/*
 * 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.tools;

import java.util.Enumeration;
import java.util.NoSuchElementException;

import org.apache.commons.discovery.ResourceClass;
import org.apache.commons.discovery.ResourceClassIterator;
import org.apache.commons.discovery.ResourceNameIterator;
import org.apache.commons.discovery.resource.ClassLoaders;
import org.apache.commons.discovery.resource.classes.DiscoverClasses;
import org.apache.commons.discovery.resource.names.DiscoverServiceNames;

/**
 * [this was ServiceDiscovery12... the 1.1 versus 1.2 issue
 * has been abstracted to org.apache.commons.discover.jdk.JDKHooks]
 *
 * <p>Implement the JDK1.3 'Service Provider' specification.
 * ( http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html )
 * </p>
 *
 * This class supports any VM, including JDK1.1, via
 * org.apache.commons.discover.jdk.JDKHooks.
 *
 * The caller will first configure the discoverer by adding ( in the desired
 * order ) all the places to look for the META-INF/services. Currently
 * we support loaders.
 *
 * The findResources() method will check every loader.
 */
public class Service {

    /**
     * Construct a new service discoverer
     */
    protected Service() {
    }

    /**
     * as described in
     * sun/jdk1.3.1/docs/guide/jar/jar.html#Service Provider,
     * Except this uses <code>Enumeration</code>
     * instead of <code>Interator</code>.
     *
     * @param <T> Service Provider Interface type
     * @param <S> Any type extends the SPI type
     * @param spiClass Service Provider Interface Class
     * @return Enumeration of class instances ({@code S})
     */
    public static <T, S extends T> Enumeration<S> providers(Class<T> spiClass) {
        return Service.<T, S>providers(new SPInterface<T>(spiClass), null);
    }

    /**
     * This version lets you specify constructor arguments..
     *
     * @param <T> Service Provider Interface type
     * @param <S> Any type extends the SPI type
     * @param spi SPI to look for and load.
     * @param loaders loaders to use in search.
     *        If <code>null</code> then use ClassLoaders.getAppLoaders().
     * @return Enumeration of class instances ({@code S})
     */
    public static <T, S extends T> Enumeration<S> providers(final SPInterface<T> spi,
                                                            ClassLoaders loaders) {
        if (loaders == null) {
            loaders = ClassLoaders.getAppLoaders(spi.getSPClass(),
                                                 Service.class,
                                                 true);
        }

        ResourceNameIterator servicesIter =
            (new DiscoverServiceNames(loaders)).findResourceNames(spi.getSPName());

        final ResourceClassIterator<T> services =
            (new DiscoverClasses<T>(loaders)).findResourceClasses(servicesIter);

        return new Enumeration<S>() {

            private S object = getNextClassInstance();

            public boolean hasMoreElements() {
                return object != null;
            }

            public S nextElement() {
                if (object == null) {
                    throw new NoSuchElementException();
                }

                S obj = object;
                object = getNextClassInstance();
                return obj;
            }

            private S getNextClassInstance() {
                while (services.hasNext()) {
                    ResourceClass<S> info = services.nextResourceClass();
                    try {
                        return spi.newInstance(info.loadClass());
                    } catch (Exception e) {
                        // ignore
                    } catch (LinkageError le) {
                        // ignore
                    }
                }
                return null;
            }
        };
    }

}