package org.apache.turbine.modules;
   
   /*
    * 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.
   */
  
  import java.util.List;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  
  import org.apache.ecs.ConcreteElement;
  
  import org.apache.turbine.Turbine;
  import org.apache.turbine.TurbineConstants;
  import org.apache.turbine.services.assemblerbroker.AssemblerBrokerService;
  import org.apache.turbine.services.assemblerbroker.TurbineAssemblerBroker;
  import org.apache.turbine.util.ObjectUtils;
  import org.apache.turbine.util.RunData;
  
  /***
   * The purpose of this class is to allow one to load and execute
   * Navigation modules.
   *
   * @author <a href="mailto:mbryson@mont.mindspring.com">Dave Bryson</a>
   * @author <a href="mailto:hps@intermeta.de">Henning P. Schmiedehausen</a>
   * @version $Id: NavigationLoader.java 534527 2007-05-02 16:10:59Z tv $
   */
  public class NavigationLoader
      extends GenericLoader
      implements Loader
  {
      /*** Serial Version UID */
      private static final long serialVersionUID = 5062299200802529612L;
  
      /*** Logging */
      private static Log log = LogFactory.getLog(NavigationLoader.class);
  
      /*** The single instance of this class. */
      private static NavigationLoader instance =
          new NavigationLoader(Turbine.getConfiguration()
                           .getInt(TurbineConstants.NAVIGATION_CACHE_SIZE_KEY,
                                   TurbineConstants.NAVIGATION_CACHE_SIZE_DEFAULT));
  
      /*** The Assembler Broker Service */
      private static AssemblerBrokerService ab = TurbineAssemblerBroker.getService();
  
      /***
       * These ctor's are private to force clients to use getInstance()
       * to access this class.
       */
      private NavigationLoader()
      {
          super();
      }
  
      /***
       * These ctor's are private to force clients to use getInstance()
       * to access this class.
       */
      private NavigationLoader(int i)
      {
          super(i);
      }
  
      /***
       * Adds an instance of an object into the hashtable.
       *
       * @param name Name of object.
       * @param navigation Navigation to be associated with name.
       */
      private void addInstance(String name, Navigation navigation)
      {
          if (cache())
          {
              this.put(name, (Navigation) navigation);
          }
      }
  
      /***
       * Attempts to load and execute the external Navigation. This is
       * used when you want to execute a Navigation which returns its
       * output via a MultiPartElement instead of out the data.getPage()
       * value.  This allows you to easily chain the execution of
      * Navigation modules together.
      *
      * @param data Turbine information.
      * @param name Name of object that will execute the navigation.
      * @exception Exception a generic exception.
      */
     public ConcreteElement eval(RunData data, String name)
             throws Exception
     {
         // Execute Navigation
         return getInstance(name).build(data);
     }
 
     /***
      * Attempts to load and execute the external Navigation.
      *
      * @param data Turbine information.
      * @param name Name of object instance.
      * @exception Exception a generic exception.
      */
     public void exec(RunData data, String name)
             throws Exception
     {
         this.eval(data, name);
     }
 
     /***
      * Pulls out an instance of the object by name.  Name is just the
      * single name of the object. This is equal to getInstance but
      * returns an Assembler object and is needed to fulfil the Loader
      * interface.
      *
      * @param name Name of object instance.
      * @return A Layout with the specified name, or null.
      * @exception Exception a generic exception.
      */
     public Assembler getAssembler(String name)
         throws Exception
     {
         return getInstance(name);
     }
 
     /***
      * Pulls out an instance of the Navigation by name.  Name is just the
      * single name of the Navigation.
      *
      * @param name Name of requested Navigation
      * @return A Navigation with the specified name, or null.
      * @exception Exception a generic exception.
      */
     public Navigation getInstance(String name)
             throws Exception
     {
         Navigation navigation = null;
 
         // Check if the navigation is already in the cache
         if (cache() && this.containsKey(name))
         {
             navigation = (Navigation) this.get(name);
             log.debug("Found Navigation " + name + " in the cache!");
         }
         else
         {
             log.debug("Loading Navigation " + name + " from the Assembler Broker");
 
             try
             {
                 if (ab != null)
                 {
                     // Attempt to load the navigation
                     navigation = (Navigation) ab.getAssembler(
                         AssemblerBrokerService.NAVIGATION_TYPE, name);
                 }
             }
             catch (ClassCastException cce)
             {
                 // This can alternatively let this exception be thrown
                 // So that the ClassCastException is shown in the
                 // browser window.  Like this it shows "Screen not Found"
                 navigation = null;
             }
 
             if (navigation == null)
             {
                 // If we did not find a screen we should try and give
                 // the user a reason for that...
                 // FIX ME: The AssemblerFactories should each add it's
                 // own string here...
                 List packages = Turbine.getConfiguration()
                     .getList(TurbineConstants.MODULE_PACKAGES);
 
                 ObjectUtils.addOnce(packages,
                         GenericLoader.getBasePackage());
 
                 throw new ClassNotFoundException(
                         "\n\n\tRequested Navigation not found: " + name +
                         "\n\tTurbine looked in the following " +
                         "modules.packages path: \n\t" + packages.toString() + "\n");
             }
             else if (cache())
             {
                 // The new instance is added to the cache
                 addInstance(name, navigation);
             }
         }
         return navigation;
     }
 
     /***
      * The method through which this class is accessed.
      *
      * @return The single instance of this class.
      */
     public static NavigationLoader getInstance()
     {
         return instance;
     }
 }