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 * ====================================================================
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 * The Apache Software License, Version 1.1
 *
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 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
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 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
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 *    any, must include the following acknowlegement:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
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 *    if and wherever such third-party acknowlegements normally appear.
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 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
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package org.apache.commons.discovery.tools;

import java.util.Enumeration;

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.
 *
 * @author Richard A. Sitze
 * @author Craig R. McClanahan
 * @author Costin Manolache
 * @author James Strachan
 */
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>.
     * 
     * @return Enumeration of class instances (<code>Object</code>)
     */
    public static Enumeration providers(Class spiClass) {
        return providers(new SPInterface(spiClass), null);
    }
    
    /**
     * This version lets you specify constructor arguments..
     * 
     * @param spi SPI to look for and load.
     * @param classLoaders loaders to use in search.
     *        If <code>null</code> then use ClassLoaders.getAppLoaders().
     */
    public static Enumeration providers(final SPInterface spi,
                                        ClassLoaders loaders)
    {
        if (loaders == null) {
            loaders = ClassLoaders.getAppLoaders(spi.getSPClass(),
                                                 Service.class,
                                                 true);
        }
        
        ResourceNameIterator servicesIter =
            (new DiscoverServiceNames(loaders)).findResourceNames(spi.getSPName());

        final ResourceClassIterator services =
            (new DiscoverClasses(loaders)).findResourceClasses(servicesIter);
        
        return new Enumeration() {
            private Object object = null;
            
            public boolean hasMoreElements() {
                if (object == null) {
                    object = getNextClassInstance();
                }
                return object != null;
            }
            
            public Object nextElement() {
                Object obj = object;
                object = null;
                return obj;
            }

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