JspContext


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9   *     http://www.apache.org/licenses/LICENSE-2.0
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11  * Unless required by applicable law or agreed to in writing, software
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13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14  * See the License for the specific language governing permissions and
15  * limitations under the License.
16  */ 
17  package javax.servlet.jsp;
18  
19  import java.util.Enumeration  ;
20  
21  import javax.el.ELContext;
22  import javax.servlet.jsp.el.ExpressionEvaluator  ;
23  import javax.servlet.jsp.el.VariableResolver  ;
24  
25  /**
26   * <p>
27   * <code>JspContext</code> serves as the base class for the 
28   * PageContext class and abstracts all information that is not specific
29   * to servlets.  This allows for Simple Tag Extensions to be used
30   * outside of the context of a request/response Servlet.
31   * <p>
32   * The JspContext provides a number of facilities to the 
33   * page/component author and page implementor, including:
34   * <ul>
35   * <li>a single API to manage the various scoped namespaces
36   * <li>a mechanism to obtain the JspWriter for output
37   * <li>a mechanism to expose page directive attributes to the 
38   *     scripting environment
39   * </ul>
40   *
41   * <p><B>Methods Intended for Container Generated Code</B>
42   * <p>
43   * The following methods enable the <B>management of nested</B> JspWriter 
44   * streams to implement Tag Extensions: <code>pushBody()</code> and
45   * <code>popBody()</code>
46   *
47   * <p><B>Methods Intended for JSP authors</B>
48   * <p>
49   * Some methods provide <B>uniform access</B> to the diverse objects
50   * representing scopes.
51   * The implementation must use the underlying machinery
52   * corresponding to that scope, so information can be passed back and
53   * forth between the underlying environment (e.g. Servlets) and JSP pages.
54   * The methods are:
55   * <code>setAttribute()</code>,  <code>getAttribute()</code>,
56   * <code>findAttribute()</code>,  <code>removeAttribute()</code>,
57   * <code>getAttributesScope()</code> and 
58   * <code>getAttributeNamesInScope()</code>.
59   * 
60   * <p>
61   * The following methods provide <B>convenient access</B> to implicit objects:
62   * <code>getOut()</code>
63   *
64   * <p>
65   * The following methods provide <B>programmatic access</b> to the 
66   * Expression Language evaluator:
67   * <code>getExpressionEvaluator()</code>, <code>getVariableResolver()</code>
68   *
69   * @since 2.0
70   */
71  
72  public abstract class JspContext {
73  
74      /**
75       * Sole constructor. (For invocation by subclass constructors, 
76       * typically implicit.)
77       */
78      public JspContext() {
79      }
80      
81      /** 
82       * Register the name and value specified with page scope semantics.
83       * If the value passed in is <code>null</code>, this has the same 
84       * effect as calling 
85       * <code>removeAttribute( name, PageContext.PAGE_SCOPE )</code>.
86       *
87       * @param name the name of the attribute to set
88       * @param value the value to associate with the name, or null if the
89       *     attribute is to be removed from the page scope.
90       * @throws NullPointerException if the name is null
91       */
92  
93      abstract public void setAttribute(String   name, Object   value);
94  
95      /**
96       * Register the name and value specified with appropriate 
97       * scope semantics.  If the value passed in is <code>null</code>, 
98       * this has the same effect as calling
99       * <code>removeAttribute( name, scope )</code>.
100      * 
101      * @param name the name of the attribute to set
102      * @param value the object to associate with the name, or null if
103      *     the attribute is to be removed from the specified scope.
104      * @param scope the scope with which to associate the name/object
105      * 
106      * @throws NullPointerException if the name is null
107      * @throws IllegalArgumentException if the scope is invalid
108      * @throws IllegalStateException if the scope is 
109      *     PageContext.SESSION_SCOPE but the page that was requested
110      *     does not participate in a session or the session has been
111      *     invalidated.
112      */
113 
114     abstract public void setAttribute(String   name, Object   value, int scope);
115 
116     /**
117      * Returns the object associated with the name in the page scope or null
118      * if not found.
119      *
120      * @param name the name of the attribute to get
121      * @return the object associated with the name in the page scope 
122      *     or null if not found.
123      * 
124      * @throws NullPointerException if the name is null
125      */
126 
127     abstract public Object   getAttribute(String   name);
128 
129     /**
130      * Return the object associated with the name in the specified
131      * scope or null if not found.
132      *
133      * @param name the name of the attribute to set
134      * @param scope the scope with which to associate the name/object
135      * @return the object associated with the name in the specified
136      *     scope or null if not found.
