ServiceRegistry.java

/*
   Copyright (C) 2005-2012, by the President and Fellows of Harvard College.

   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.

   Dataverse Network - A web application to share, preserve and analyze research data.
   Developed at the Institute for Quantitative Social Science, Harvard University.
   Version 3.0.
*/
package edu.harvard.iq.dataverse.ingest.plugin.spi;



// This file was Taken out from openjdk-6-src-b16-24_apr_2009.tar.gz
// http://download.java.net/openjdk/jdk6/promoted/b16/openjdk-6-src-b16-24_apr_2009.tar.gz
// downloaded: 2009-05-07


/*
 * Copyright 2000-2007 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

//package javax.imageio.spi;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.ServiceLoader;

/**
 * A registry for service provider instances.
 *
 * <p> A <i>service</i> is a well-known set of interfaces and (usually
 * abstract) classes.  A <i>service provider</i> is a specific
 * implementation of a service.  The classes in a provider typically
 * implement the interface or subclass the class defined by the
 * service itself.
 *
 * <p> Service providers are stored in one or more <i>categories</i>,
 * each of which is defined by a class of interface (described by a
 * <code>Class</code> object) that all of its members must implement.
 * The set of categories may be changed dynamically.
 *
 * <p> Only a single instance of a given leaf class (that is, the
 * actual class returned by <code>getClass()</code>, as opposed to any
 * inherited classes or interfaces) may be registered.  That is,
 * suppose that the
 * <code>com.mycompany.mypkg.GreenServiceProvider</code> class
 * implements the <code>com.mycompany.mypkg.MyService</code>
 * interface.  If a <code>GreenServiceProvider</code> instance is
 * registered, it will be stored in the category defined by the
 * <code>MyService</code> class.  If a new instance of
 * <code>GreenServiceProvider</code> is registered, it will replace
 * the previous instance.  In practice, service provider objects are
 * usually singletons so this behavior is appropriate.
 *
 * <p> To declare a service provider, a <code>services</code>
 * subdirectory is placed within the <code>META-INF</code> directory
 * that is present in every JAR file.  This directory contains a file
 * for each service provider interface that has one or more
 * implementation classes present in the JAR file.  For example, if
 * the JAR file contained a class named
 * <code>com.mycompany.mypkg.MyServiceImpl</code> which implements the
 * <code>javax.someapi.SomeService</code> interface, the JAR file
 * would contain a file named: <pre>
 * META-INF/services/javax.someapi.SomeService </pre>
 *
 * containing the line:
 *
 * <pre>
 * com.mycompany.mypkg.MyService
 * </pre>
 *
 * <p> The service provider classes should be to be lightweight and
 * quick to load.  Implementations of these interfaces should avoid
 * complex dependencies on other classes and on native code. The usual
 * pattern for more complex services is to register a lightweight
 * proxy for the heavyweight service.
 *
 * <p> An application may customize the contents of a registry as it
 * sees fit, so long as it has the appropriate runtime permission.
 *
 * <p> For more details on declaring service providers, and the JAR
 * format in general, see the <a
 * href="../../../../technotes/guides/jar/jar.html">
 * JAR File Specification</a>.
 *
 * @see RegisterableService
 *
 */
public class ServiceRegistry {

    // Class -> Registry
    private Map categoryMap = new HashMap();

    /**
     * Constructs a <code>ServiceRegistry</code> instance with a
     * set of categories taken from the <code>categories</code>
     * argument.
     *
     * @param categories an <code>Iterator</code> containing
     * <code>Class</code> objects to be used to define categories.
     *
     * @exception IllegalArgumentException if
     * <code>categories</code> is <code>null</code>.
     */
    public ServiceRegistry(Iterator<Class<?>> categories) {
        if (categories == null) {
            throw new IllegalArgumentException("categories == null!");
        }
        while (categories.hasNext()) {
            Class category = (Class)categories.next();
            SubRegistry reg = new SubRegistry(this, category);
            categoryMap.put(category, reg);
        }
    }

    // The following two methods expose functionality from
    // sun.misc.Service.  If that class is made public, they may be
    // removed.
    //
    // The sun.misc.ServiceConfigurationError class may also be
    // exposed, in which case the references to 'an
    // <code>Error</code>' below should be changed to 'a
    // <code>ServiceConfigurationError</code>'.

