001 /*
002 * Copyright 2004 The Apache Software Foundation
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016 package javax.servlet.jsp;
017
018 import java.util.Enumeration;
019
020 import javax.servlet.jsp.el.ExpressionEvaluator;
021 import javax.servlet.jsp.el.VariableResolver;
022
023 /**
024 * <p>
025 * <code>JspContext</code> serves as the base class for the
026 * PageContext class and abstracts all information that is not specific
027 * to servlets. This allows for Simple Tag Extensions to be used
028 * outside of the context of a request/response Servlet.
029 * <p>
030 * The JspContext provides a number of facilities to the
031 * page/component author and page implementor, including:
032 * <ul>
033 * <li>a single API to manage the various scoped namespaces
034 * <li>a mechanism to obtain the JspWriter for output
035 * <li>a mechanism to expose page directive attributes to the
036 * scripting environment
037 * </ul>
038 *
039 * <p><B>Methods Intended for Container Generated Code</B>
040 * <p>
041 * The following methods enable the <B>management of nested</B> JspWriter
042 * streams to implement Tag Extensions: <code>pushBody()</code> and
043 * <code>popBody()</code>
044 *
045 * <p><B>Methods Intended for JSP authors</B>
046 * <p>
047 * Some methods provide <B>uniform access</B> to the diverse objects
048 * representing scopes.
049 * The implementation must use the underlying machinery
050 * corresponding to that scope, so information can be passed back and
051 * forth between the underlying environment (e.g. Servlets) and JSP pages.
052 * The methods are:
053 * <code>setAttribute()</code>, <code>getAttribute()</code>,
054 * <code>findAttribute()</code>, <code>removeAttribute()</code>,
055 * <code>getAttributesScope()</code> and
056 * <code>getAttributeNamesInScope()</code>.
057 *
058 * <p>
059 * The following methods provide <B>convenient access</B> to implicit objects:
060 * <code>getOut()</code>
061 *
062 * <p>
063 * The following methods provide <B>programmatic access</b> to the
064 * Expression Language evaluator:
065 * <code>getExpressionEvaluator()</code>, <code>getVariableResolver()</code>
066 *
067 * @since 2.0
068 */
069
070 public abstract class JspContext {
071
072 /**
073 * Sole constructor. (For invocation by subclass constructors,
074 * typically implicit.)
075 */
076 public JspContext() {
077 }
078
079 /**
080 * Register the name and value specified with page scope semantics.
081 * If the value passed in is <code>null</code>, this has the same
082 * effect as calling
083 * <code>removeAttribute( name, PageContext.PAGE_SCOPE )</code>.
084 *
085 * @param name the name of the attribute to set
086 * @param value the value to associate with the name, or null if the
087 * attribute is to be removed from the page scope.
088 * @throws NullPointerException if the name is null
089 */
090
091 abstract public void setAttribute(String name, Object value);
092
093 /**
094 * Register the name and value specified with appropriate
095 * scope semantics. If the value passed in is <code>null</code>,
096 * this has the same effect as calling
097 * <code>removeAttribute( name, scope )</code>.
098 *
099 * @param name the name of the attribute to set
100 * @param value the object to associate with the name, or null if
101 * the attribute is to be removed from the specified scope.
102 * @param scope the scope with which to associate the name/object
103 *
104 * @throws NullPointerException if the name is null
105 * @throws IllegalArgumentException if the scope is invalid
106 * @throws IllegalStateException if the scope is
107 * PageContext.SESSION_SCOPE but the page that was requested
108 * does not participate in a session or the session has been
109 * invalidated.
110 */
111
112 abstract public void setAttribute(String name, Object value,
113 int scope);
114
115 /**
116 * Returns the object associated with the name in the page scope or null
117 * if not found.
118 *
119 * @param name the name of the attribute to get
120 * @return the object associated with the name in the page scope
121 * or null if not found.
122 *
123 * @throws NullPointerException if the name is null
124 */
125
126 abstract public Object getAttribute(String name);
127
128 /**
129 * Return the object associated with the name in the specified
130 * scope or null if not found.
131 *
132 * @param name the name of the attribute to set
133 * @param scope the scope with which to associate the name/object
134 * @return the object associated with the name in the specified
135 * scope or null if not found.
