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Ajax4jsf is an open source extension to the JavaServer Faces standard that adds AJAX capability to JSF applications without requiring the writing of any JavaScript.
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/**
 * Licensed under the Common Development and Distribution License,
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *   http://www.sun.com/cddl/
 *   
 * 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 org.ajax4jsf.framework;

import java.io.IOException;
import java.lang.reflect.Constructor;
import java.util.Locale;

import javax.faces.FacesException;
import javax.faces.application.ViewHandler;
import javax.faces.component.UIViewRoot;
import javax.faces.context.FacesContext;

import org.ajax4jsf.framework.util.message.Messages;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Base wrapper for {@link javax.faces.application.ViewHandler} . By default, delegate all
 * method calls to wrapped handler.
 * @author [email protected] (latest modification by $Author: slava_kabanovich $)
 * @version $Revision: 1.4 $ $Date: 2006/07/11 15:54:01 $
 *
 */
public class ViewHandlerWrapper extends ViewHandler {
	private static final String HANDLERS = "org.ajax4jsf.VIEW_HANDLERS";
	
	private static final Log _log = LogFactory.getLog(ViewHandlerWrapper.class);

	/**
	 * Wrapped ViewHandler
	 */
	protected ViewHandler _handler;
	
	private boolean _initialized = false; 

	/**
	 * @param handler - to wrap.
	 */
	public ViewHandlerWrapper(ViewHandler handler) {
		_handler = handler;
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#calculateLocale(javax.faces.context.FacesContext)
	 */
	public Locale calculateLocale(FacesContext context) {
		fillChain(context);
		return _handler.calculateLocale(context);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#calculateRenderKitId(javax.faces.context.FacesContext)
	 */
	public String calculateRenderKitId(FacesContext context) {
		fillChain(context);
		return _handler.calculateRenderKitId(context);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#createView(javax.faces.context.FacesContext, java.lang.String)
	 */
	public UIViewRoot createView(FacesContext context, String viewId) {
		fillChain(context);
		return _handler.createView(context, viewId);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#getActionURL(javax.faces.context.FacesContext, java.lang.String)
	 */
	public String getActionURL(FacesContext context, String url) {
		fillChain(context);
		return _handler.getActionURL(context, url);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#getResourceURL(javax.faces.context.FacesContext, java.lang.String)
	 */
	public String getResourceURL(FacesContext context, String url) {
		fillChain(context);
		return _handler.getResourceURL(context, url);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#renderView(javax.faces.context.FacesContext, javax.faces.component.UIViewRoot)
	 */
	public void renderView(FacesContext context, UIViewRoot root) throws IOException, FacesException {
		fillChain(context);
		_handler.renderView(context, root);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#restoreView(javax.faces.context.FacesContext, java.lang.String)
	 */
	public UIViewRoot restoreView(FacesContext context, String viewId) {
		fillChain(context);
		return _handler.restoreView(context, viewId);
	}

	/* (non-Javadoc)
	 * @see javax.faces.application.ViewHandler#writeState(javax.faces.context.FacesContext)
	 */
	public void writeState(FacesContext context) throws IOException {
		fillChain(context);
		_handler.writeState(context);
	}

	/**
	 * @return Returns the handler.
	 */
	protected ViewHandler getHandler() {
		return _handler;
	}
	
	/**
	 * Fill view-handlers chain for alternate handlers.
	 * @param context
	 */
	synchronized protected void fillChain( FacesContext context){
		if(!_initialized){
			_initialized = true;
			String handlers = context.getExternalContext().getInitParameter(HANDLERS);
			if(null != handlers){
				ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
				String[] classes = handlers.split(",");
				for (int i = 0; i < classes.length; i++) {
					String handlerClass = classes[i];
					if (_log.isDebugEnabled()) {
						_log.debug(Messages.getMessage(Messages.CREATE_ALTERNATE_HANDLER, handlerClass));
					}
					try {
						Class clazz = classLoader.loadClass(handlerClass);
						try {
							Constructor constructor = clazz
									.getConstructor(new Class[] { ViewHandler.class });
							_handler = (ViewHandler) constructor
									.newInstance(new Object[] { _handler });
						} catch (NoSuchMethodException e) {
							// No constructor with parent class - create simple instance
							if (_log.isWarnEnabled()) {
								_log.warn(Messages.getMessage(Messages.ALTERNATE_HANDLER_CONSTRUCTOR_WARNING));
							}
							_handler = (ViewHandler) clazz.newInstance();
						}
					} catch (Exception e) {
						_log.error(Messages.getMessage(Messages.VIEW_HANDLER_INSTANTIATION_ERROR), e);
					}
				}
			}
		}
	}

}