Java/GWT/JSON

Class that acts as a client to a JSON service

   <source lang="java">

/*

* Copyright 2006 Google Inc.
* 
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
* 
* http://www.apache.org/licenses/LICENSE-2.0
* 
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/

package com.google.gwt.sample.json.client; import com.google.gwt.json.client.JSONArray; import com.google.gwt.json.client.JSONException; import com.google.gwt.json.client.JSONObject; import com.google.gwt.json.client.JSONParser; import com.google.gwt.json.client.JSONString; import com.google.gwt.json.client.JSONValue; import com.google.gwt.user.client.HTTPRequest; import com.google.gwt.user.client.ResponseTextHandler; import com.google.gwt.user.client.Window; import com.google.gwt.user.client.ui.Button; import com.google.gwt.user.client.ui.ClickListener; import com.google.gwt.user.client.ui.RootPanel; import com.google.gwt.user.client.ui.Tree; import com.google.gwt.user.client.ui.TreeItem; import com.google.gwt.user.client.ui.Widget; import java.util.Iterator; import java.util.Set; /**

* Class that acts as a client to a JSON service. Currently, this client just
* requests a text which contains a JSON encoding of a search result set from
* yahoo. We use a text file to demonstrate how the pieces work without tripping
* on cross-site scripting issues.
* 
* If you would like to make this a more dynamic example, you can associate a
* servlet with this example and simply have it hit the yahoo service and return
* the results.
*/

public class JSON {

 /**
  * Class for handling the response text associated with a request for a JSON
  * object.
  * 
  */
 private class JSONResponseTextHandler implements ResponseTextHandler {
   public void onCompletion(String responseText) {
     try {
       JSONValue jsonValue = JSONParser.parse(responseText);
       displayJSONObject(jsonValue);
     } catch (JSONException e) {
       displayError(responseText);
     }
     searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
   }
 }
 /*
  * Class for handling the fetch button"s click event.
  */
 private class SearchButtonClickListener implements ClickListener {
   public void onClick(Widget sender) {
     jsonTree.setVisible(false);
     doFetchURL();
   }
 }
 /*
  * Default URL to use to fetch JSON objects. Note that the contents of this
  * JSON result were as a result of requesting the following URL:
  * 
  * http://api.search.yahoo.ru/ImageSearchService/V1/imageSearch?appid=YahooDemo&query=potato&results=2&output=json
  * 
  */
 private static final String DEFAULT_SEARCH_URL = "search-results.js";
 /*
  * Text displayed on the fetch button when we are in a default state.
  */
 private static final String SEARCH_BUTTON_DEFAULT_TEXT = "Search";
 /*
  * Text displayed on the fetch button when we are waiting for a JSON reply.
  */
 private static final String SEARCH_BUTTON_WAITING_TEXT = "Waiting for JSON Response...";
 private Tree jsonTree = new Tree();
 private Button searchButton = new Button();
 /**
  * Entry point for this simple application. Currently, we build the
  * application"s form and wait for events.
  */
 public void onModuleLoad() {
   initializeMainForm();
 }
 /*
  * Add the object presented by the JSONValue as a children to the requested
  * TreeItem.
  */
 private void addChildren(TreeItem treeItem, JSONValue jsonValue) {
   JSONArray jsonArray;
   JSONObject jsonObject;
   JSONString jsonString;
   if ((jsonArray = jsonValue.isArray()) != null) {
     for (int i = 0; i < jsonArray.size(); ++i) {
       TreeItem child = treeItem.addItem(getChildText("["
           + Integer.toString(i) + "]"));
       addChildren(child, jsonArray.get(i));
     }
   } else if ((jsonObject = jsonValue.isObject()) != null) {
     Set keys = jsonObject.keySet();
     for (Iterator iter = keys.iterator(); iter.hasNext();) {
       String key = (String) iter.next();
       TreeItem child = treeItem.addItem(getChildText(key));
       addChildren(child, jsonObject.get(key));
     }
   } else if ((jsonString = jsonValue.isString()) != null) {
     // Use stringValue instead of toString() because we don"t want escaping
     treeItem.addItem(jsonString.stringValue());
   } else {
     // JSONBoolean, JSONNumber, and JSONNull work well with toString().
     treeItem.addItem(getChildText(jsonValue.toString()));
   }
 }
 private void displayError(String responseText) {
   jsonTree.removeItems();
   jsonTree.setVisible(true);
   TreeItem treeItem = jsonTree.addItem("Failed to parse JSON response");
   treeItem.addItem(responseText);
   treeItem.setStyleName("JSON-JSONResponseObject");
   treeItem.setState(true);
 }
 /*
  * Update the treeview of a JSON object.
  */
 private void displayJSONObject(JSONValue jsonValue) {
   jsonTree.removeItems();
   jsonTree.setVisible(true);
   TreeItem treeItem = jsonTree.addItem("JSON Response");
   addChildren(treeItem, jsonValue);
   treeItem.setStyleName("JSON-JSONResponseObject");
   treeItem.setState(true);
 }
 /*
  * Fetch the requested URL.
  */
 private void doFetchURL() {
   searchButton.setText(SEARCH_BUTTON_WAITING_TEXT);
   if (!HTTPRequest.asyncGet(DEFAULT_SEARCH_URL, new JSONResponseTextHandler())) {
     // Reset the caption.
     //
     searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
   }
 }
 /*
  * Causes the text of child elements to wrap.
  */
 private String getChildText(String text) {
   return "" + text + "";
 }
 /**
  * Initialize the main form"s layout and content.
  */
 private void initializeMainForm() {
   searchButton.setStyleName("JSON-SearchButton");
   searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
   searchButton.addClickListener(new SearchButtonClickListener());
   // Avoids showing an "empty" cell
   jsonTree.setVisible(false);
   // Find out where the host page wants the button.
   //
   RootPanel searchButtonSlot = RootPanel.get("search");
   if (searchButtonSlot == null) {
     Window.alert("Please define a container element whose id is "search"");
     return;
   }
   // Find out where the host page wants the tree view.
   //
   RootPanel treeViewSlot = RootPanel.get("tree");
   if (treeViewSlot == null) {
     Window.alert("Please define a container element whose id is "tree"");
     return;
   }
   // Add both widgets.
   //
   searchButtonSlot.add(searchButton);
   treeViewSlot.add(jsonTree);
 }

}

      </source>