org.apache.commons.discovery.resource.ClassLoaders.java Source code

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Here is the source code for org.apache.commons.discovery.resource.ClassLoaders.java

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/*
 * 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.
 */
package org.apache.commons.discovery.resource;

import java.util.Vector;

import org.apache.commons.discovery.jdk.JDKHooks;

/**
 * There are many different contexts in which
 * loaders can be used.  This provides a holder
 * for a set of class loaders, so that they
 * don't have to be build back up everytime...
 *
 * @author Richard A. Sitze
 * @author Craig R. McClanahan
 * @author Costin Manolache
 */
public class ClassLoaders {
    protected Vector classLoaders = new Vector();

    /** Construct a new class loader set
     */
    public ClassLoaders() {
    }

    public int size() {
        return classLoaders.size();
    }

    public ClassLoader get(int idx) {
        return (ClassLoader) classLoaders.elementAt(idx);
    }

    /**
     * Specify a new class loader to be used in searching.
     * The order of loaders determines the order of the result.
     * It is recommended to add the most specific loaders first.
     */
    public void put(ClassLoader classLoader) {
        if (classLoader != null) {
            classLoaders.addElement(classLoader);
        }
    }

    /**
     * Specify a new class loader to be used in searching.
     * The order of loaders determines the order of the result.
     * It is recommended to add the most specific loaders first.
     * 
     * @param prune if true, verify that the class loader is
     *              not an Ancestor (@see isAncestor) before
     *              adding it to our list.
     */
    public void put(ClassLoader classLoader, boolean prune) {
        if (classLoader != null && !(prune && isAncestor(classLoader))) {
            classLoaders.addElement(classLoader);
        }
    }

    /**
     * Check to see if <code>classLoader</code> is an
     * ancestor of any contained class loader.
     * 
     * This can be used to eliminate redundant class loaders
     * IF all class loaders defer to parent class loaders
     * before resolving a class.
     * 
     * It may be that this is not always true.  Therefore,
     * this check is not done internally to eliminate
     * redundant class loaders, but left to the discretion
     * of the user.
     */
    public boolean isAncestor(final ClassLoader classLoader) {
        /* bootstrap classloader, at root of all trees! */
        if (classLoader == null)
            return true;

        for (int idx = 0; idx < size(); idx++) {
            for (ClassLoader walker = get(idx); walker != null; walker = walker.getParent()) {
                if (walker == classLoader) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Utility method.  Returns a preloaded ClassLoaders instance
     * containing the following class loaders, in order:
     * 
     * <ul>
     *   <li>spi.getClassLoader</li>
     *   <li>seeker.getClassLoader</li>
     *   <li>System Class Loader</li>
     * </ul>
     * 
     * Note that the thread context class loader is NOT present.
     * This is a reasonable set of loaders to try if the resource to be found
     * should be restricted to a libraries containing the SPI and Factory.
     * 
     * @param spi WHAT is being looked for (an implementation of this class,
     *            a default property file related to this class).
     * @param factory WHO is performing the lookup.
     * @param prune Determines if ancestors are allowed to be loaded or not.
     */
    public static ClassLoaders getLibLoaders(Class spi, Class factory, boolean prune) {
        ClassLoaders loaders = new ClassLoaders();

        if (spi != null)
            loaders.put(spi.getClassLoader());
        if (factory != null)
            loaders.put(factory.getClassLoader(), prune);
        loaders.put(JDKHooks.getJDKHooks().getSystemClassLoader(), prune);

        return loaders;
    }

    /**
     * Utility method.  Returns a preloaded ClassLoaders instance
     * containing the following class loaders, in order:
     * 
     * <ul>
     *   <li>Thread Context Class Loader</li>
     *   <li>spi.getClassLoader</li>
     *   <li>seeker.getClassLoader</li>
     *   <li>System Class Loader</li>
     * </ul>
     * 
     * Note that the thread context class loader IS  present.
     * This is a reasonable set of loaders to try if the resource to be found
     * may be provided by an application.
     * 
     * @param spi WHAT is being looked for (an implementation of this class,
     *            a default property file related to this class).
     * @param factory WHO is performing the lookup (factory).
     * @param prune Determines if ancestors are allowed to be loaded or not.
     */
    public static ClassLoaders getAppLoaders(Class spi, Class factory, boolean prune) {
        ClassLoaders loaders = new ClassLoaders();

        loaders.put(JDKHooks.getJDKHooks().getThreadContextClassLoader());
        if (spi != null)
            loaders.put(spi.getClassLoader(), prune);
        if (factory != null)
            loaders.put(factory.getClassLoader(), prune);
        loaders.put(JDKHooks.getJDKHooks().getSystemClassLoader(), prune);

        return loaders;
    }
}