001    /*
002    * Licensed to the Apache Software Foundation (ASF) under one or more
003    * contributor license agreements.  See the NOTICE file distributed with
004    * this work for additional information regarding copyright ownership.
005    * The ASF licenses this file to You under the Apache License, Version 2.0
006    * (the "License"); you may not use this file except in compliance with
007    * the License.  You may obtain a copy of the License at
008    *
009    *     http://www.apache.org/licenses/LICENSE-2.0
010    *
011    * Unless required by applicable law or agreed to in writing, software
012    * distributed under the License is distributed on an "AS IS" BASIS,
013    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014    * See the License for the specific language governing permissions and
015    * limitations under the License.
016    */
017    
018    package javax.servlet.jsp.tagext;
019    
020    import javax.servlet.jsp.*;
021    
022    /**
023     * The IterationTag interface extends Tag by defining one additional
024     * method that controls the reevaluation of its body.
025     *
026     * <p> A tag handler that implements IterationTag is treated as one that
027     * implements Tag regarding  the doStartTag() and doEndTag() methods.
028     * IterationTag provides a new method: <code>doAfterBody()</code>.
029     *
030     * <p> The doAfterBody() method is invoked after every body evaluation
031     * to control whether the body will be reevaluated or not.  If doAfterBody()
032     * returns IterationTag.EVAL_BODY_AGAIN, then the body will be reevaluated.
033     * If doAfterBody() returns Tag.SKIP_BODY, then the body will be skipped
034     * and doEndTag() will be evaluated instead.
035     *
036     * <p><B>Properties</B>
037     * There are no new properties in addition to those in Tag.
038     *
039     * <p><B>Methods</B>
040     * There is one new methods: doAfterBody().
041     *
042     * <p><B>Lifecycle</B>
043     *
044     * <p> Lifecycle details are described by the transition diagram
045     * below.  Exceptions that are thrown during the computation of
046     * doStartTag(), BODY and doAfterBody() interrupt the execution
047     * sequence and are propagated up the stack, unless the tag handler
048     * implements the TryCatchFinally interface; see that interface for
049     * details.
050     *
051     * <p>
052     * <IMG src="doc-files/IterationTagProtocol.gif"
053     *      alt="Lifecycle Details Transition Diagram for IterationTag"/>
054     *
055     * <p><B>Empty and Non-Empty Action</B>
056     * <p> If the TagLibraryDescriptor file indicates that the action must
057     * always have an empty element body, by a &lt;body-content&gt; entry of 
058     * "empty", then the doStartTag() method must return SKIP_BODY.
059     *
060     * <p>Note that which methods are invoked after the doStartTag() depends on
061     * both the return value and on if the custom action element is empty
062     * or not in the JSP page, not on how it's declared in the TLD.
063     *
064     * <p>
065     * If SKIP_BODY is returned the body is not evaluated, and then doEndTag()
066     * is invoked.
067     *
068     * <p>
069     * If EVAL_BODY_INCLUDE is returned, and the custom action element is not
070     * empty, the body is evaluated and "passed through" to the current out, 
071     * then doAfterBody() is invoked and, after zero or more iterations, 
072     * doEndTag() is invoked.
073     */
074    
075    public interface IterationTag extends Tag {
076    
077        /**
078         * Request the reevaluation of some body.
079         * Returned from doAfterBody.
080         *
081         * For compatibility with JSP 1.1, the value is carefully selected
082         * to be the same as the, now deprecated, BodyTag.EVAL_BODY_TAG,
083         * 
084         */
085     
086        public final static int EVAL_BODY_AGAIN = 2;
087    
088        /**
089         * Process body (re)evaluation.  This method is invoked by the
090         * JSP Page implementation object after every evaluation of
091         * the body into the BodyEvaluation object. The method is
092         * not invoked if there is no body evaluation.
093         *
094         * <p>
095         * If doAfterBody returns EVAL_BODY_AGAIN, a new evaluation of the
096         * body will happen (followed by another invocation of doAfterBody).
097         * If doAfterBody returns SKIP_BODY, no more body evaluations will occur,
098         * and the doEndTag method will be invoked.
099         *
100         * <p>
101         * If this tag handler implements BodyTag and doAfterBody returns
102         * SKIP_BODY, the value of out will be restored using the popBody 
103         * method in pageContext prior to invoking doEndTag.
104         *
105         * <p>
106         * The method re-invocations may be lead to different actions because
107         * there might have been some changes to shared state, or because
108         * of external computation.
109         *
110         * <p>
111         * The JSP container will resynchronize the values of any AT_BEGIN and
112         * NESTED variables (defined by the associated TagExtraInfo or TLD) after
113         * the invocation of doAfterBody().
114         *
115         * @return whether additional evaluations of the body are desired
116         * @throws JspException if an error occurred while processing this tag
117         */
118    
119        int doAfterBody() throws JspException;
120    }