JSONParser


1   /*
2    * Copyright 2006 Google Inc.
3    * 
4    * Licensed under the Apache License, Version 2.0 (the "License"); you may not
5    * use this file except in compliance with the License. You may obtain a copy of
6    * the License at
7    * 
8    * http://www.apache.org/licenses/LICENSE-2.0
9    * 
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13   * License for the specific language governing permissions and limitations under
14   * the License.
15   */
16  package com.google.gwt.json.client;
17  
18  import com.google.gwt.core.client.JavaScriptException;
19  import com.google.gwt.core.client.JavaScriptObject;
20  
21  /**
22   * Parses the string representation of a JSON object into a set of
23   * JSONValue-derived objects.
24   * 
25   * @see com.google.gwt.json.client.JSONValue
26   */
27  public class JSONParser {
28  
29    /**
30     * Given a jsonString, returns the JSONObject representation. For efficiency,
31     * parsing occurs lazily as the structure is requested.
32     * 
33     * @param jsonString
34     * @return a JSONObject that has been built by parsing the JSON string
35     * @throws NullPointerException if <code>jsonString</code> is
36     *           <code>null</code>
37     * @throws IllegalArgumentException if <code>jsonString</code> is empty
38     */
39    public static JSONValue parse(String   jsonString) {
40      // Create a JavaScriptObject from the JSON string.
41      //
42      if (jsonString == null) {
43        throw new NullPointerException  ();
44      }
45      if (jsonString == "") {
46        throw new IllegalArgumentException  ("empty argument");
47      }
48      try {
49        JavaScriptObject jsonObject = evaluate(jsonString);
50        return buildValue(jsonObject);
51      } catch (JavaScriptException ex) {
52        throw new JSONException(ex);
53      }
54    }
55  
56    /**
57     * Returns the {@link JSONValue} for a given {@link JavaScriptObject}.  
58     * 
59     * @param jsValue {@link JavaScriptObject} to build a {@link JSONValue} for, 
60     *     this object cannot be a primitive JavaScript type
61     * @return a {@link JSONValue} instance for the {@link JavaScriptObject}
62     */
63    static JSONValue buildValue(JavaScriptObject jsValue) throws JSONException {
64      
65      if (isNull(jsValue)) {
66        return JSONNull.getInstance();
67      }
68  
69      if (isArray(jsValue)) {
70        return new JSONArray(jsValue);
71      }
72  
73      if (isBoolean(jsValue)) {
74        return JSONBoolean.getInstance(asBoolean(jsValue));
75      }
76  
77      if (isString(jsValue)) {
78        return new JSONString(asString(jsValue));
79      }
80  
81      if (isDouble(jsValue)) {
82        return new JSONNumber(asDouble(jsValue));
83      }
84  
85      if (isObject(jsValue)) {
86        return new JSONObject(jsValue);
87      }
88  
89      /*
90       * In practice we should never reach this point.  If we do, we cannot make 
91       * any assumptions about the jsValue.
92       */
93      throw new JSONException("Unknown JavaScriptObject type");
94    }
95  
96    /**
97     * Returns the boolean represented by the jsValue. This method
98     * assumes that {@link #isBoolean(JavaScriptObject)} returned
99     * <code>true</code>.
100    * 
101    * @param jsValue JavaScript object to convert
102    * @return the boolean represented by the jsValue
103    */
104   private static native boolean asBoolean(JavaScriptObject jsValue) /*-{
105     return jsValue.valueOf();
106   }-*/;
107 
108   /**
109    * Returns the double represented by jsValue. This method assumes that
110    * {@link #isDouble(JavaScriptObject)} returned <code>true</code>.
111    * 
112    * @param jsValue JavaScript object to convert
113    * @return the double represented by the jsValue
114    */
115   private static native double asDouble(JavaScriptObject jsValue) /*-{
116     return jsValue.valueOf();
117   }-*/;
118 
119   /**
120    * Returns the Javascript String as a Java String. This method assumes that
121    * {@link #isString(JavaScriptObject)} returned <code>true</code>.
122    * 
123    * @param jsValue JavaScript object to convert
124    * @return the String represented by the jsValue
125    */
126   private static native String   asString(JavaScriptObject jsValue) /*-{
127     return jsValue;
128   }-*/;
129 
130   /*
131    * This method converts the json string into a JavaScriptObject inside of JSNI
132    * method by simply evaluating the string in JavaScript.
133    */
134   private static native JavaScriptObject evaluate(String   jsonString) /*-{
135     var x = eval('(' + jsonString + ')');
136     if (typeof x == 'number' || typeof x == 'string' || typeof x == 'array' || typeof x == 'boolean') {
137       x = (Object(x));
138     }
139     return x;
140   }-*/;
141 
142   /**
143    * Returns <code>true</code> if the {@link JavaScriptObject} is a wrapped
144    * JavaScript Array.
145    * 
146    * @param jsValue JavaScript object to test
147    * @return <code>true</code> if jsValue is a wrapped JavaScript Array
148    */
149   private static native boolean isArray(JavaScriptObject jsValue) /*-{
150     return jsValue instanceof Array; 
151   }-*/;
152 
153   /**
154    * Returns <code>true</code> if the {@link JavaScriptObject} is a wrapped
155    * JavaScript Boolean.
156    * 
157    * @param jsValue JavaScript object to test
158    * @return <code>true</code> if jsValue is a wrapped JavaScript Boolean
159    */
160   private static native boolean isBoolean(JavaScriptObject jsValue) /*-{
161     return jsValue instanceof Boolean; 
162   }-*/;
163 
164   /**
165    * Returns <code>true</code> if the {@link JavaScriptObject} is a wrapped
166    * JavaScript Double.
167    * 
168    * @param jsValue JavaScript object to test
169    * @return <code>true</code> if jsValue is a wrapped JavaScript Double
170    */
171   private static native boolean isDouble(JavaScriptObject jsValue) /*-{
172     return jsValue instanceof Number;
173   }-*/;
174 
175   /**
176    * Returns <code>true</code> if the {@link JavaScriptObject} is <code>null</code>
177    * or <code>undefined</code>.
178    * 
179    * @param jsValue JavaScript object to test
180    * @return <code>true</code> if jsValue is <code>null</code> or
181    *         <code>undefined</code>
182    */
183   private static native boolean isNull(JavaScriptObject jsValue) /*-{
184     return jsValue == null;
185   }-*/;
186 
187   /**
188    * Returns <code>true</code> if the {@link JavaScriptObject} is a JavaScript
189    * Object.
190    * 
191    * @param jsValue JavaScript object to test
192    * @return <code>true</code> if jsValue is a JavaScript Object
193    */
194   private static native boolean isObject(JavaScriptObject jsValue) /*-{
195     return jsValue instanceof Object;
196   }-*/;
197 
198   /**
199    * Returns <code>true</code> if the {@link JavaScriptObject} is a JavaScript
200    * String.
201    * 
202    * @param jsValue JavaScript object to test
203    * @return <code>true</code> if jsValue is a JavaScript String
204    */
205   private static native boolean isString(JavaScriptObject jsValue) /*-{
206     return jsValue instanceof String;
207   }-*/;
208 
209   /**
210    * Not instantiable.
211    */
212   private JSONParser() {
213   }
214 }
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