xmlwalker.java

webwork source

/*
 * WebWork, Web Application Framework
 *
 * Distributable under Apache license.
 * See terms of license at opensource.org
 */

package webwork.view.xslt.SAXAdapter;

import org.apache.commons.logging.*;
import org.xml.sax.ContentHandler;
import org.xml.sax.SAXException;
import webwork.view.xslt.SAXAdapter.dom.NodeWalker;
import webwork.view.xslt.SAXAdapter.dom.TextWalker;

import java.util.*;

/**
 * This class is used by XMLReaderAdapter to do the actual walking of the object
 * tree. Because the XMLReader Interface is stateful in respect of the systemId
 * (here the walked bean) this class implements the stateless part.
 *
 */
public class XMLWalker
{
   // Walkers that are used without been registered due to
   // the nature of the classes they walk.
   private Walker arrayWalker = new ArrayWalker();
   private Walker beanWalker = new BeanWalker();
   private Walker toStringWalker = new ToStringWalker();
   private Walker nullWalker = new NullWalker();

   // walker registry
   private List walkers = new ArrayList();
   private Map walkerCache = new HashMap();

   public XMLWalker()
   {

      // Add default walkers
      registerWalker(new CollectionWalker());
      registerWalker(new EnumerationWalker());
      registerWalker(new MapWalker());
      registerWalker(new BooleanWalker());
      registerWalker(new CharacterWalker());

      // Add DOM walkers
      registerWalker(new TextWalker());
      registerWalker(new NodeWalker());
      // Arrays and JavaBeans are being processed specially using Class.isArray() but register it
      // here for completeness.
      registerWalker(arrayWalker);
      registerWalker(beanWalker);
      registerWalker(toStringWalker);
   }

   public void walk(ContentHandler contentHandler, Object object, String name, List walkedInstances) throws SAXException
   {
      //Null property handling:
      if (object == null) {
         nullWalker.walk(this, contentHandler, object, name, walkedInstances);
         return; // fast exit.
      }

      //Extract the object's class.
      Class klass = object.getClass();

      //Try to find a registered walker
      Walker walker = findWalker(klass);

      if (walker != null)
      {
         //Found special walker
         walker.walk(this, contentHandler, object, name, walkedInstances);
      }
      else if (klass.isArray())
      {
         //Use arrayWalker to process the object
         arrayWalker.walk(this, contentHandler, object, name, walkedInstances);
      }
      else if (String.class.isAssignableFrom(klass) || klass.isPrimitive() || Number.class.isAssignableFrom(klass))
      {
         //Primitives and java.lang.Numbers are handled by toStringWalker, too.
         toStringWalker.walk(this, contentHandler, object, name, walkedInstances);
      }
      else
      {
         //Fallback: dissect the object using the JavaBeans API.
         beanWalker.walk(this, contentHandler, object, name, walkedInstances);
      }
   }

   /**
    * Registers locally the class the walker returns as
    * it's walkable type.
    */
   public void registerWalker(Walker walker)
   {
      //Registering a new walker will render the cached information obsolete.
      walkerCache.clear();
      this.walkers.add(walker);
   }

   /**
    * Find a walker registered for object's class.
    * To speed up the walking of the collection of
    * registered Walkers, a cache is used.
    * If no Walker was found null is returned.
    *
    * @return the Walker which was registered for the object's class or null
    *         if no walker was registered.
    **/
   private synchronized Walker findWalker(Class klass)
   {

      // Try to look up walker in cache.
      if (walkerCache.containsKey(klass))
      {
         return (Walker) walkerCache.get(klass);
      }

      // Try to find walker in registred walkers.
      Walker walker = null;
      findWalker: for (Iterator i = walkers.iterator(); i.hasNext();)
      {
         Walker tryWalker = (Walker) i.next();
         if (tryWalker.getWalkedType().isAssignableFrom(klass))
         {
            walker = tryWalker;
            break findWalker;
         }
      }

      LogFactory.getLog(this.getClass()).debug("Caching " + (walker != null ? "walker " + walker.getClass().getName() :
                "no walker") + " for " + klass.getName());
      walkerCache.put(klass, walker);
      return walker;
   }

   /**
    * Expose toStringWalker to public for BeanWalker can use it as a
    * fall back when to properties are found.
    */
   public Walker getToStringWalker() {
      return toStringWalker;
   }
}