/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you 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.apache.myfaces.webapp;

 import javax.el.ELContext;

 import javax.el.ExpressionFactory;

 import javax.el.MethodExpression;

 import javax.el.ValueExpression;

 import javax.faces.FacesException;

 import javax.faces.context.ExternalContext;

 import javax.servlet.ServletContext;

 import org.apache.commons.logging.Log;

 import org.apache.commons.logging.LogFactory;

 /**

  * Initializes MyFaces in a JSP 2.0 (or less) environment.

  *

  */

 public class Jsp20FacesInitializer extends AbstractFacesInitializer

 {

     /**

      * The logger instance for this class.

      */

     private static final Log log = LogFactory.getLog(Jsp20FacesInitializer.class);

     /**

      * The ExpressionFactory implementation of the EL-RI.

      */

     private static final String EL_RI_EXPRESSION_FACTORY_IMPL = "com.sun.el.ExpressionFactoryImpl";

     /**

      * Jasper's ExpressionFactory implementation.

      */

     private static final String JASPER_EL_EXPRESSION_FACTORY_IMPL = "org.apache.el.ExpressionFactoryImpl";

     /**

      * All known ExpressionFactory implementations.

      */

     private static final String[] KNOWN_EXPRESSION_FACTORIES = 

         new String[] { EL_RI_EXPRESSION_FACTORY_IMPL, JASPER_EL_EXPRESSION_FACTORY_IMPL };

     @Override

     protected void initContainerIntegration(

             ServletContext servletContext, ExternalContext externalContext)

     {

         if (log.isInfoEnabled()) {

             log.info("This application isn't running in a JSP 2.1 container."); 

         }

         // It's possible to run MyFaces in a JSP 2.0 Container, but the user has to provide

         // the ExpressionFactory implementation to use as there is no JspApplicationContext

         // we could ask for. Having said that, though, the user only has to provide it, if

         // there is no known ExpressionFactory available (i.e. if neither 

         // "com.sun.el.ExpressionFactoryImpl" nor "org.apache.el.ExpressionFactoryImpl"

         // are available).

         ExpressionFactory expressionFactory = getUserDefinedExpressionFactory(externalContext);

         if (expressionFactory == null) {

             if (log.isInfoEnabled()) {

                 log.info("Either you haven't specified the ExpressionFactory implementation, or an " 

                         + "error occured while instantiating the implementation you've specified. "

                         + "However, attempting to load a known implementation.");

             }

             expressionFactory = findExpressionFactory(KNOWN_EXPRESSION_FACTORIES);

             if (expressionFactory == null) { // if we still haven't got a valid implementation

                 if (log.isErrorEnabled()) {

                     log.error("No valid ExpressionFactory implementation is available "

                             + "but that's required as this application isn't running in a JSP 2.1 container.");

                 }

                 // use a dummy implementation that reports the error again

                 expressionFactory = new ErrorExpressionFactory();

             }

         }

         if (log.isDebugEnabled()) {

             log.debug("The following ExpressionFactory implementation will " 

                     + "be used: '" + expressionFactory + "'.");

         }

         buildConfiguration(servletContext, externalContext, expressionFactory);

     }

     /**

      * Attempts to find a valid ExpressionFactory implementation. Each of the

      * given "ExpressionFactory implementation candidates" will be tried to

      * instantiate. If an attempt succeeded, the ExpressionFactory implementation

      * will be returned (i.e. the first valid ExpressionFactory implementation

      * will be returned) and if no attempt succeeds, <code>null</code> will be

      * returned.

      * 

      * @param expressionFactoryClassNames

      *            "ExpresionFactory implementation candidates"

      * 

      * @return the newly created ExpressionFactory implementation, or

      *         <code>null</code>, if there is no valid implementation

      */

     private static ExpressionFactory findExpressionFactory(String[] expressionFactoryClassNames) 

     {

         for (String expressionFactoryClassName : expressionFactoryClassNames) {

             ExpressionFactory expressionFactory = 

                 loadExpressionFactory(expressionFactoryClassName);

             if (expressionFactory != null) {

                 return expressionFactory;

             }

         }

         return null;

     }

     /**

      * Dummy implementation informing the user that there is no valid

      * ExpressionFactory implementation available. This class makes it easier

      * for the user to understand why the application crashes. Otherwise he

      * would have to deal with NullPointerExceptions.

      * 

      */

     private class ErrorExpressionFactory extends ExpressionFactory

     {

         public Object coerceToType(Object obj, Class<?> targetType)

         {

             throw new FacesException("No valid ExpressionFactory implementation is available "

                             + "but that's required as this application isn't running in a JSP 2.1 container.");

         }

         public MethodExpression createMethodExpression(ELContext context,

                 String expression, Class<?> expectedReturnType, Class<?>[] expectedParamTypes)

         {

             throw new FacesException("No valid ExpressionFactory implementation is available "

                             + "but that's required as this application isn't running in a JSP 2.1 container.");

         }

         public ValueExpression createValueExpression(Object instance, Class<?> expectedType)

         {

             throw new FacesException("No valid ExpressionFactory implementation is available "

                             + "but that's required as this application isn't running in a JSP 2.1 container.");

         }

         public ValueExpression createValueExpression(

                 ELContext context, String expression, Class<?> expectedType)

         {

             throw new FacesException("No valid ExpressionFactory implementation is available "

                             + "but that's required as this application isn't running in a JSP 2.1 container.");

         }

     }

 }