    /**
     * Searches for implementations of a particular service class
     * using the given class loader.
     *
     * <p> This method transforms the name of the given service class
     * into a provider-configuration filename as described in the
     * class comment and then uses the <code>getResources</code>
     * method of the given class loader to find all available files
     * with that name.  These files are then read and parsed to
     * produce a list of provider-class names.  The iterator that is
     * returned uses the given class loader to look up and then
     * instantiate each element of the list.
     *
     * <p> Because it is possible for extensions to be installed into
     * a running Java virtual machine, this method may return
     * different results each time it is invoked.
     *
     * @param providerClass a <code>Class</code>object indicating the
     * class or interface of the service providers being detected.
     *
     * @param loader the class loader to be used to load
     * provider-configuration files and instantiate provider classes,
     * or <code>null</code> if the system class loader (or, failing that
     * the bootstrap class loader) is to be used.
     *
     * @return An <code>Iterator</code> that yields provider objects
     * for the given service, in some arbitrary order.  The iterator
     * will throw an <code>Error</code> if a provider-configuration
     * file violates the specified format or if a provider class
     * cannot be found and instantiated.
     *
     * @exception IllegalArgumentException if
     * <code>providerClass</code> is <code>null</code>.
     */
    public static <T> Iterator<T> lookupProviders(Class<T> providerClass,
                                                  ClassLoader loader)
    {
        if (providerClass == null) {
            throw new IllegalArgumentException("providerClass == null!");
        }
        return ServiceLoader.load(providerClass, loader).iterator();
    }

    /**
     * Locates and incrementally instantiates the available providers
     * of a given service using the context class loader.  This
     * convenience method is equivalent to:
     *
     * <pre>
     *   ClassLoader cl = Thread.currentThread().getContextClassLoader();
     *   return Service.providers(service, cl);
     * </pre>
     *
     * @param providerClass a <code>Class</code>object indicating the
     * class or interface of the service providers being detected.
     *
     * @return An <code>Iterator</code> that yields provider objects
     * for the given service, in some arbitrary order.  The iterator
     * will throw an <code>Error</code> if a provider-configuration
     * file violates the specified format or if a provider class
     * cannot be found and instantiated.
     *
     * @exception IllegalArgumentException if
     * <code>providerClass</code> is <code>null</code>.
     */
    public static <T> Iterator<T> lookupProviders(Class<T> providerClass) {
        if (providerClass == null) {
            throw new IllegalArgumentException("providerClass == null!");
        }
        return ServiceLoader.load(providerClass).iterator();
    }

    /**
     * Returns an <code>Iterator</code> of <code>Class</code> objects
     * indicating the current set of categories.  The iterator will be
     * empty if no categories exist.
     *
     * @return an <code>Iterator</code> containing
     * <code>Class</code>objects.
     */
    public Iterator<Class<?>> getCategories() {
        Set keySet = categoryMap.keySet();
        return keySet.iterator();
    }

    /**
     * Returns an Iterator containing the subregistries to which the
     * provider belongs.
     */
    private Iterator getSubRegistries(Object provider) {
        List l = new ArrayList();
        Iterator iter = categoryMap.keySet().iterator();
        while (iter.hasNext()) {
            Class c = (Class)iter.next();
            if (c.isAssignableFrom(provider.getClass())) {
                l.add((SubRegistry)categoryMap.get(c));
            }
        }
        return l.iterator();
    }

