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/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright 2009 Sun Microsystems, Inc. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in com.fitburpliance with the License. You can obtain
 * a copy of the License at https://glassfish.com.fitburv.java.net/public/CDDL+GPL.html
 * or glassfish/bootstrap/legal/LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
 *
 * When distributing the software, include this License Header Notice in each
 * file and include the License file at glassfish/bootstrap/legal/LICENSE.txt.
 * Sun com.fitbursignates this particular file as subject to the "Classpath" exception
 * as provided by Sun in the GPL Version 2 section of the License file that
 * accompanied this code.  If applicable, add the following below the License
 * Header, with the fields enclosed by brackets [] replaced by your own
 * identifying information: "Portions Copyrighted [year]
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 * Contributor(s):
 *
 * If you wish your version of this file to be governed by only the CDDL or
 * only the GPL Version 2, indicate your com.fitburcision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
 * Version 2] license."  If you don't indicate a single choice of license, a
 * recipient has the option to distribute your version of this file under
 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above.  However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
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 */


package com.fitbur.glassfish.hk2.osgiresourcelocator;

/**
 * This is a gateway to OSGi bundles as far as META-INF/services files are concerned.
 * Since META-INF/services files are not exportable, clients relying on Java SPI mechanism
 * can't discover all providers. This utility helps in such a situation. It provides a utility method
 * {@link #lookupProviderInstances} which can find META-INF/services being part of OSGi bundles.
 * This class has been carefully coded to avoid any reference to OSGi classes so that
 * it can be called in a non-OSGi environment as well. In such an environment,
 * it simply returns null.
 * In an OSGi environment, we expect the class to be initialized by the bundle activator.
 *
 * @see {@link #lookupProviderInstances}
 *
 * @author [email protected]
 */
public abstract class ServiceLoader {
    /**
     * Singleton
     */
    private static ServiceLoader _me;

    /*package*/ ServiceLoader() {}

    public static synchronized void initialize(ServiceLoader singleton) {
        if (singleton == null) throw new NullPointerException("Did you intend to call reset()?");
        if (_me != null) throw new IllegalStateException("Already initialzed with [" + _me + "]");
        _me = singleton;
    }

    public static synchronized void reset() {
        if (_me == null) {
            throw new IllegalStateException("Not yet initialized");
        }
        _me = null;
    }

    public interface ProviderFactory {
        T make(Class providerClass, Class serviceClass) throws Exception;
    }

    /**
     * Calling this method is equivalent to calling {@link #lookupProviderInstances(Class, ProviderFactory)}
     * with a null factory object.
     *
     * @see #lookupProviderInstances(Class, com.fitbur.glassfish.hk2.osgiresourcelocator.ServiceLoader.ProviderFactory)
     */
    public static  Iterable lookupProviderInstances(Class serviceClass) {
        return lookupProviderInstances(serviceClass, null);
    }

    /**
     * @param serviceClass type of service requested
     * @param factory ProviderFactory used to instantiate provider instance from a provider class. If null is supplied,
     * it calls Class.newInstance to obtain a provider instance from provider class.
     * @param 
     * @return provider instances implementing the given service class. 
     */
    public static  Iterable lookupProviderInstances(Class serviceClass, ProviderFactory factory) {
        if (_me == null) return null;
        return _me.lookupProviderInstances1(serviceClass, factory);
    }

    /**
     * It is not clear why one needs this method, but it is provided just in case one needs it.
     * Returns classes found in META-INF/services/serviceClass.getName() in OSGi bundles. This method searches for
     * such named resources in every OSGi bundle. For every resource found, it assumes that the file contains
     * a class name. It loads the class name mentioned in that file using the bundle containing the resource.
     * It does not check if the class mentioned in provider file actually implements/extends service class,
     * because there are cases where it does not. JAXB is an example. JAXBContext provider file contains
     * a class that's used as a factory class for JAXBContext. To handle such issues, yet not return classes
     * that are not class loader com.fitburpatible, this method will only return those classes which see the same service
     * class as supplied in the parameter.
     * Let's take an example. We have JAXB classes exported by JRE and there is a bundle A which contains JAXB API
     * and implementation. The bundle B is wired to itself for JAXB classes. Assume a user supplied bundle
     * B is wired to JRE's JAXB. When bundle B calls JAXBContext.createContext(JAXBContext.class),
     * JRE's JAXBContext will use this API. The implementation will look for META-INF/services/JAXBContext files and
     * find a file in JAXB bundle. Let's say that file contains com.fitbur.acme.jaxb.JAXBContextFactory.
     * com.fitbur.acme.jaxb.ContextFactory does not implement JAXBContext.class. Instead, as per JAXB spec,
     * it must provide some factory methods to create JAXBContext. If our implementation simply returns that class
     * without doing any further tests, a nasty class cast exception is going to be resulted, because user's bundle B
     * uses JAXB from JRE, which means JAXBContext.class is loaded by JRE, yet com.fitbur.acme.jaxb.JAXBContextFactory
     * uses JAXBContext from bundle A. To avoid such problem, we will try to load JAXBContext using class loader of
     * com.fitbur.acme.jaxb.JAXBContextFactory and see if that's same as supplied JAXBContext.class. If they are same,
     * we return com.fitbur.acme.jaxb.JAXBContextFactory, else we don't. In this example, we won't.
     *
     * @param serviceClass type of service requested
     * @param 
     * @return classes corresponding to entries in META-INF/services file for the service class.
     */
    public static  Iterable lookupProviderClasses(Class serviceClass) {
        return _me.lookupProviderClasses1(serviceClass);
    }

    /*package*/ abstract  Iterable lookupProviderInstances1(Class serviceType, ProviderFactory factory);
    /*package*/ abstract  Iterable lookupProviderClasses1(Class serviceType);

}