136 *
137 * @throws NullPointerException if the name is null
138 * @throws IllegalArgumentException if the scope is invalid
139 * @throws IllegalStateException if the scope is
140 * PageContext.SESSION_SCOPE but the page that was requested
141 * does not participate in a session or the session has been
142 * invalidated.
143 */
144
145 abstract public Object getAttribute(String name, int scope);
146
147 /**
148 * Searches for the named attribute in page, request, session (if valid),
149 * and application scope(s) in order and returns the value associated or
150 * null.
151 *
152 * @param name the name of the attribute to search for
153 * @return the value associated or null
154 * @throws NullPointerException if the name is null
155 */
156
157 abstract public Object findAttribute(String name);
158
159 /**
160 * Remove the object reference associated with the given name
161 * from all scopes. Does nothing if there is no such object.
162 *
163 * @param name The name of the object to remove.
164 * @throws NullPointerException if the name is null
165 */
166
167 abstract public void removeAttribute(String name);
168
169 /**
170 * Remove the object reference associated with the specified name
171 * in the given scope. Does nothing if there is no such object.
172 *
173 * @param name The name of the object to remove.
174 * @param scope The scope where to look.
175 * @throws IllegalArgumentException if the scope is invalid
176 * @throws IllegalStateException if the scope is
177 * PageContext.SESSION_SCOPE but the page that was requested
178 * does not participate in a session or the session has been
179 * invalidated.
180 * @throws NullPointerException if the name is null
181 */
182
183 abstract public void removeAttribute(String name, int scope);
184
185 /**
186 * Get the scope where a given attribute is defined.
187 *
188 * @param name the name of the attribute to return the scope for
189 * @return the scope of the object associated with the name specified or 0
190 * @throws NullPointerException if the name is null
191 */
192
193 abstract public int getAttributesScope(String name);
194
195 /**
196 * Enumerate all the attributes in a given scope.
197 *
198 * @param scope the scope to enumerate all the attributes for
199 * @return an enumeration of names (java.lang.String) of all the
200 * attributes the specified scope
201 * @throws IllegalArgumentException if the scope is invalid
202 * @throws IllegalStateException if the scope is
203 * PageContext.SESSION_SCOPE but the page that was requested
204 * does not participate in a session or the session has been
205 * invalidated.
206 */
207
208 abstract public Enumeration getAttributeNamesInScope(int scope);
209
210 /**
211 * The current value of the out object (a JspWriter).
212 *
213 * @return the current JspWriter stream being used for client response
214 */
215 abstract public JspWriter getOut();
216
217 /**
218 * Provides programmatic access to the ExpressionEvaluator.
219 * The JSP Container must return a valid instance of an
220 * ExpressionEvaluator that can parse EL expressions.
221 *
222 * @return A valid instance of an ExpressionEvaluator.
223 * @since 2.0
224 */
225 public abstract ExpressionEvaluator getExpressionEvaluator();
226
227 /**
228 * Returns an instance of a VariableResolver that provides access to the
229 * implicit objects specified in the JSP specification using this JspContext
230 * as the context object.
231 *
232 * @return A valid instance of a VariableResolver.
233 * @since 2.0
234 */
235 public abstract VariableResolver getVariableResolver();
236
237 /**
238 * Return a new JspWriter object that sends output to the
239 * provided Writer. Saves the current "out" JspWriter,
240 * and updates the value of the "out" attribute in the
241 * page scope attribute namespace of the JspContext.
242 * <p>The returned JspWriter must implement all methods and
243 * behave as though it were unbuffered. More specifically:
244 * <ul>
245 * <li>clear() must throw an IOException</li>
246 * <li>clearBuffer() does nothing</li>
247 * <li>getBufferSize() always returns 0</li>
248 * <li>getRemaining() always returns 0</li>
249 * </ul>
250 * </p>
251 *
252 * @param writer The Writer for the returned JspWriter to send
253 * output to.
254 * @return a new JspWriter that writes to the given Writer.
255 * @since 2.0
256 */
257 public JspWriter pushBody(java.io.Writer writer) {
258 return null; // XXX to implement
259 }
260
261 /**
262 * Return the previous JspWriter "out" saved by the matching
263 * pushBody(), and update the value of the "out" attribute in
264 * the page scope attribute namespace of the JspContext.
265 *
266 * @return the saved JspWriter.
267 */
268 public JspWriter popBody() {
269 return null; // XXX to implement
270 }
271 }
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