Java tutorial
/* * Copyright 2007 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.core.client.GWT; import com.google.gwt.event.dom.client.ClickEvent; import com.google.gwt.event.dom.client.ClickHandler; import com.google.gwt.http.client.Request; import com.google.gwt.http.client.RequestBuilder; import com.google.gwt.http.client.RequestCallback; import com.google.gwt.http.client.RequestException; import com.google.gwt.http.client.Response; 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.Window; import com.google.gwt.user.client.ui.Button; 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 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 RequestCallback { public void onError(Request request, Throwable exception) { displayRequestError(exception.toString()); resetSearchButtonCaption(); } public void onResponseReceived(Request request, Response response) { String responseText = response.getText(); try { JSONValue jsonValue = JSONParser.parse(responseText); displayJSONObject(jsonValue); } catch (JSONException e) { displayParseError(responseText); } resetSearchButtonCaption(); } } /* * Class for handling the fetch button's click event. */ private class SearchButtonHandler implements ClickHandler { public void onClick(ClickEvent event) { 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.com/ImageSearchService/V1/imageSearch?appid=YahooDemo * &query=potato&results=2&output=json */ private static final String DEFAULT_SEARCH_URL = GWT.getModuleBaseURL() + "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(); /* * RequestBuilder used to issue HTTP GET requests. */ private final RequestBuilder requestBuilder = new RequestBuilder(RequestBuilder.GET, DEFAULT_SEARCH_URL); 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<String> keys = jsonObject.keySet(); for (String key : keys) { 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 errorType, String errorMessage) { jsonTree.removeItems(); jsonTree.setVisible(true); TreeItem treeItem = jsonTree.addItem(errorType); treeItem.addItem(errorMessage); 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); } private void displayParseError(String responseText) { displayError("Failed to parse JSON response", responseText); } private void displayRequestError(String message) { displayError("Request failed.", message); } private void displaySendError(String message) { displayError("Failed to send the request.", message); } /* * Fetch the requested URL. */ private void doFetchURL() { searchButton.setText(SEARCH_BUTTON_WAITING_TEXT); try { requestBuilder.sendRequest(null, new JSONResponseTextHandler()); } catch (RequestException ex) { displaySendError(ex.toString()); resetSearchButtonCaption(); } } /* * Causes the text of child elements to wrap. */ private String getChildText(String text) { return "<span style='white-space:normal'>" + text + "</span>"; } /** * Initialize the main form's layout and content. */ private void initializeMainForm() { searchButton.setStyleName("JSON-SearchButton"); searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT); searchButton.addClickHandler(new SearchButtonHandler()); // 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); } private void resetSearchButtonCaption() { searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT); } }