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A Lightweight Java Job Scheduling Framework
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/* 
 * Copyright 2001-2009 Terracotta, Inc. 
 * 
 * Licensed 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.quartz.simpl;

import java.io.InputStream;
import java.net.URL;
import java.util.Iterator;
import java.util.LinkedList;

import org.quartz.spi.ClassLoadHelper;

/**
 * A ClassLoadHelper uses all of the ClassLoadHelper types that are found in this package in its attempts to load a class, when one scheme is found to work, it is promoted to the scheme that will be used first the next time
 * a class is loaded (in order to improve performance).
 * 
 * This approach is used because of the wide variance in class loader behavior between the various environments in which Quartz runs (e.g. disparate application servers, stand-alone, mobile devices, etc.). Because of this disparity, Quartz ran into
 * difficulty with a one class-load style fits-all design. Thus, this class loader finds the approach that works, then 'remembers' it.
 * 
 * 
 * @see org.quartz.spi.ClassLoadHelper
 * @see org.quartz.simpl.LoadingLoaderClassLoadHelper
 * @see org.quartz.simpl.SimpleClassLoadHelper
 * @see org.quartz.simpl.ThreadContextClassLoadHelper
 * @see org.quartz.simpl.InitThreadContextClassLoadHelper
 * @author jhouse
 * @author pl47ypus
 */
public class CascadingClassLoadHelper implements ClassLoadHelper {

  /*
   * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Data members. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   */

  private LinkedList loadHelpers;

  private ClassLoadHelper bestCandidate;

  /*
   * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Interface. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   */

  /**
   * Called to give the ClassLoadHelper a chance to initialize itself, including the opportunity to "steal" the class loader off of the calling thread, which is the thread that is initializing Quartz.
   */
  public void initialize() {

    loadHelpers = new LinkedList();

    loadHelpers.add(new LoadingLoaderClassLoadHelper());
    loadHelpers.add(new SimpleClassLoadHelper());
    loadHelpers.add(new ThreadContextClassLoadHelper());
    loadHelpers.add(new InitThreadContextClassLoadHelper());

    for (ClassLoadHelper loadHelper : loadHelpers) {
      loadHelper.initialize();
    }
  }

  /**
   * Return the class with the given name.
   */
  public Class loadClass(String name) throws ClassNotFoundException {

    if (bestCandidate != null) {
      try {
        return bestCandidate.loadClass(name);
      } catch (Throwable t) {
        bestCandidate = null;
      }
    }

    Throwable throwable = null;
    Class clazz = null;
    ClassLoadHelper loadHelper = null;

    Iterator iter = loadHelpers.iterator();
    while (iter.hasNext()) {
      loadHelper = (ClassLoadHelper) iter.next();

      try {
        clazz = loadHelper.loadClass(name);
        break;
      } catch (Throwable t) {
        throwable = t;
      }
    }

    if (clazz == null) {
      if (throwable instanceof ClassNotFoundException) {
        throw (ClassNotFoundException) throwable;
      } else {
        throw new ClassNotFoundException(String.format("Unable to load class %s by any known loaders.", name), throwable);
      }
    }

    bestCandidate = loadHelper;

    return clazz;
  }

  /**
   * Finds a resource with a given name. This method returns null if no resource with this name is found.
   * 
   * @param name name of the desired resource
   * @return a java.net.URL object
   */
  public URL getResource(String name) {

    URL result = null;

    if (bestCandidate != null) {
      result = bestCandidate.getResource(name);
      if (result == null)
        bestCandidate = null;
    }

    ClassLoadHelper loadHelper = null;

    Iterator iter = loadHelpers.iterator();
    while (iter.hasNext()) {
      loadHelper = (ClassLoadHelper) iter.next();

      result = loadHelper.getResource(name);
      if (result != null) {
        break;
      }
    }

    bestCandidate = loadHelper;
    return result;
  }

  /**
   * Finds a resource with a given name. This method returns null if no resource with this name is found.
   * 
   * @param name name of the desired resource
   * @return a java.io.InputStream object
   */
  public InputStream getResourceAsStream(String name) {

    InputStream result = null;

    if (bestCandidate != null) {
      result = bestCandidate.getResourceAsStream(name);
      if (result == null)
        bestCandidate = null;
    }

    ClassLoadHelper loadHelper = null;

    Iterator iter = loadHelpers.iterator();
    while (iter.hasNext()) {
      loadHelper = (ClassLoadHelper) iter.next();

      result = loadHelper.getResourceAsStream(name);
      if (result != null) {
        break;
      }
    }

    bestCandidate = loadHelper;
    return result;
  }

  /**
   * Enable sharing of the "best" class-loader with 3rd party.
   * 
   * @return the class-loader user be the helper.
   */
  public ClassLoader getClassLoader() {

    return (this.bestCandidate == null) ? Thread.currentThread().getContextClassLoader() : this.bestCandidate.getClassLoader();
  }

}