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package javax.servlet.jsp.tagext;
import javax.servlet.jsp.*;
/**
* 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:
* <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></p>
*
* <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 <body-content> 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();
}
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