Tree Model in Jackson

The tree model is another important concept in Jackson. It is used to get the in-memory representation of the JSON document. ObjectMapper class is used for building the tree of nodes (JsonNode).

There are the following two tree operations:

Creating a tree from JSON
Traversing tree model

Creating a Tree from JSON

We use ObjectMapper class for creating Tree Model. It provides a pointer for the root node of the tree after reading the JSON. The root node provided by ObjectMapper class allows us to traverse the complete tree.

Syntax of reading data from Model Tree is as follows:

 //Create an instance of the ObjectMapper class
ObjectMapper obj = new ObjectMapper();	
String jsonString = "{\"stdName\":\"Nicholas\", \"course\":24,\"enroll\":TCA1501218,\"marks\": [70,90,85]}";
//create tree from JSON
JsonNode node = obj.readTree(jsonString);

We use the try-catch block to create a tree because it can throw JsonParseException, JsonMappingException, or IOException.

Traversing Tree Model

After creating Tree from JSON, it is required to traverse it for printing the data. At the time of traversing Tree, we get each node and process data using a path relative to the root node. We traverse Tree model in the following way:

JsonNode stdName = rootNode.path("stdName");
System.out.println("Name: "+ stdName.textValue());
JsonNode enrollNode = rootNode.path("enroll");
Iterator<JsonNode> iterator = enrollNode.elements();

Let's take an example to understand how we can create a Tree model from JSON and iterate it to show the data of each node.

TreeModelExample.java

import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Scanner;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class TreeModelExample {
   public static void main(String args[]){
	   ObjectMapper mapper = new ObjectMapper();
	   Scanner sc1 = new Scanner(System.in);
		try {
			Player player = new Player();
			String playerId, playerName;
			System.out.println("Enter Player Id:");
			playerId = sc1.nextLine();
			player.setId(playerId);
			System.out.println("Enter Player Name");
			playerName = sc1.nextLine();
			player.setName(playerName);
			List<String> scoresList = new ArrayList<String>();
			for(int i = 0; i < 3; i++ ) {
				System.out.println("Enter Score"+(i+1));
				scoresList.add(sc1.nextLine());
			}
			player.setScores(scoresList);
			String jsonString = mapper.writeValueAsString(player);
			System.out.println("Json String ====> "+jsonString);
			JsonNode rootNode = mapper.readTree(jsonString);
			JsonNode idNode = rootNode.path("id");
			System.out.println("Player Id: "+ idNode.textValue());
			JsonNode nameNode = rootNode.path("name");
			System.out.println("Player Name: " + nameNode.textValue());
			JsonNode scores = rootNode.path("scores");
			Iterator<JsonNode> iterator = scores.elements();
			System.out.print("Scores: [ ");
			while (iterator.hasNext()) {
				JsonNode data = iterator.next();
				System.out.print(data.textValue() + " "); 
			}
			System.out.println("]");
		}catch (JsonParseException e) {
			e.printStackTrace(); 
		}catch (JsonMappingException e) {
			e.printStackTrace(); 
		}catch (IOException e) {
			e.printStackTrace(); 
		}
		// close scanner class object
		sc1.close();
   	}
}
class Player {  
	//Creating properties of Product class  
    	private String playerId;    
    	private String playerName;    
    	private List<String> playerScores;  
    
    	//Getter and Setters  
    	public String getId() {  
        	return playerId;  
    	}  
    	public void setId(String playerId) {  
        	this.playerId = playerId;  
    	}  
    	public String getName() {  
        	return playerName;  
    	}  
    	public void setName(String playerName) {  
        	this.playerName = playerName;  
    	}  
    	public List<String> getScores() {  
        	return playerScores;  
    	}  
    	public void setScores(List<String> playerScores) {  
        	this.playerScores = playerScores;  
    	}  
    	@Override    
    	public String toString() {    
        	// TODO Auto-generated method stub    
        	return "\"Player [Player Id "+ playerId + ", Player Name = " + playerName + ", Player Scores = " + playerScores + "]";    
    	}  
}  

Output:

Explanation:

In the above code, first, we take input from the user to create a Java object of Player. After that, we convert the Java object into JSON to make a Tree Model of the Player object. We use the readTree() method of the ObjectMapper class to get the root node. By using this root node, we get values of id, name, and scores node with the help of the path() method of JsonNode. We use an iterator for traversing each element of the scores node. The intValue() and the textvalue() methods are used for reading node value as number or text.

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