cascadingclassloadhelper.java

定时器开源项目, 相对于 jcrontab, Quartz 算是更完整的一个项目, 随著开发的版本上来, 他已经脱离只是写在程序里面的计时器, 在指定的时间或区间, 处理所指定的事件. 也加入了 se

/*
 * Copyright James House (c) 2001-2004
 * 
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met: 1.
 * Redistributions of source code must retain the above copyright notice, this
 * list of conditions and the following disclaimer. 2. Redistributions in
 * binary form must reproduce the above copyright notice, this list of
 * conditions and the following disclaimer in the documentation and/or other
 * materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *  
 */
package org.quartz.simpl;

import java.util.Iterator;
import java.util.LinkedList;

import org.quartz.spi.ClassLoadHelper;

/**
 * A <code>ClassLoadHelper</code> uses all of the <code>ClassLoadHelper</code>
 * 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 perfomance).
 * 
 * @see org.quartz.spi.ClassLoadHelper
 * @see org.quartz.simpl.SimpleClassLoadHelper
 * @see org.quartz.simpl.ThreadContextClassLoadHelper
 * @see org.quartz.simpl.InitThreadContextClassLoadHelper
 * 
 * @author jhouse
 */
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 oportunity 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 SimpleClassLoadHelper());
        loadHelpers.add(new ThreadContextClassLoadHelper());
        loadHelpers.add(new InitThreadContextClassLoadHelper());

        Iterator iter = loadHelpers.iterator();
        while (iter.hasNext()) {
            ClassLoadHelper loadHelper = (ClassLoadHelper) iter.next();
            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 (Exception e) {
                bestCandidate = null;
            }
        }

        ClassNotFoundException cnfe = 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 (ClassNotFoundException e) {
                cnfe = e;
            }
        }

        if (clazz == null) throw cnfe;

        bestCandidate = loadHelper;

        return clazz;
    }

}