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