DefaultClassLoadStrategy.java

/*
Name: 
    - DefaultClassLoadStrategy

Description: 
    -  

Requires:
    - 
    
Provides:
    - 

Part of: ProcessPuzzle Framework, Domain and Business Model Ready Architecture. Provides content, workflow and social networking functionality. 
http://www.processpuzzle.com

ProcessPuzzle - Content and Workflow Management Integration Business Platform

Author(s): 
    - Zsolt Zsuffa

Copyright: (C) 2011 This program is free software: you can redistribute it and/or modify it under the terms of the 
GNU General Public License as published by the Free Software Foundation, either version 3 of the License, 
or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package com.processpuzzle.application.configuration.domain;


// ----------------------------------------------------------------------------
/**
 * A default implementation of {@link IClassLoadStrategy} that should be
 * suitable for a variety of situations. See {@link #getClassLoader} for
 * details.
 * 
 * @author (C) <a
 *         href="http://www.javaworld.com/columns/jw-qna-index.shtml">Vlad
 *         Roubtsov</a>, 2003
 */
public class DefaultClassLoadStrategy implements IClassLoadStrategy {
   // public: ................................................................

   // inherit javadoc
   public ClassLoader getClassLoader(final ClassLoadContext ctx) {
      if (ctx == null) throw new IllegalArgumentException("null input: ctx");

      final ClassLoader callerLoader = ctx.getCallerClass().getClassLoader();
      final ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();

      ClassLoader result;

      // if 'callerLoader' and 'contextLoader' are in a parent-child
      // relationship, always choose the child:

      if (isChild(contextLoader, callerLoader)) result = callerLoader;
      else if (isChild(callerLoader, contextLoader)) result = contextLoader;
      else {
         // this else branch could be merged into the previous one,
         // but I show it here to emphasize the ambiguous case:
         result = contextLoader;
      }

      final ClassLoader systemLoader = ClassLoader.getSystemClassLoader();

      // precaution for when deployed as a bootstrap or extension class:
      if (isChild(result, systemLoader)) result = systemLoader;

      return result;
   }

   // protected: .............................................................

   // package: ...............................................................

   // private: ...............................................................

   /**
    * Returns 'true' if 'loader2' is a delegation child of 'loader1' [or if
    * 'loader1'=='loader2']. Of course, this works only for classloaders that
    * set their parent pointers correctly. 'null' is interpreted as the
    * primordial loader [i.e., everybody's parent].
    */
   private static boolean isChild(final ClassLoader loader1, ClassLoader loader2) {
      if (loader1 == loader2) return true;
      if (loader2 == null) return false;
      if (loader1 == null) return true;

      for (; loader2 != null; loader2 = loader2.getParent()) {
         if (loader2 == loader1) return true;
      }

      return false;
   }

}