servletapi/jsr152/src/share/javax/servlet/jsp/tagext/IterationTag.java

/*
* Copyright 2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package javax.servlet.jsp.tagext;

import javax.servlet.jsp.*;

/**
 * The IterationTag interface extends Tag by defining one additional
 * method that controls the reevaluation of its body.
 *
 * <p> A tag handler that implements IterationTag is treated as one that
 * implements Tag regarding  the doStartTag() and doEndTag() methods.
 * IterationTag provides a new method: <code>doAfterBody()</code>.
 *
 * <p> The doAfterBody() method is invoked after every body evaluation
 * to control whether the body will be reevaluated or not.  If doAfterBody()
 * returns IterationTag.EVAL_BODY_AGAIN, then the body will be reevaluated.
 * If doAfterBody() returns Tag.SKIP_BODY, then the body will be skipped
 * and doEndTag() will be evaluated instead.
 *
 * <p><B>Properties</B>
 * There are no new properties in addition to those in Tag.
 *
 * <p><B>Methods</B>
 * There is one new methods: doAfterBody().
 *
 * <p><B>Lifecycle</B>
 *
 * <p> Lifecycle details are described by the transition diagram
 * below.  Exceptions that are thrown during the computation of
 * doStartTag(), BODY and doAfterBody() interrupt the execution
 * sequence and are propagated up the stack, unless the tag handler
 * implements the TryCatchFinally interface; see that interface for
 * details.
 *
 * <p>
 * <IMG src="doc-files/IterationTagProtocol.gif"
 *      alt="Lifecycle Details Transition Diagram for IterationTag"/>
 *
 * <p><B>Empty and Non-Empty Action</B>
 * <p> If the TagLibraryDescriptor file indicates that the action must
 * always have an empty element body, by a &lt;body-content&gt; entry of 
 * "empty", then the doStartTag() method must return SKIP_BODY.
 *
 * <p>Note that which methods are invoked after the doStartTag() depends on
 * both the return value and on if the custom action element is empty
 * or not in the JSP page, not on how it's declared in the TLD.
 *
 * <p>
 * If SKIP_BODY is returned the body is not evaluated, and then doEndTag()
 * is invoked.
 *
 * <p>
 * If EVAL_BODY_INCLUDE is returned, and the custom action element is not
 * empty, the body is evaluated and "passed through" to the current out, 
 * then doAfterBody() is invoked and, after zero or more iterations, 
 * doEndTag() is invoked.
 */

public interface IterationTag extends Tag {

    /**
     * Request the reevaluation of some body.
     * Returned from doAfterBody.
     *
     * For compatibility with JSP 1.1, the value is carefully selected
     * to be the same as the, now deprecated, BodyTag.EVAL_BODY_TAG,
     * 
     */
 
    public final static int EVAL_BODY_AGAIN = 2;

    /**
     * Process body (re)evaluation.  This method is invoked by the
     * JSP Page implementation object after every evaluation of
     * the body into the BodyEvaluation object. The method is
     * not invoked if there is no body evaluation.
     *
     * <p>
     * If doAfterBody returns EVAL_BODY_AGAIN, a new evaluation of the
     * body will happen (followed by another invocation of doAfterBody).
     * If doAfterBody returns SKIP_BODY, no more body evaluations will occur,
     * and the doEndTag method will be invoked.
     *
     * <p>
     * If this tag handler implements BodyTag and doAfterBody returns
     * SKIP_BODY, the value of out will be restored using the popBody 
     * method in pageContext prior to invoking doEndTag.
     *
     * <p>
     * The method re-invocations may be lead to different actions because
     * there might have been some changes to shared state, or because
     * of external computation.
     *
     * <p>
     * The JSP container will resynchronize the values of any AT_BEGIN and
     * NESTED variables (defined by the associated TagExtraInfo or TLD) after
     * the invocation of doAfterBody().
     *
     * @return whether additional evaluations of the body are desired
     * @throws JspException if an error occurred while processing this tag
     */

    int doAfterBody() throws JspException;
}