/*
 * Copyright 2002-2007 the original author or authors.
 *
 * 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 org.springframework.web.jsf;

import javax.faces.context.FacesContext;
import javax.faces.el.EvaluationException;
import javax.faces.el.VariableResolver;

import org.springframework.util.Assert;
import org.springframework.web.context.WebApplicationContext;

/**
 * Special JSF 1.1 <code>VariableResolver</code> that exposes the Spring
 * <code>WebApplicationContext</code> instance under a variable named
 * "webApplicationContext".
 *
 * <p>In contrast to {@link DelegatingVariableResolver}, this VariableResolver
 * does <i>not</i> resolve JSF variable names as Spring bean names. It rather
 * exposes Spring's root WebApplicationContext <i>itself</i> under a special name.
 * JSF-managed beans can then use Spring's WebApplicationContext API to retrieve
 * Spring-managed beans, access resources, etc.
 *
 * <p>Configure this resolver in your <code>faces-config.xml</code> file as follows:
 *
 * <pre>
 * &lt;application>
 *   ...
 *   &lt;variable-resolver>org.springframework.web.jsf.WebApplicationContextVariableResolver&lt;/variable-resolver>
 * &lt;/application></pre>
 *
 * @author Colin Sampaleanu
 * @author Juergen Hoeller
 * @since 1.2.5
 * @see DelegatingVariableResolver
 * @see FacesContextUtils#getWebApplicationContext
 */
public class WebApplicationContextVariableResolver extends VariableResolver {

	/**
	 * Name of the exposed WebApplicationContext variable: "webApplicationContext".
	 */
	public static final String WEB_APPLICATION_CONTEXT_VARIABLE_NAME = "webApplicationContext";


	protected final VariableResolver originalVariableResolver;


	/**
	 * Create a new WebApplicationContextVariableResolver, using the given
	 * original VariableResolver.
	 * <p>A JSF implementation will automatically pass its original resolver into the
	 * constructor of a configured resolver, provided that there is a corresponding
	 * constructor argument.
	 * @param originalVariableResolver the original VariableResolver
	 */
	public WebApplicationContextVariableResolver(VariableResolver originalVariableResolver) {
		Assert.notNull(originalVariableResolver, "Original JSF VariableResolver must not be null");
		this.originalVariableResolver = originalVariableResolver;
	}

	/**
	 * Return the original JSF VariableResolver that this resolver delegates to.
	 * Used to resolve standard JSF-managed beans.
	 */
	protected final VariableResolver getOriginalVariableResolver() {
		return this.originalVariableResolver;
	}


	/**
	 * Check for the special "webApplicationContext" variable first,
	 * then delegate to the original VariableResolver.
	 * <p>If no WebApplicationContext is available, all requests
	 * will be delegated to the original VariableResolver.
	 */
	public Object resolveVariable(FacesContext context, String name) throws EvaluationException {
		Object value = null;
		if (WEB_APPLICATION_CONTEXT_VARIABLE_NAME.equals(name)) {
			value = getWebApplicationContext(context);
		}
		if (value == null) {
			value = getOriginalVariableResolver().resolveVariable(context, name);
		}
		return value;
	}

	/**
	 * Retrieve the WebApplicationContext reference to expose.
	 * <p>The default implementation delegates to FacesContextUtils,
	 * returning <code>null</code> if no WebApplicationContext found.
	 * @param facesContext the current JSF context
	 * @return the Spring web application context
	 * @see FacesContextUtils#getWebApplicationContext
	 */
	protected WebApplicationContext getWebApplicationContext(FacesContext facesContext) {
		return FacesContextUtils.getWebApplicationContext(facesContext);
	}

}