/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 jakarta.servlet.jsp.tagext;

import jakarta.servlet.jsp.JspException;


/**
 * The BodyTag interface extends IterationTag by defining additional methods that let a tag handler manipulate the
 * content of evaluating its body.
 * <p>
 * It is the responsibility of the tag handler to manipulate the body content. For example the tag handler may take the
 * body content, convert it into a String using the bodyContent.getString method and then use it. Or the tag handler may
 * take the body content and write it out into its enclosing JspWriter using the bodyContent.writeOut method.
 * <p>
 * A tag handler that implements BodyTag is treated as one that implements IterationTag, except that the doStartTag
 * method can return SKIP_BODY, EVAL_BODY_INCLUDE or EVAL_BODY_BUFFERED.
 * <p>
 * If EVAL_BODY_INCLUDE is returned, then evaluation happens as in IterationTag.
 * <p>
 * If EVAL_BODY_BUFFERED is returned, then a BodyContent object will be created (by code generated by the JSP compiler)
 * to capture the body evaluation. The code generated by the JSP compiler obtains the BodyContent object by calling the
 * pushBody method of the current pageContext, which additionally has the effect of saving the previous out value. The
 * page compiler returns this object by calling the popBody method of the PageContext class; the call also restores the
 * value of out.
 * <p>
 * The interface provides one new property with a setter method and one new action method.
 * <p>
 * <B>Properties</B>
 * <p>
 * There is a new property: bodyContent, to contain the BodyContent object, where the JSP Page implementation object
 * will place the evaluation (and reevaluation, if appropriate) of the body. The setter method (setBodyContent) will
 * only be invoked if doStartTag() returns EVAL_BODY_BUFFERED and the corresponding action element does not have an
 * empty body.
 * <p>
 * <B>Methods</B>
 * <p>
 * In addition to the setter method for the bodyContent property, there is a new action method: doInitBody(), which is
 * invoked right after setBodyContent() and before the body evaluation. This method is only invoked if doStartTag()
 * returns EVAL_BODY_BUFFERED.
 * <p>
 * <B>Lifecycle</B>
 * <p>
 * Lifecycle details are described by the transition diagram below. Exceptions that are thrown during the computation of
 * doStartTag(), setBodyContent(), doInitBody(), BODY, 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/BodyTagProtocol.gif" alt="Lifecycle Details Transition Diagram for BodyTag">
 * <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. Otherwise, the
 * doStartTag() method may return SKIP_BODY, EVAL_BODY_INCLUDE, or EVAL_BODY_BUFFERED.
 * <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 how it's declared in the TLD.
 * <p>
 * If SKIP_BODY is returned the body is not evaluated, and doEndTag() is invoked.
 * <p>
 * If EVAL_BODY_INCLUDE is returned, and the custom action element is not empty, setBodyContent() is not invoked,
 * doInitBody() is not invoked, the body is evaluated and "passed through" to the current out, doAfterBody() is invoked
 * and then, after zero or more iterations, doEndTag() is invoked. If the custom action element is empty, only doStart()
 * and doEndTag() are invoked.
 * <p>
 * If EVAL_BODY_BUFFERED is returned, and the custom action element is not empty, setBodyContent() is invoked,
 * doInitBody() is invoked, the body is evaluated, doAfterBody() is invoked, and then, after zero or more iterations,
 * doEndTag() is invoked. If the custom action element is empty, only doStart() and doEndTag() are invoked.
 */
public interface BodyTag extends IterationTag {

    /**
     * Request the creation of new buffer, a BodyContent on which to evaluate the body of this tag. Returned from
     * doStartTag when it implements BodyTag. This is an illegal return value for doStartTag when the class does not
     * implement BodyTag.
     */
    int EVAL_BODY_BUFFERED = 2;

    /**
     * Set the bodyContent property. This method is invoked by the JSP page implementation object at most once per
     * action invocation. This method will be invoked before doInitBody. This method will not be invoked for empty tags
     * or for non-empty tags whose doStartTag() method returns SKIP_BODY or EVAL_BODY_INCLUDE.
     * <p>
     * When setBodyContent is invoked, the value of the implicit object out has already been changed in the pageContext
     * object. The BodyContent object passed will have no data on it but may have been reused (and cleared) from some
     * previous invocation.
     * <p>
     * The BodyContent object is available and with the appropriate content until after the invocation of the doEndTag
     * method, at which case it may be reused.
     *
     * @param b the BodyContent
     *
     * @see #doInitBody
     * @see #doAfterBody
     */
    void setBodyContent(BodyContent b);

    /**
     * Prepare for evaluation of the body. This method is invoked by the JSP page implementation object after
     * setBodyContent and before the first time the body is to be evaluated. This method will not be invoked for empty
     * tags or for non-empty tags whose doStartTag() method returns SKIP_BODY or EVAL_BODY_INCLUDE.
     * <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 doInitBody().
     *
     * @throws JspException if an error occurred while processing this tag
     *
     * @see #doAfterBody
     */
    void doInitBody() throws JspException;
}
