servletapi/jsr152/src/share/javax/servlet/jsp/tagext/TagAdapter.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.*;


/**
 * Wraps any SimpleTag and exposes it using a Tag interface.  This is used
 * to allow collaboration between classic Tag handlers and SimpleTag
 * handlers.
 * <p>
 * Because SimpleTag does not extend Tag, and because Tag.setParent()
 * only accepts a Tag instance, a classic tag handler (one
 * that implements Tag) cannot have a SimpleTag as its parent.  To remedy
 * this, a TagAdapter is created to wrap the SimpleTag parent, and the
 * adapter is passed to setParent() instead.  A classic Tag Handler can
 * call getAdaptee() to retrieve the encapsulated SimpleTag instance.
 *
 * @since 2.0
 */
public class TagAdapter 
    implements Tag
{
    /** The simple tag that's being adapted. */
    private SimpleTag simpleTagAdaptee;

    /** The parent, of this tag, converted (if necessary) to be of type Tag. */
    private Tag parent;

    // Flag indicating whether we have already determined the parent
    private boolean parentDetermined;

    /**
     * Creates a new TagAdapter that wraps the given SimpleTag and 
     * returns the parent tag when getParent() is called.
     *
     * @param adaptee The SimpleTag being adapted as a Tag.
     */
    public TagAdapter( SimpleTag adaptee ) {
        if( adaptee == null ) {
	    // Cannot wrap a null adaptee.
	    throw new IllegalArgumentException();
        }
        this.simpleTagAdaptee = adaptee;
    }
    
    /**
     * Must not be called.
     *
     * @param pc ignored.
     * @throws UnsupportedOperationException Must not be called
     */
    public void setPageContext(PageContext pc) {
        throw new UnsupportedOperationException( 
            "Illegal to invoke setPageContext() on TagAdapter wrapper" );
    }


    /**
     * Must not be called.  The parent of this tag is always 
     * getAdaptee().getParent().
     *
     * @param parentTag ignored.
     * @throws UnsupportedOperationException Must not be called.
     */
    public void setParent( Tag parentTag ) {
        throw new UnsupportedOperationException( 
            "Illegal to invoke setParent() on TagAdapter wrapper" );
    }


    /**
     * Returns the parent of this tag, which is always
     * getAdaptee().getParent().  
     *
     * This will either be the enclosing Tag (if getAdaptee().getParent()
     * implements Tag), or an adapter to the enclosing Tag (if 
     * getAdaptee().getParent() does not implement Tag).
     *
     * @return The parent of the tag being adapted.
     */
    public Tag getParent() {
	if (!parentDetermined) {
	    JspTag adapteeParent = simpleTagAdaptee.getParent();
	    if (adapteeParent != null) {
		if (adapteeParent instanceof Tag) {
		    this.parent = (Tag) adapteeParent;
		} else {
		    // Must be SimpleTag - no other types defined.
		    this.parent = new TagAdapter((SimpleTag) adapteeParent);
		}
	    }
	    parentDetermined = true;
	}

	return this.parent;
    }
    
    /**
     * Gets the tag that is being adapted to the Tag interface.
     * This should be an instance of SimpleTag in JSP 2.0, but room
     * is left for other kinds of tags in future spec versions.
     *
     * @return the tag that is being adapted
     */
    public JspTag getAdaptee() {
        return this.simpleTagAdaptee;
    }

    /**
     * Must not be called.
     *
     * @return always throws UnsupportedOperationException
     * @throws UnsupportedOperationException Must not be called
     * @throws JspException never thrown
     */
    public int doStartTag() throws JspException {
        throw new UnsupportedOperationException( 
            "Illegal to invoke doStartTag() on TagAdapter wrapper" );
    }
 
    /**
     * Must not be called.
     *
     * @return always throws UnsupportedOperationException
     * @throws UnsupportedOperationException Must not be called
     * @throws JspException never thrown
     */
    public int doEndTag() throws JspException {
        throw new UnsupportedOperationException( 
            "Illegal to invoke doEndTag() on TagAdapter wrapper" );
    }

    /**
     * Must not be called.
     *
     * @throws UnsupportedOperationException Must not be called
     */
    public void release() {
        throw new UnsupportedOperationException( 
            "Illegal to invoke release() on TagAdapter wrapper" );
    }
}