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1   /**
2    *
3    * Copyright 2003-2004 The Apache Software Foundation
4    *
5    *  Licensed under the Apache License, Version 2.0 (the "License");
6    *  you may not use this file except in compliance with the License.
7    *  You may obtain a copy of the License at
8    *
9    *     http://www.apache.org/licenses/LICENSE-2.0
10   *
11   *  Unless required by applicable law or agreed to in writing, software
12   *  distributed under the License is distributed on an "AS IS" BASIS,
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  
18  //
19  // This source code implements specifications defined by the Java
20  // Community Process. In order to remain compliant with the specification
21  // DO NOT add / change / or delete method signatures!
22  //
23  
24  package javax.servlet.jsp.tagext;
25  
26  import javax.servlet.jsp.JspException;
27  import javax.servlet.jsp.PageContext;
28  
29  
30  /**
31   * The interface of a classic tag handler that does not want to manipulate 
32   * its body.  The Tag interface defines the basic protocol between a Tag 
33   * handler and JSP page implementation class.  It defines the life cycle 
34   * and the methods to be invoked at start and end tag.
35   *
36   * <p><B>Properties</B></p>
37   *
38   * <p>The Tag interface specifies the setter and getter methods for the core
39   * pageContext and parent properties.</p>
40   *
41   * <p>The JSP page implementation object invokes setPageContext and
42   * setParent, in that order, before invoking doStartTag() or doEndTag().</p>
43   *
44   * <p><B>Methods</B></p>
45   *
46   * <p>There are two main actions: doStartTag and doEndTag.  Once all
47   * appropriate properties have been initialized, the doStartTag and
48   * doEndTag methods can be invoked on the tag handler.  Between these
49   * invocations, the tag handler is assumed to hold a state that must
50   * be preserved.  After the doEndTag invocation, the tag handler is
51   * available for further invocations (and it is expected to have
52   * retained its properties).</p>
53   *
54   * <p><B>Lifecycle</B></p>
55   *
56   * <p>Lifecycle details are described by the transition diagram below,
57   * with the following comments:
58   * <ul>
59   * <li> [1] This transition is intended to be for releasing long-term data.
60   * no guarantees are assumed on whether any properties have been retained
61   * or not.
62   * <li> [2] This transition happens if and only if the tag ends normally
63   * without raising an exception
64   * <li> [3] Some setters may be called again before a tag handler is 
65   * reused.  For instance, <code>setParent()</code> is called if it's 
66   * reused within the same page but at a different level, 
67   * <code>setPageContext()</code> is called if it's used in another page, 
68   * and attribute setters are called if the values differ or are expressed 
69   * as request-time attribute values.
70   * <li> Check the TryCatchFinally interface for additional details related
71   * to exception handling and resource management.
72   * </ul></p>
73   *
74   * <IMG src="doc-files/TagProtocol.gif"
75   *      alt="Lifecycle Details Transition Diagram for Tag"/>
76   * 
77   * <p>Once all invocations on the tag handler
78   * are completed, the release method is invoked on it.  Once a release
79   * method is invoked <em>all</em> properties, including parent and
80   * pageContext, are assumed to have been reset to an unspecified value.
81   * The page compiler guarantees that release() will be invoked on the Tag
82   * handler before the handler is released to the GC.</p>
83   *
84   * <p><B>Empty and Non-Empty Action</B></p>
85   * <p>If the TagLibraryDescriptor file indicates that the action must
86   * always have an empty action, by an &lt;body-_content&gt; entry of "empty",
87   * then the doStartTag() method must return SKIP_BODY.</p>
88   *
89   * <p>Otherwise, the doStartTag() method may return SKIP_BODY or
90   * EVAL_BODY_INCLUDE.</p>
91   *
92   * <p>If SKIP_BODY is returned the body, if present, is not evaluated.</p>
93   * 
94   * <p>If EVAL_BODY_INCLUDE is returned, the body is evaluated and
95   * "passed through" to the current out.</p>
96  */
97  
98  public interface Tag extends JspTag {
99  
100     /**
101      * Skip body evaluation.
102      * Valid return value for doStartTag and doAfterBody.
103      */
104  
105     public final static int SKIP_BODY = 0;
106  
107     /**
108      * Evaluate body into existing out stream.
109      * Valid return value for doStartTag.
110      */
111  
112     public final static int EVAL_BODY_INCLUDE = 1;
113 
114     /**
115      * Skip the rest of the page.
116      * Valid return value for doEndTag.
117      */
118 
119     public final static int SKIP_PAGE = 5;
120 
121     /**
122      * Continue evaluating the page.
123      * Valid return value for doEndTag().
124      */
125 
126     public final static int EVAL_PAGE = 6;
127 
128     // Setters for Tag handler data
129 
130 
131     /**
132      * Set the current page context.
