</artifactId>
    <version>1.1</version>
</dependency>

Let's create a JSON file named "posts.json":

{
    "id": 100,
    "title": "JSONP Tutorial",
    "description": "Post about JSONP",
     "content": "HTML content here",
    "tags": [
        "Java",
        "JSON"
    ]
}

We have Java bean classes that represent above JSON format as:

package net.javaguides.jsonp.tutorial;

import java.util.Arrays;

public class Post {
    private int id;
    private String title;
    private String description;
    private String content;
    private String[] tags;

    public Post() {

    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String[] getTags() {
        return tags;
    }

    public void setTags(String[] tags) {
        this.tags = tags;
    }

    @Override
    public String toString() {
        return "Post [id=" + id + ", title=" + title + ", description=" + description + ", content=" + content +
            ", tags=" + Arrays.toString(tags) + "]";
    }
}

Java Create JSON Example

In the below example, JsonGenerator writes JSON data to an output source in a streaming way. The JsonGeneratorFactory contains methods to create JsonGenerator instances.

package net.javaguides.jsonp.tutorial;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.HashMap;
import java.util.Map;

import javax.json.Json;
import javax.json.stream.JsonGenerator;
import javax.json.stream.JsonGeneratorFactory;


/**
 * Class to create a json using jsonp library.
 * @author Ramesh fadatare
 *
 */
public class CreateJSON {
    public static void main(String[] args) throws FileNotFoundException {
        OutputStream fos = new FileOutputStream("posts.json");

        Map < String, Boolean > config = new HashMap < String, Boolean > ();
        config.put(JsonGenerator.PRETTY_PRINTING, true);

        JsonGeneratorFactory factory = Json.createGeneratorFactory(config);

        JsonGenerator jsonGenerator = factory.createGenerator(fos);

        Post post = createPost();
        jsonGenerator.writeStartObject(); // {
        jsonGenerator.write("id", post.getId()); // "id":123
        jsonGenerator.write("title", post.getTitle());
        jsonGenerator.write("description", post.getDescription());
        jsonGenerator.write("content", post.getContent());

        jsonGenerator.writeStartArray("tags");
        for (String tag: post.getTags()) {
            jsonGenerator.write(tag);
        }

        jsonGenerator.writeEnd(); // end of phone num array
        jsonGenerator.writeEnd(); // }

        jsonGenerator.close();
    }

    private static Post createPost() {
        // create a post
        Post post = new Post();
        post.setTitle("JSONP Tutorial");
        post.setId(100);
        post.setDescription("Post about JSONP");
        post.setContent("HTML content here");

        String[] tags = {
            "Java",
            "JSON"
        };
        // create some predefined tags
        post.setTags(tags);

        // set tags to post
        return post;
    }
}

Let's understand the above Java create JSON example using JSON-P library.

We create a JSON and store in "posts.json" file:

OutputStream fos = new FileOutputStream("posts.json");

A JsonGenerator is created with a JsonGeneratorFactory. The factory receives configuration data which enable pretty printing:

Map<String, Boolean> config = new HashMap<String, Boolean>();
config.put(JsonGenerator.PRETTY_PRINTING, true);

JsonGeneratorFactory factory = Json.createGeneratorFactory(config);

JsonGenerator jsonGenerator = factory.createGenerator(fos);
``
An object is started with writeStartObject(). It is later ended with writeEnd():
```java
jsonGenerator.writeStartObject();

An array is started with writeStartArray(). It is later ended with writeEnd():

jsonGenerator.writeStartArray("tags");

Convert post object into JSON object:

Post post = createPost();
jsonGenerator.writeStartObject(); // {
jsonGenerator.write("id", post.getId()); // "id":123
jsonGenerator.write("title", post.getTitle());
jsonGenerator.write("description", post.getDescription());
jsonGenerator.write("content", post.getContent());

private static Post createPost() {
		// create a post
        Post post = new Post();
        post.setTitle("JSONP Tutorial");
        post.setId(100);
        post.setDescription("Post about JSONP");
        post.setContent("HTML content here");

        String[] tags = {"Java", "JSON"};
        // create some predefined tags
        post.setTags(tags);
        
        // set tags to post
		return post;		
	}