/*
 * Copyright (c) 1997, 2020 Oracle and/or its affiliates and others.
 * All rights reserved.
 * 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 jakarta.servlet.jsp.tagext;

import jakarta.servlet.jsp.JspException;
import jakarta.servlet.jsp.PageContext;

/**
 * The interface of a classic tag handler that does not want to manipulate its body. The Tag interface defines the basic
 * protocol between a Tag handler and JSP page implementation class. It defines the life cycle and the methods to be
 * invoked at start and end tag.
 *
 * <p>
 * <B>Properties</B>
 * </p>
 *
 * <p>
 * The Tag interface specifies the setter and getter methods for the core pageContext and parent properties.
 * </p>
 *
 * <p>
 * The JSP page implementation object invokes setPageContext and setParent, in that order, before invoking doStartTag()
 * or doEndTag().
 * </p>
 *
 * <p>
 * <B>Methods</B>
 * </p>
 *
 * <p>
 * There are two main actions: doStartTag and doEndTag. Once all appropriate properties have been initialized, the
 * doStartTag and doEndTag methods can be invoked on the tag handler. Between these invocations, the tag handler is
 * assumed to hold a state that must be preserved. After the doEndTag invocation, the tag handler is available for
 * further invocations (and it is expected to have retained its properties).
 * </p>
 *
 * <p>
 * <B>Lifecycle</B>
 * </p>
 *
 * <p>
 * Lifecycle details are described by the transition diagram below, with the following comments:
 * </p>
 * <ul>
 * <li>[1] This transition is intended to be for releasing long-term data. no guarantees are assumed on whether any
 * properties have been retained or not.
 * <li>[2] This transition happens if and only if the tag ends normally without raising an exception
 * <li>[3] Some setters may be called again before a tag handler is reused. For instance, <code>setParent()</code> is
 * called if it's reused within the same page but at a different level, <code>setPageContext()</code> is called if it's
 * used in another page, and attribute setters are called if the values differ or are expressed as request-time
 * attribute values.
 * <li>Check the TryCatchFinally interface for additional details related to exception handling and resource management.
 * </ul>
 *
 * <IMG src="doc-files/TagProtocol.gif" alt="Lifecycle Details Transition Diagram for Tag">
 * 
 * <p>
 * Once all invocations on the tag handler are completed, the release method is invoked on it. Once a release method is
 * invoked <em>all</em> properties, including parent and pageContext, are assumed to have been reset to an unspecified
 * value. The page compiler guarantees that release() will be invoked on the Tag handler before the handler is released
 * to the GC.
 * </p>
 *
 * <p>
 * <B>Empty and Non-Empty Action</B>
 * </p>
 * <p>
 * If the TagLibraryDescriptor file indicates that the action must always have an empty action, by an
 * &lt;body-content&gt; entry of "empty", then the doStartTag() method must return SKIP_BODY.
 * </p>
 *
 * <p>
 * Otherwise, the doStartTag() method may return SKIP_BODY or EVAL_BODY_INCLUDE.
 * </p>
 *
 * <p>
 * If SKIP_BODY is returned the body, if present, is not evaluated.
 * </p>
 * 
 * <p>
 * If EVAL_BODY_INCLUDE is returned, the body is evaluated and "passed through" to the current out.
 * </p>
 */
public interface Tag extends JspTag {

    /**
     * Skip body evaluation. Valid return value for doStartTag and doAfterBody.
     */
    public final static int SKIP_BODY = 0;

    /**
     * Evaluate body into existing out stream. Valid return value for doStartTag.
     */
    public final static int EVAL_BODY_INCLUDE = 1;

    /**
     * Skip the rest of the page. Valid return value for doEndTag.
     */
    public final static int SKIP_PAGE = 5;

    /**
     * Continue evaluating the page. Valid return value for doEndTag().
     */
    public final static int EVAL_PAGE = 6;

