src/java/org/apache/commons/jelly/impl/DefaultTagLibraryResolver.java

/*
 * Copyright 2002,2004 The Apache Software Foundation.
 *
 * 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.apache.commons.jelly.impl;

import org.apache.commons.discovery.ResourceClass;
import org.apache.commons.discovery.ResourceClassIterator;
import org.apache.commons.discovery.resource.ClassLoaders;
import org.apache.commons.discovery.resource.classes.DiscoverClasses;
import org.apache.commons.jelly.TagLibrary;
import org.apache.commons.jelly.util.ClassLoaderUtils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;


/**
 * <p><code>DefaultTagLibraryResolver</code> is a default implemenation
 * which attempts to interpret the URI as a String called 'jelly:className'
 * and class load the given Java class. Otherwise META-INF/services/jelly/uri
 * is searched for on the thread context's class path and, if found, that
 * class will be loaded.</p>
 *
 * @author <a href="mailto:jstrachan@apache.org">James Strachan</a>
 * @version $Revision: 1.12 $
 */
public class DefaultTagLibraryResolver implements TagLibraryResolver {

    /** The Log to which logging calls will be made. */
    private static final Log log = LogFactory.getLog(DefaultTagLibraryResolver.class);

    private DiscoverClasses discovery;

    /**
     * The class loader to use for instantiating application objects.
     * If not specified, the context class loader, or the class loader
     * used to load this class itself, is used, based on the value of the
     * <code>useContextClassLoader</code> variable.
     */
    private ClassLoader classLoader;

    /**
     * Do we want to use the Context ClassLoader when loading classes
     * for instantiating new objects?  Default is <code>false</code>.
     */
    private boolean useContextClassLoader = false;


    public DefaultTagLibraryResolver() {
    }


    // TagLibraryResolver interface
    //-------------------------------------------------------------------------

    /**
     * Attempts to resolve the given URI to be associated with a TagLibrary
     * otherwise null is returned to indicate no tag library could be found
     * so that the namespace URI should be treated as just vanilla XML.
     */
    public TagLibrary resolveTagLibrary(String uri) {
        DiscoverClasses discovery = getDiscoverClasses();
        String name = uri;
        if ( uri.startsWith( "jelly:" ) ) {
            name = "jelly." + uri.substring(6);
        }

        log.info( "Looking up service name: " + name );

/*
        ClassLoaders loaders = ClassLoaders.getAppLoaders(TagLibrary.class, getClass(), false);

        DiscoverClass discover = new DiscoverClass(loaders);
        Class implClass = discover.find(TestInterface2.class);



        TagLibrary answer = null;
        try {
            answer = (TagLibrary) DiscoverSingleton.find(TagLibrary.class, name);
        }
        catch (Exception e) {
            log.error( "Could not load service: " + name );
        }
        return answer;
*/
        ResourceClassIterator iter = discovery.findResourceClasses(name);
        while (iter.hasNext()) {
            ResourceClass resource = iter.nextResourceClass();
            try {
                Class typeClass = resource.loadClass();
                if ( typeClass != null ) {
                    return newInstance(uri, typeClass);
                }
            }
            catch (Exception e) {
                log.error( "Could not load service: " + resource );
            }
        }
        log.info( "Could not find any services for name: " + name );
        return null;
    }

    // Properties
    //-------------------------------------------------------------------------

    /**
     * Return the class loader to be used for instantiating application objects
     * when required.  This is determined based upon the following rules:
     * <ul>
     * <li>The class loader set by <code>setClassLoader()</code>, if any</li>
     * <li>The thread context class loader, if it exists and the
     *     <code>useContextClassLoader</code> property is set to true</li>
     * <li>The class loader used to load the XMLParser class itself.
     * </ul>
     */
    public ClassLoader getClassLoader() {
        return ClassLoaderUtils.getClassLoader(classLoader, useContextClassLoader, getClass());
    }

    /**
     * Set the class loader to be used for instantiating application objects
     * when required.
     *
     * @param classLoader The new class loader to use, or <code>null</code>
     *  to revert to the standard rules
     */
    public void setClassLoader(ClassLoader classLoader) {
        this.classLoader = classLoader;
    }

    /**
     * Return the boolean as to whether the context classloader should be used.
     */
    public boolean getUseContextClassLoader() {
        return useContextClassLoader;
    }

    /**
     * Determine whether to use the Context ClassLoader (the one found by
     * calling <code>Thread.currentThread().getContextClassLoader()</code>)
     * to resolve/load classes.  If not
     * using Context ClassLoader, then the class-loading defaults to
     * using the calling-class' ClassLoader.
     *
     * @param boolean determines whether to use JellyContext ClassLoader.
     */
    public void setUseContextClassLoader(boolean use) {
        useContextClassLoader = use;
    }

    /**
     * @return the DiscoverClasses instance to use to locate services.
     *  This object is lazily created if it has not been configured.
     */
    public DiscoverClasses getDiscoverClasses() {
        if ( discovery == null ) {
            ClassLoaders loaders = ClassLoaders.getAppLoaders(TagLibrary.class, getClass(), false);
            discovery = new DiscoverClasses(loaders);
        }
        return discovery;
    }

    /**
     * Sets the fully configured DiscoverClasses instance to be used to
     * lookup services
     */
    public void setDiscoverClasses(DiscoverClasses discovery) {
        this.discovery = discovery;
    }

    // Implementation methods
    //-------------------------------------------------------------------------

    /**
     * Instantiates the given class name. Otherwise an exception is logged
     * and null is returned
     */
    protected TagLibrary loadClass(String uri, String className) {
        try {
            Class theClass = getClassLoader().loadClass(className);
            if ( theClass != null ) {
                return newInstance(uri, theClass);
            }
        }
        catch (ClassNotFoundException e) {
            log.error("Could not find the class: " + className + " when trying to resolve URI: " + uri, e);
        }
        return null;
    }


    /**
     * Creates a new instance of the given TagLibrary class or
     * return null if it could not be instantiated.
     */
    protected TagLibrary newInstance(String uri, Class theClass) {
        try {
            Object object = theClass.newInstance();
            if (object instanceof TagLibrary) {
                return (TagLibrary) object;
            }
            else {
                log.error(
                    "The tag library object mapped to: "
                        + uri
                        + " is not a TagLibrary. Object = "
                        + object);
            }
        }
        catch (Exception e) {
            log.error(
                "Could not instantiate instance of class: " + theClass.getName() + ". Reason: " + e,
                e);
        }
        return null;
    }

}