Jackson - Tree Model



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Tree Model prepares a in-memory tree representation of the JSON document. ObjectMapper build tree of JsonNode nodes. It is most flexible approach. It is analogus to DOM parser for XML.

Create Tree from JSON

ObjectMapper provides a pointer to root node of the tree after reading the JSON. Root Node can be used to traverse the complete tree. Consider the following code snippet to get the root node of a provided JSON String.

//Create an ObjectMapper instance
ObjectMapper mapper = new ObjectMapper();	
String jsonString = "{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
//create tree from JSON
JsonNode rootNode = mapper.readTree(jsonString);

Traversing Tree Model

Get each node using relative path to the root node while traversing tree and process the data. Consider the following code snippet traversing the tree provided the root node.

JsonNode nameNode = rootNode.path("name");
System.out.println("Name: "+ nameNode.textValue());
 
JsonNode marksNode = rootNode.path("marks");
Iterator<JsonNode> iterator = marksNode.elements();

Example

Create a java class file named JacksonTester in C:\>Jackson_WORKSPACE.

File: JacksonTester.java

import java.io.IOException;
import java.util.Iterator;

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 JacksonTester {
   public static void main(String args[]){

      try {
         ObjectMapper mapper = new ObjectMapper();
         String jsonString = "{\"name\":\"Mahesh Kumar\",  \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
         JsonNode rootNode = mapper.readTree(jsonString);

         JsonNode nameNode = rootNode.path("name");
         System.out.println("Name: "+ nameNode.textValue());

         JsonNode ageNode = rootNode.path("age");
         System.out.println("Age: " + ageNode.intValue());

         JsonNode verifiedNode = rootNode.path("verified");
         System.out.println("Verified: " + (verifiedNode.booleanValue() ? "Yes":"No"));

         JsonNode marksNode = rootNode.path("marks");
         Iterator<JsonNode> iterator = marksNode.elements();
         System.out.print("Marks: [ ");

         while (iterator.hasNext()) {
            JsonNode marks = iterator.next();
            System.out.print(marks.intValue() + " "); 
         }

         System.out.println("]");
      }
      catch (JsonParseException e) { e.printStackTrace(); }
      catch (JsonMappingException e) { e.printStackTrace(); }
      catch (IOException e) { e.printStackTrace(); }
   }
}

Verify the result

Compile the classes using javac compiler as follows:

C:\Jackson_WORKSPACE>javac JacksonTester.java

Now run the jacksonTester to see the result:

C:\Jackson_WORKSPACE>java JacksonTester

Verify the Output

Name: Mahesh Kumar
Age: 21
Verified: No
Marks: [ 100 90 85 ]

Tree to JSON

Tree to Java Objects



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