    // Setters for Tag handler data

    /**
     * Set the current page context. This method is invoked by the JSP page implementation object prior to doStartTag().
     * <p>
     * This value is *not* reset by doEndTag() and must be explicitly reset by a page implementation if it changes
     * between calls to doStartTag().
     *
     * @param pc The page context for this tag handler.
     */
    void setPageContext(PageContext pc);

    /**
     * Set the parent (closest enclosing tag handler) of this tag handler. Invoked by the JSP page implementation object
     * prior to doStartTag().
     * <p>
     * This value is *not* reset by doEndTag() and must be explicitly reset by a page implementation.
     *
     * @param t The parent tag, or null.
     */
    void setParent(Tag t);

    /**
     * Get the parent (closest enclosing tag handler) for this tag handler.
     *
     * <p>
     * The getParent() method can be used to navigate the nested tag handler structure at runtime for cooperation among
     * custom actions; for example, the findAncestorWithClass() method in TagSupport provides a convenient way of doing
     * this.
     *
     * <p>
     * The current version of the specification only provides one formal way of indicating the observable type of a tag
     * handler: its tag handler implementation class, described in the tag-class subelement of the tag element. This is
     * extended in an informal manner by allowing the tag library author to indicate in the description subelement an
     * observable type. The type should be a subtype of the tag handler implementation class or void. This addititional
     * constraint can be exploited by a specialized container that knows about that specific tag library, as in the case
     * of the JSP standard tag library.
     *
     * @return the current parent, or null if none.
     * @see TagSupport#findAncestorWithClass
     */
    Tag getParent();

    // Actions for basic start/end processing.

    /**
     * Process the start tag for this instance. This method is invoked by the JSP page implementation object.
     *
     * <p>
     * The doStartTag method assumes that the properties pageContext and parent have been set. It also assumes that any
     * properties exposed as attributes have been set too. When this method is invoked, the body has not yet been
     * evaluated.
     *
     * <p>
     * This method returns Tag.EVAL_BODY_INCLUDE or BodyTag.EVAL_BODY_BUFFERED to indicate that the body of the action
     * should be evaluated or SKIP_BODY to indicate otherwise.
     *
     * <p>
     * When a Tag returns EVAL_BODY_INCLUDE the result of evaluating the body (if any) is included into the current
     * "out" JspWriter as it happens and then doEndTag() is invoked.
     *
     * <p>
     * BodyTag.EVAL_BODY_BUFFERED is only valid if the tag handler implements BodyTag.
     *
     * <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 doStartTag(), except for a tag handler implementing BodyTag whose
     * doStartTag() method returns BodyTag.EVAL_BODY_BUFFERED.
     *
     * @return EVAL_BODY_INCLUDE if the tag wants to process body, SKIP_BODY if it does not want to process it.
     * @throws JspException if an error occurred while processing this tag
     * @see BodyTag
     */
    int doStartTag() throws JspException;

    /**
     * Process the end tag for this instance. This method is invoked by the JSP page implementation object on all Tag
     * handlers.
     *
     * <p>
     * This method will be called after returning from doStartTag. The body of the action may or may not have been
     * evaluated, depending on the return value of doStartTag.
     *
     * <p>
     * If this method returns EVAL_PAGE, the rest of the page continues to be evaluated. If this method returns
     * SKIP_PAGE, the rest of the page is not evaluated, the request is completed, and the doEndTag() methods of
     * enclosing tags are not invoked. If this request was forwarded or included from another page (or Servlet), only
     * the current page evaluation is stopped.
     *
     * <p>
     * The JSP container will resynchronize the values of any AT_BEGIN and AT_END variables (defined by the associated
     * TagExtraInfo or TLD) after the invocation of doEndTag().
     *
     * @return indication of whether to continue evaluating the JSP page.
     * @throws JspException if an error occurred while processing this tag
     */
    int doEndTag() throws JspException;

    /**
     * Called on a Tag handler to release state. The page compiler guarantees that JSP page implementation objects will
     * invoke this method on all tag handlers, but there may be multiple invocations on doStartTag and doEndTag in
     * between.
     */
    void release();
}