    /**
     * Adds a service provider object to the registry.  The provider
     * is associated with the given category.
     *
     * <p> If <code>provider</code> implements the
     * <code>RegisterableService</code> interface, its
     * <code>onRegistration</code> method will be called.  Its
     * <code>onDeregistration</code> method will be called each time
     * it is deregistered from a category, for example if a
     * category is removed or the registry is garbage collected.
     *
     * @param provider the service provide object to be registered.
     * @param category the category under which to register the
     * provider.
     *
     * @return true if no provider of the same class was previously
     * registered in the same category category.
     *
     * @exception IllegalArgumentException if <code>provider</code> is
     * <code>null</code>.
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     * @exception ClassCastException if provider does not implement
     * the <code>Class</code> defined by <code>category</code>.
     */
    public <T> boolean registerServiceProvider(T provider,
                                               Class<T> category) {
        if (provider == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        if (!category.isAssignableFrom(provider.getClass())) {
            throw new ClassCastException();
        }

        return reg.registerServiceProvider(provider);
    }

    /**
     * Adds a service provider object to the registry.  The provider
     * is associated within each category present in the registry
     * whose <code>Class</code> it implements.
     *
     * <p> If <code>provider</code> implements the
     * <code>RegisterableService</code> interface, its
     * <code>onRegistration</code> method will be called once for each
     * category it is registered under.  Its
     * <code>onDeregistration</code> method will be called each time
     * it is deregistered from a category or when the registry is
     * finalized.
     *
     * @param provider the service provider object to be registered.
     *
     * @exception IllegalArgumentException if
     * <code>provider</code> is <code>null</code>.
     */
    public void registerServiceProvider(Object provider) {
        if (provider == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        Iterator regs = getSubRegistries(provider);
        while (regs.hasNext()) {
            SubRegistry reg = (SubRegistry)regs.next();
            reg.registerServiceProvider(provider);
        }
    }

    /**
     * Adds a set of service provider objects, taken from an
     * <code>Iterator</code> to the registry.  Each provider is
     * associated within each category present in the registry whose
     * <code>Class</code> it implements.
     *
     * <p> For each entry of <code>providers</code> that implements
     * the <code>RegisterableService</code> interface, its
     * <code>onRegistration</code> method will be called once for each
     * category it is registered under.  Its
     * <code>onDeregistration</code> method will be called each time
     * it is deregistered from a category or when the registry is
     * finalized.
     *
     * @param providers an Iterator containing service provider
     * objects to be registered.
     *
     * @exception IllegalArgumentException if <code>providers</code>
     * is <code>null</code> or contains a <code>null</code> entry.
     */
    public void registerServiceProviders(Iterator<?> providers) {
        if (providers == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        while (providers.hasNext()) {
            registerServiceProvider(providers.next());
        }
    }

    /**
     * Removes a service provider object from the given category.  If
     * the provider was not previously registered, nothing happens and
     * <code>false</code> is returned.  Otherwise, <code>true</code>
     * is returned.  If an object of the same class as
     * <code>provider</code> but not equal (using <code>==</code>) to
     * <code>provider</code> is registered, it will not be
     * deregistered.
     *
     * <p> If <code>provider</code> implements the
     * <code>RegisterableService</code> interface, its
     * <code>onDeregistration</code> method will be called.
     *
     * @param provider the service provider object to be deregistered.
     * @param category the category from which to deregister the
     * provider.
     *
     * @return <code>true</code> if the provider was previously
     * registered in the same category category,
     * <code>false</code> otherwise.
     *
     * @exception IllegalArgumentException if <code>provider</code> is
     * <code>null</code>.
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     * @exception ClassCastException if provider does not implement
     * the class defined by <code>category</code>.
     */
    public <T> boolean deregisterServiceProvider(T provider,
                                                 Class<T> category) {
        if (provider == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        if (!category.isAssignableFrom(provider.getClass())) {
            throw new ClassCastException();
        }
        return reg.deregisterServiceProvider(provider);
    }

