北野

Jackson: build java tree model from json

Jackson provides a tree node called com.fasterxml.jackson.databind.JsonNode. The ObjectMapper provides a method to read the json into a tree with the root being a JsonNode. This can be thought of as being similar to DOM nodes in XML DOM trees. The example below demonstrates building a tree from the json string

package com.studytrails.json.jackson;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Iterator;

import org.apache.commons.io.IOUtils;

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class TreeModelParser1 {
	public static void main(String[] args) throws MalformedURLException, IOException {
		// Get a list of albums from free music archive. limit the results to 5
		String url = "http://freemusicarchive.org/api/get/albums.json?api_key=60BLHNQCAOUFPIBZ&limit=2";
		// Get the contents of json as a string using commons IO IOUTils class.
		String genreJson = IOUtils.toString(new URL(url));

		// create an ObjectMapper instance.
		ObjectMapper mapper = new ObjectMapper();
		// use the ObjectMapper to read the json string and create a tree
		JsonNode node = mapper.readTree(genreJson);

		// lets see what type the node is
		System.out.println(node.getNodeType()); // prints OBJECT
		// is it a container
		System.out.println(node.isContainerNode()); // prints true
		// lets find out what fields it has
		Iterator<String> fieldNames = node.fieldNames();
		while (fieldNames.hasNext()) {
			String fieldName = fieldNames.next();
			System.out.println(fieldName);// prints title, message, errors,
											// total,
											// total_pages, page, limit, dataset
		}

		// we now know what elemets the container has. lets get the value for
		// one of them
		System.out.println(node.get("title").asText()); // prints
														// "Free Music Archive".

		// Lets look at the dataset now.
		JsonNode dataset = node.get("dataset");

		// what is its type?
		System.out.println(dataset.getNodeType()); // Prints ARRAY

		// so the dataset is an array. Lets iterate through the array and see
		// what each of the elements are
		Iterator<JsonNode> datasetElements = dataset.iterator();
		while (datasetElements.hasNext()) {
			JsonNode datasetElement = datasetElements.next();
			// what is its type
			System.out.println(datasetElement.getNodeType());// Prints Object
			// it is again a container . what are the elements ?
			Iterator<String> datasetElementFields = datasetElement.fieldNames();
			while (datasetElementFields.hasNext()) {
				String datasetElementField = datasetElementFields.next();
				System.out.println(datasetElementField); // prints album_id,
															// album_title,
															// album_handle,
															// album_url,
															// album_type,
															// artist_name,
															// artist_url,
															// album_producer,
															// album_engineer,
															// album_information,
															// album_date_released,
															// album_comments,
															// album_favorites,
															// album_tracks,
															// album_listens,
															// album_date_created,
															// album_image_file,
															// album_images

			}
			// break from the loop, since we just want to see the structure
			break;

		}

	}
}

Lets look at another example that shows the use of the path method. The path method is similar to the get() method but if the node does not exist, it does not return null but returns an object of type MissingNode

package com.studytrails.json.jackson;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Iterator;

import org.apache.commons.io.IOUtils;

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class TreeModelParser2 {
	public static void main(String[] args) throws MalformedURLException, IOException {
		// Get a list of albums from free music archive. limit the results to 5
		String url = "http://freemusicarchive.org/api/get/albums.json?api_key=60BLHNQCAOUFPIBZ&limit=10";
		// Get the contents of json as a string using commons IO IOUTils class.
		String genreJson = IOUtils.toString(new URL(url));

		// create an ObjectMapper instance.
		ObjectMapper mapper = new ObjectMapper();
		// use the ObjectMapper to read the json string and create a tree
		JsonNode node = mapper.readTree(genreJson);

		// not the use of path. this does not cause the code to break if the
		// code does not exist
		Iterator<JsonNode> albums = node.path("dataset").iterator();
		while (albums.hasNext()) {
			System.out.println(albums.next().path("album_title"));
		}

	}
}

(Note: This article’s original links is here )