Here's an example of how to use the JsonFactory class from the Apache Commons IO library to create a JSON object from a Java object and write it to a file:

import org.apache.commons.io.FileUtils;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerator;

import java.io.File;
import java.io.IOException;

public class Main {
    public static void main(String[] args) throws IOException {
        // Create a Java object to be converted to JSON
        Person person = new Person("John", "Doe", 30);

        // Create a JSON factory
        JsonFactory jsonFactory = new JsonFactory();

        // Create a JSON generator
        JsonGenerator jsonGenerator = jsonFactory.createJsonGenerator(new File("person.json"), JsonEncoding.UTF8);

        // Write the Java object to the JSON generator
        jsonGenerator.writeObject(person);

        // Close the JSON generator
        jsonGenerator.close();
    }
}

Here, Person is a class that represents a person.

class Person {
    private String firstName;
    private String lastName;
    private int age;

    public Person(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    // getters and setters here
}

This will write a JSON object to a file called "person.json" that looks like this:

{
    "firstName": "John",
    "lastName": "Doe",
    "age": 30
}

You can also use jsonGenerator.writeStartObject() and jsonGenerator.writeEndObject() to create JSON object manually.

You can read the json file using FileUtils.readFileToString(new File("person.json"), "UTF-8")