    /**
     * Removes a service provider object from all categories that
     * contain it.
     *
     * @param provider the service provider object to be deregistered.
     *
     * @exception IllegalArgumentException if <code>provider</code> is
     * <code>null</code>.
     */
    public void deregisterServiceProvider(Object provider) {
        if (provider == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        Iterator regs = getSubRegistries(provider);
        while (regs.hasNext()) {
            SubRegistry reg = (SubRegistry)regs.next();
            reg.deregisterServiceProvider(provider);
        }
    }

    /**
     * Returns <code>true</code> if <code>provider</code> is currently
     * registered.
     *
     * @param provider the service provider object to be queried.
     *
     * @return <code>true</code> if the given provider has been
     * registered.
     *
     * @exception IllegalArgumentException if <code>provider</code> is
     * <code>null</code>.
     */
    public boolean contains(Object provider) {
        if (provider == null) {
            throw new IllegalArgumentException("provider == null!");
        }
        Iterator regs = getSubRegistries(provider);
        while (regs.hasNext()) {
            SubRegistry reg = (SubRegistry)regs.next();
            if (reg.contains(provider)) {
                return true;
            }
        }

        return false;
    }

    /**
     * Returns an <code>Iterator</code> containing all registered
     * service providers in the given category.  If
     * <code>useOrdering</code> is <code>false</code>, the iterator
     * will return all of the server provider objects in an arbitrary
     * order.  Otherwise, the ordering will respect any pairwise
     * orderings that have been set.  If the graph of pairwise
     * orderings contains cycles, any providers that belong to a cycle
     * will not be returned.
     *
     * @param category the category to be retrieved from.
     * @param useOrdering <code>true</code> if pairwise orderings
     * should be taken account in ordering the returned objects.
     *
     * @return an <code>Iterator</code> containing service provider
     * objects from the given category, possibly in order.
     *
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     */
    public <T> Iterator<T> getServiceProviders(Class<T> category,
                                               boolean useOrdering) {
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        return reg.getServiceProviders(useOrdering);
    }

    /**
     * A simple filter interface used by
     * <code>ServiceRegistry.getServiceProviders</code> to select
     * providers matching an arbitrary criterion.  Classes that
     * implement this interface should be defined in order to make use
     * of the <code>getServiceProviders</code> method of
     * <code>ServiceRegistry</code> that takes a <code>Filter</code>.
     *
     * @see ServiceRegistry#getServiceProviders(Class, ServiceRegistry.Filter, boolean)
     */
    public interface Filter {

        /**
         * Returns <code>true</code> if the given
         * <code>provider</code> object matches the criterion defined
         * by this <code>Filter</code>.
         *
         * @param provider a service provider <code>Object</code>.
         *
         * @return true if the provider matches the criterion.
         */
        boolean filter(Object provider);
    }

    /**
     * Returns an <code>Iterator</code> containing service provider
     * objects within a given category that satisfy a criterion
     * imposed by the supplied <code>ServiceRegistry.Filter</code>
     * object's <code>filter</code> method.
     *
     * <p> The <code>useOrdering</code> argument controls the
     * ordering of the results using the same rules as
     * <code>getServiceProviders(Class, boolean)</code>.
     *
     * @param category the category to be retrieved from.
     * @param filter an instance of <code>ServiceRegistry.Filter</code>
     * whose <code>filter</code> method will be invoked.
     * @param useOrdering <code>true</code> if pairwise orderings
     * should be taken account in ordering the returned objects.
     *
     * @return an <code>Iterator</code> containing service provider
     * objects from the given category, possibly in order.
     *
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     */
    public <T> Iterator<T> getServiceProviders(Class<T> category,
                                               Filter filter,
                                               boolean useOrdering) {
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        Iterator iter = getServiceProviders(category, useOrdering);
        return new FilterIterator(iter, filter);
    }