133      * This method is invoked by the JSP page implementation object
134      * prior to doStartTag().
135      * <p>
136      * This value is *not* reset by doEndTag() and must be explicitly reset
137      * by a page implementation if it changes between calls to doStartTag().
138      *
139      * @param pc The page context for this tag handler.
140      */
141 
142     void setPageContext(PageContext pc);
143 
144 
145     /**
146      * Set the parent (closest enclosing tag handler) of this tag handler.
147      * Invoked by the JSP page implementation object prior to doStartTag().
148      * <p>
149      * This value is *not* reset by doEndTag() and must be explicitly reset
150      * by a page implementation.
151      *
152      * @param t The parent tag, or null.
153      */
154 
155 
156     void setParent(Tag t);
157 
158 
159     /**
160      * Get the parent (closest enclosing tag handler) for this tag handler.
161      *
162      * <p>
163      * The getParent() method can be used to navigate the nested tag
164      * handler structure at runtime for cooperation among custom actions;
165      * for example, the findAncestorWithClass() method in TagSupport
166      * provides a convenient way of doing this.
167      *
168      * <p>
169      * The current version of the specification only provides one formal
170      * way of indicating the observable type of a tag handler: its
171      * tag handler implementation class, described in the tag-class
172      * subelement of the tag element.  This is extended in an
173      * informal manner by allowing the tag library author to
174      * indicate in the description subelement an observable type.
175      * The type should be a subtype of the tag handler implementation
176      * class or void.
177      * This addititional constraint can be exploited by a
178      * specialized container that knows about that specific tag library,
179      * as in the case of the JSP standard tag library.
180      *
181      * @return the current parent, or null if none.
182      * @see TagSupport#findAncestorWithClass
183      */
184 
185     Tag getParent();
186 
187 
188     // Actions for basic start/end processing.
189 
190 
191     /**
192      * Process the start tag for this instance.
193      * This method is invoked by the JSP page implementation object.
194      *
195      * <p>
196      * The doStartTag method assumes that the properties pageContext and
197      * parent have been set. It also assumes that any properties exposed as
198      * attributes have been set too.  When this method is invoked, the body
199      * has not yet been evaluated.
200      *
201      * <p>
202      * This method returns Tag.EVAL_BODY_INCLUDE or
203      * BodyTag.EVAL_BODY_BUFFERED to indicate
204      * that the body of the action should be evaluated or SKIP_BODY to
205      * indicate otherwise.
206      *
207      * <p>
208      * When a Tag returns EVAL_BODY_INCLUDE the result of evaluating
209      * the body (if any) is included into the current "out" JspWriter as it
210      * happens and then doEndTag() is invoked.
211      *
212      * <p>
213      * BodyTag.EVAL_BODY_BUFFERED is only valid  if the tag handler
214      * implements BodyTag.
215      *
216      * <p>
217      * The JSP container will resynchronize the values of any AT_BEGIN and
218      * NESTED variables (defined by the associated TagExtraInfo or TLD)
219      * after the invocation of doStartTag(), except for a tag handler
220      * implementing BodyTag whose doStartTag() method returns
221      * BodyTag.EVAL_BODY_BUFFERED.
222      *
223      * @return EVAL_BODY_INCLUDE if the tag wants to process body, SKIP_BODY 
224      *     if it does not want to process it.
225      * @throws JspException if an error occurred while processing this tag
226      * @see BodyTag
227      */
228  
229     int doStartTag() throws JspException;
230  
231 
232     /**
233      * Process the end tag for this instance.
234      * This method is invoked by the JSP page implementation object
235      * on all Tag handlers.
236      *
237      * <p>
238      * This method will be called after returning from doStartTag. The
239      * body of the action may or may not have been evaluated, depending on
240      * the return value of doStartTag.
241      *
242      * <p>
243      * If this method returns EVAL_PAGE, the rest of the page continues
244      * to be evaluated.  If this method returns SKIP_PAGE, the rest of
245      * the page is not evaluated, the request is completed, and 
246      * the doEndTag() methods of enclosing tags are not invoked.  If this
247      * request was forwarded or included from another page (or Servlet),
248      * only the current page evaluation is stopped.
249      *
250      * <p>
251      * The JSP container will resynchronize the values of any AT_BEGIN and
252      * AT_END variables (defined by the associated TagExtraInfo or TLD)
253      * after the invocation of doEndTag().
254      *
255      * @return indication of whether to continue evaluating the JSP page.
256      * @throws JspException if an error occurred while processing this tag
257      */
258 
259     int doEndTag() throws JspException;
260 
261     /**
262      * Called on a Tag handler to release state.
263      * The page compiler guarantees that JSP page implementation
264      * objects will invoke this method on all tag handlers,
265      * but there may be multiple invocations on doStartTag and doEndTag in between.
266      */
267 
268     void release();
269 
270 }