/*
    * 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.");
         }
 
     }
 
 }