    /**
     * Returns the currently registered service provider object that
     * is of the given class type.  At most one object of a given
     * class is allowed to be registered at any given time.  If no
     * registered object has the desired class type, <code>null</code>
     * is returned.
     *
     * @param providerClass the <code>Class</code> of the desired
     * service provider object.
     *
     * @return a currently registered service provider object with the
     * desired <code>Class</code>type, or <code>null</code> is none is
     * present.
     *
     * @exception IllegalArgumentException if <code>providerClass</code> is
     * <code>null</code>.
     */
    public <T> T getServiceProviderByClass(Class<T> providerClass) {
        if (providerClass == null) {
            throw new IllegalArgumentException("providerClass == null!");
        }
        Iterator iter = categoryMap.keySet().iterator();
        while (iter.hasNext()) {
            Class c = (Class)iter.next();
            if (c.isAssignableFrom(providerClass)) {
                SubRegistry reg = (SubRegistry)categoryMap.get(c);
                T provider = reg.getServiceProviderByClass(providerClass);
                if (provider != null) {
                    return provider;
                }
            }
        }
        return null;
    }

    /**
     * Sets a pairwise ordering between two service provider objects
     * within a given category.  If one or both objects are not
     * currently registered within the given category, or if the
     * desired ordering is already set, nothing happens and
     * <code>false</code> is returned.  If the providers previously
     * were ordered in the reverse direction, that ordering is
     * removed.
     *
     * <p> The ordering will be used by the
     * <code>getServiceProviders</code> methods when their
     * <code>useOrdering</code> argument is <code>true</code>.
     *
     * @param category a <code>Class</code> object indicating the
     * category under which the preference is to be established.
     * @param firstProvider the preferred provider.
     * @param secondProvider the provider to which
     * <code>firstProvider</code> is preferred.
     *
     * @return <code>true</code> if a previously unset ordering
     * was established.
     *
     * @exception IllegalArgumentException if either provider is
     * <code>null</code> or they are the same object.
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     */
    public <T> boolean setOrdering(Class<T> category,
                                   T firstProvider,
                                   T secondProvider) {
        if (firstProvider == null || secondProvider == null) {
            throw new IllegalArgumentException("provider is null!");
        }
        if (firstProvider == secondProvider) {
            throw new IllegalArgumentException("providers are the same!");
        }
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        if (reg.contains(firstProvider) &&
            reg.contains(secondProvider)) {
            return reg.setOrdering(firstProvider, secondProvider);
        }
        return false;
    }

    /**
     * Sets a pairwise ordering between two service provider objects
     * within a given category.  If one or both objects are not
     * currently registered within the given category, or if no
     * ordering is currently set between them, nothing happens
     * and <code>false</code> is returned.
     *
     * <p> The ordering will be used by the
     * <code>getServiceProviders</code> methods when their
     * <code>useOrdering</code> argument is <code>true</code>.
     *
     * @param category a <code>Class</code> object indicating the
     * category under which the preference is to be disestablished.
     * @param firstProvider the formerly preferred provider.
     * @param secondProvider the provider to which
     * <code>firstProvider</code> was formerly preferred.
     *
     * @return <code>true</code> if a previously set ordering was
     * disestablished.
     *
     * @exception IllegalArgumentException if either provider is
     * <code>null</code> or they are the same object.
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     */
    public <T> boolean unsetOrdering(Class<T> category,
                                     T firstProvider,
                                     T secondProvider) {
        if (firstProvider == null || secondProvider == null) {
            throw new IllegalArgumentException("provider is null!");
        }
        if (firstProvider == secondProvider) {
            throw new IllegalArgumentException("providers are the same!");
        }
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        if (reg.contains(firstProvider) &&
            reg.contains(secondProvider)) {
            return reg.unsetOrdering(firstProvider, secondProvider);
        }
        return false;
    }

