In the previous section, we discussed the APIs provided by JSR 353 to convert the JSON data into the Java object model. In this section, you will learn how to convert a Java object model into the JSON format, which is the opposite of the operation discussed in the previous section.

The very first step is to build an object model. You can use either of the following classes to generate the object model. The choice of the builder class depends on whether you want to generate a JSON object or a JSON array:

• javax.json.JsonObjectBuilder: This builder class is used for generating the JSON object model from scratch. This class provides methods to add the name-value pairs to the object model and to return the final object.
• javax.json.JsonArrayBuilder: This builder class is used for generating an array of JSON objects from scratch. This class provides methods to add objects or values to the array model and to return the final array.

The builder classes can be created either from the javax.json.Json factory class or from the javax.json.JsonBuilderFactory class. You may go for JsonBuilderFactory if you want to override the default configurations for the generated objects (configurations are specific to vendors) or if you need to create multiple instances of the builder classes.

The following code snippet illustrates the use of the JsonArrayBuilder APIs for converting an array of the employee objects into the JSON array. The client builds the JSON objects by using JsonObjectBuilder and adds them to JsonArrayBuilder. Finally, when the client calls the build() method, JsonArrayBuilder returns the associated JSON array:

//import statements for the core classes used this example 
//Other imports are removed for brevity 
import javax.json.Json; 
import javax.json.JsonArray; 
import javax.json.JsonArrayBuilder; 
import javax.json.JsonObject; 
import javax.json.JsonObjectBuilder; 
import javax.json.JsonWriter; 
import java.io.FileOutputStream; 
import java.io.OutputStream; 
 
 
// Creates a JsonArrayBuilder instance that is 
// used to build JsonArray 
JsonArrayBuilder jsonArrayBuilder = Json.createArrayBuilder(); 
 
//Get a list of Employee instances. 
// We are not interested in the implementation details of the 
// getEmployeeList() method used in this example. 
List<Employee> employees = getEmployeeList(); 
 
//Iterate over the employee list and create JsonObject for each item 
for (Employee employee : employees) { 
 
//Add desired name-value pairs to the JSON object and push 
// each object in to the array. 
  jsonArrayBuilder.add( 
    Json.createObjectBuilder() 
    .add("employeeId", employee.getEmployeeId()) 
    .add("firstName", employee.getFirstName()) 
    .add("lastName", employee.getLastName()) 
    .add("email", employee.getEmail()) 
    .add("hireDate", employee.getHireDate()) 
  ); 
 
} 
//Return the json array holding employee details 
JsonArray employeesArray = jsonArrayBuilder.build(); 
 
//write the array to file 
OutputStream outputStream = new FileOutputStream 
  ("emp-array.json"); 
JsonWriter jsonWriter = Json.createWriter(outputStream); 
jsonWriter.writeArray(employeesArray); 
 
//Close the stream to clean up the associated resources 
outputStream.close(); 
jsonWriter.close(); 

The resulting file output content may look like the following:

[ {"employeeId":100,"firstName":"John","lastName":"Chen", 
  "email":"[email protected]","hireDate":"2008-10-16"}, 
  {"employeeId":101,"firstName":"Ameya","lastName":"Job", 
"email":"[email protected]","hireDate":"2013-03-06"}]