Utility class containing static methods that are useful for reading in and marshalling JSON

JsonUtil.java

package net.blerg.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.UnsupportedEncodingException;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLEncoder;
import java.util.Map;
import java.util.Map.Entry;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.gson.Gson;

/**
 * Utility class containing static methods that are useful for reading in and
 * marshalling JSON
 * 
 * @author arthur
 * 
 */
public class JsonUtil {

	private static final Logger log = LoggerFactory.getLogger(JsonUtil.class);
	private static final String UTF8 = "UTF-8";

	/**
	 * Gets the JSON from a given URL with the contents of postData POSTed in
	 * and marshals it into an object of type T
	 * 
	 * @param urlString
	 *            The URL to open
	 * @param classOfT
	 *            Class of the object to marshal the JSON into
	 * @param postData
	 *            Map of data to POST to the URL
	 * @return An object of type T if the JSON was successfully retrieved from
	 *         the URL, otherwise null is returned.
	 */
	public static <T> T getJsonFromUrl(String urlString, Class<T> classOfT, Map<String, String> postData) {
		try {
			URL url = new URL(urlString);

			InputStream inputStream = null;
			if (postData != null && postData.size() > 0) {
				URLConnection urlConnection = url.openConnection();
				urlConnection.setDoOutput(true);
				OutputStreamWriter writer = new OutputStreamWriter(urlConnection.getOutputStream());
				writer.write(mapToData(postData));
				writer.flush();
				inputStream = urlConnection.getInputStream();
			} else {
				inputStream = url.openStream();
			}
			BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
			Gson gson = new Gson();
			return gson.fromJson(reader, classOfT);
		} catch (MalformedURLException e) {
			log.error("Could not parse " + urlString + " into a URL object.", e);
		} catch (IOException e) {
			log.error("Could not open " + urlString, e);
		}

		return null;
	}

	/**
	 * Gets the JSON from a given URL and marshals it into an object of type T
	 * 
	 * @param urlString
	 *            The URL to open
	 * @param classOfT
	 *            Class of the object to marshal the JSON into
	 * @return An object of type T if the JSON was successfully retrieved from
	 *         the URL, otherwise null is returned.
	 */
	public static <T> T getJsonFromUrl(String urlString, Class<T> classOfT) {
		return getJsonFromUrl(urlString, classOfT, null);
	}

	/**
	 * URL encodes the keys and values into UTF-8 format and concatenates the
	 * key=value pairs together with an &. This has been created in order to
	 * POST data.
	 * 
	 * @param map
	 *            A map of keys and values to be URL encoded
	 * @return A string such as key1=foo&key2=bar where key1 and key2 are keys
	 *         in the map and foo and bar are their corresponding values.
	 */
	private static String mapToData(Map<String, String> map) {

		boolean first = true;
		String data = "";

		for (Entry<String, String> entry : map.entrySet()) {
			if (first) {
				first = false;
			} else {
				data += "&";
			}

			try {
				data += URLEncoder.encode(entry.getKey(), UTF8) + "=" + URLEncoder.encode(entry.getValue(), UTF8);
			} catch (UnsupportedEncodingException e) {
				log.error("URLEncoding failed, with either " + entry.getKey() + " and/or " + entry.getValue(), e);
			}
		}

		return data;
	}
}