137      * 
138      * @throws NullPointerException if the name is null
139      * @throws IllegalArgumentException if the scope is invalid 
140      * @throws IllegalStateException if the scope is 
141      *     PageContext.SESSION_SCOPE but the page that was requested
142      *     does not participate in a session or the session has been
143      *     invalidated.
144      */
145 
146     abstract public Object   getAttribute(String   name, int scope);
147 
148     /**
149      * Searches for the named attribute in page, request, session (if valid),
150      * and application scope(s) in order and returns the value associated or
151      * null.
152      *
153      * @param name the name of the attribute to search for
154      * @return the value associated or null
155      * @throws NullPointerException if the name is null
156      */
157 
158     abstract public Object   findAttribute(String   name);
159 
160     /**
161      * Remove the object reference associated with the given name
162      * from all scopes.  Does nothing if there is no such object.
163      *
164      * @param name The name of the object to remove.
165      * @throws NullPointerException if the name is null
166      */
167 
168     abstract public void removeAttribute(String   name);
169 
170     /**
171      * Remove the object reference associated with the specified name
172      * in the given scope.  Does nothing if there is no such object.
173      *
174      * @param name The name of the object to remove.
175      * @param scope The scope where to look.
176      * @throws IllegalArgumentException if the scope is invalid
177      * @throws IllegalStateException if the scope is 
178      *     PageContext.SESSION_SCOPE but the page that was requested
179      *     does not participate in a session or the session has been
180      *     invalidated.
181      * @throws NullPointerException if the name is null
182      */
183 
184     abstract public void removeAttribute(String   name, int scope);
185 
186     /**
187      * Get the scope where a given attribute is defined.
188      *
189      * @param name the name of the attribute to return the scope for
190      * @return the scope of the object associated with the name specified or 0
191      * @throws NullPointerException if the name is null
192      */
193 
194     abstract public int getAttributesScope(String   name);
195 
196     /**
197      * Enumerate all the attributes in a given scope.
198      *
199      * @param scope the scope to enumerate all the attributes for
200      * @return an enumeration of names (java.lang.String) of all the 
201      *     attributes the specified scope
202      * @throws IllegalArgumentException if the scope is invalid
203      * @throws IllegalStateException if the scope is 
204      *     PageContext.SESSION_SCOPE but the page that was requested
205      *     does not participate in a session or the session has been
206      *     invalidated.
207      */
208 
209     abstract public Enumeration  <String  > getAttributeNamesInScope(int scope);
210 
211     /**
212      * The current value of the out object (a JspWriter).
213      *
214      * @return the current JspWriter stream being used for client response
215      */
216     abstract public JspWriter   getOut();
217     
218     /**
219      * Provides programmatic access to the ExpressionEvaluator.
220      * The JSP Container must return a valid instance of an 
221      * ExpressionEvaluator that can parse EL expressions.
222      *
223      * @return A valid instance of an ExpressionEvaluator.
224      * @since 2.0
225      */
226     public abstract ExpressionEvaluator   getExpressionEvaluator();
227     
228     
229     public abstract ELContext getELContext();
230     
231     /**
232      * Returns an instance of a VariableResolver that provides access to the
233      * implicit objects specified in the JSP specification using this JspContext
234      * as the context object.
235      *
236      * @return A valid instance of a VariableResolver.
237      * @since 2.0
238      */
239     public abstract VariableResolver   getVariableResolver();
240     
241     /**
242      * Return a new JspWriter object that sends output to the
243      * provided Writer.  Saves the current "out" JspWriter,
244      * and updates the value of the "out" attribute in the
245      * page scope attribute namespace of the JspContext.
246      * <p>The returned JspWriter must implement all methods and
247      * behave as though it were unbuffered.  More specifically:
248      * <ul>
249      *   <li>clear() must throw an IOException</li>
250      *   <li>clearBuffer() does nothing</li>
251      *   <li>getBufferSize() always returns 0</li>
252      *   <li>getRemaining() always returns 0</li>
253      * </ul>
254      * </p>
255      *
256      * @param writer The Writer for the returned JspWriter to send
257      *     output to.
258      * @return a new JspWriter that writes to the given Writer.
259      * @since 2.0
260      */
261     public JspWriter   pushBody( java.io.Writer   writer ) {
262         return null; // XXX to implement
263     }
264     
265     /**
266      * Return the previous JspWriter "out" saved by the matching
267      * pushBody(), and update the value of the "out" attribute in
268      * the page scope attribute namespace of the JspContext.
269      *
270      * @return the saved JspWriter.
271      */
272     public JspWriter   popBody() {
273         return null; // XXX to implement
274     }
275 }
276
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