About Jackson API

Jackson API is a high performance JSON processor for Java. We can perform serialization, deserialization , reading a JSON file, writing a JSON file and a lot more things using Jackson API.

To use Jackson API, we need to add it in java project build path. You can add using Maven or download a jar file with transitive jars.

Class ObjectMapper

This is the most powerful class provided by Jackson API and maximum time we will use this class. As of now just know that ObjectMapper provides functionalities for reading and writing JSON

Let’s start with our familiar JSON payload which we will create using ObjectMapper.

{
 	"firstname": "Jim",
 	"lastname": "Brown",
 	"totalprice": 111,
 	"depositpaid": true,
 	"additionalneeds": "Breakfast",
 	"bookingdates": {
     	"checkin": "2021-07-01",
     	"checkout": "2021-07-01"
 	}
}

Already I have explained that above JSON is a nested JSON Object. One JSON object is root node which holds fields like firstname, lastname, totalprice, depositpaid,additionalneeds and bookingdates where bookingdates is another JSON object.

Create a JSON Object or Object Node

To create a JSON Object using Jackson, we need to use createObjectNode() method of ObjectMapper class which returns an ObjectNode class instance. ObjectNode class has overloaded methods put(String fieldName, T fieldValue ) which takes field Name as String and values of different primitive and wrapper class types like String, Boolean etc. All field names should be unique. If you pass duplicate field name, it will not throw any error but override field values by latest. It is similar to put() method of Map in Java.

To get the created JSON Object as string, use writeValueAsString() provided by ObjectMapper class. If you want in proper JSON format, use writerWithDefaultPrettyPrinter() method for formatting.

Example Code

// Create an object to ObjectMapper
ObjectMapper objectMapper = new ObjectMapper();
		
// Creating Node that maps to JSON Object structures in JSON content
ObjectNode bookingDetails = objectMapper.createObjectNode();
		
// It is similar to map put method. put method is overloaded to accept different types of data
// String as field value
bookingDetails.put("firstname", "Jim");
bookingDetails.put("lastname", "Brown");
// integer as field value
bookingDetails.put("totalprice", 111);
// boolean as field value
bookingDetails.put("depositpaid", true);
bookingDetails.put("additionalneeds", "Breakfast");
// Duplicate field name. Will override value
bookingDetails.put("additionalneeds", "Lunch");
// To print created json object
String createdPlainJsonObject = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(bookingDetails);
System.out.println("Created plain JSON Object is : \n"+ createdPlainJsonObject);

Output :–

Created plain JSON Object is :
{
  "firstname" : "Jim",
  "lastname" : "Brown",
  "totalprice" : 111,
  "depositpaid" : true,
  "additionalneeds" : "Lunch"
}

Create a nested JSON Object or Object Node

To create a nested JSON Object or put another JSON Object as field value, we can not use put(String fieldName, JsonNode fieldValue) as it is deprecated. We use set(String fieldName, JsonNode fieldValue) or replace(String fieldName, JsonNode fieldValue)

Example Code

// Since requirement is to create a nested JSON Object
ObjectNode bookingDateDetails = objectMapper.createObjectNode();
bookingDateDetails.put("checkin", "2021-07-01");
bookingDateDetails.put("checkout", "2021-07-01");
		
// Since 2.4 , put(String fieldName, JsonNode value) is deprecated. So use either set(String fieldName, JsonNode value) or replace(String fieldName, JsonNode value)
bookingDetails.set("bookingdates", bookingDateDetails);
		
// To get the created json object as string. Use writerWithDefaultPrettyPrinter() for proper formatting
String createdNestedJsonObject = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(bookingDetails);
System.out.println("Created nested JSON Object is : \n"+ createdNestedJsonObject);

Output

Created nested JSON Object is :
{
  "firstname" : "Jim",
  "lastname" : "Brown",
  "totalprice" : 111,
  "depositpaid" : true,
  "additionalneeds" : "Lunch",
  "bookingdates" : {
    "checkin" : "2021-07-01",
    "checkout" : "2021-07-01"
  }
}