    /**
     * Deregisters all service provider object currently registered
     * under the given category.
     *
     * @param category the category to be emptied.
     *
     * @exception IllegalArgumentException if there is no category
     * corresponding to <code>category</code>.
     */
    public void deregisterAll(Class<?> category) {
        SubRegistry reg = (SubRegistry)categoryMap.get(category);
        if (reg == null) {
            throw new IllegalArgumentException("category unknown!");
        }
        reg.clear();
    }

    /**
     * Deregisters all currently registered service providers from all
     * categories.
     */
    public void deregisterAll() {
        Iterator iter = categoryMap.values().iterator();
        while (iter.hasNext()) {
            SubRegistry reg = (SubRegistry)iter.next();
            reg.clear();
        }
    }

    /**
     * Finalizes this object prior to garbage collection.  The
     * <code>deregisterAll</code> method is called to deregister all
     * currently registered service providers.  This method should not
     * be called from application code.
     *
     * @exception Throwable if an error occurs during superclass
     * finalization.
     */
    public void finalize() throws Throwable {
        deregisterAll();
        super.finalize();
    }
}


/**
 * A portion of a registry dealing with a single superclass or
 * interface.
 */
class SubRegistry {

    ServiceRegistry registry;

    Class category;

    // Provider Objects organized by partial oridering
    PartiallyOrderedSet poset = new PartiallyOrderedSet();

    // Class -> Provider Object of that class
    Map<Class<?>,Object> map = new HashMap();

    public SubRegistry(ServiceRegistry registry, Class category) {
        this.registry = registry;
        this.category = category;
    }

    public boolean registerServiceProvider(Object provider) {
        Object oprovider = map.get(provider.getClass());
        boolean present =  oprovider != null;

        if (present) {
            deregisterServiceProvider(oprovider);
        }
        map.put(provider.getClass(), provider);
        poset.add(provider);
        if (provider instanceof RegisterableService) {
            RegisterableService rs = (RegisterableService)provider;
            rs.onRegistration(registry, category);
        }

        return !present;
    }

    /**
     * If the provider was not previously registered, do nothing.
     *
     * @return true if the provider was previously registered.
     */
    public boolean deregisterServiceProvider(Object provider) {
        Object oprovider = map.get(provider.getClass());

        if (provider == oprovider) {
            map.remove(provider.getClass());
            poset.remove(provider);
            if (provider instanceof RegisterableService) {
                RegisterableService rs = (RegisterableService)provider;
                rs.onDeregistration(registry, category);
            }

            return true;
        }
        return false;
    }

    public boolean contains(Object provider) {
        Object oprovider = map.get(provider.getClass());
        return oprovider == provider;
    }

    public boolean setOrdering(Object firstProvider,
                               Object secondProvider) {
        return poset.setOrdering(firstProvider, secondProvider);
    }

    public boolean unsetOrdering(Object firstProvider,
                                 Object secondProvider) {
        return poset.unsetOrdering(firstProvider, secondProvider);
    }

    public Iterator getServiceProviders(boolean useOrdering) {
        if (useOrdering) {
            return poset.iterator();
        } else {
            return map.values().iterator();
        }
    }

    public <T> T getServiceProviderByClass(Class<T> providerClass) {
        return (T)map.get(providerClass);
    }

    public void clear() {
        Iterator iter = map.values().iterator();
        while (iter.hasNext()) {
            Object provider = iter.next();
            iter.remove();

            if (provider instanceof RegisterableService) {
                RegisterableService rs = (RegisterableService)provider;
                rs.onDeregistration(registry, category);
            }
        }
        poset.clear();
    }

    public void finalize() {
        clear();
    }
}


/**
 * A class for wrapping <code>Iterators</code> with a filter function.
 * This provides an iterator for a subset without duplication.
 */
class FilterIterator<T> implements Iterator<T> {

    private Iterator<T> iter;
    private ServiceRegistry.Filter filter;

    private T next = null;

    public FilterIterator(Iterator<T> iter,
                          ServiceRegistry.Filter filter) {
        this.iter = iter;
        this.filter = filter;
        advance();
    }

    private void advance() {
        while (iter.hasNext()) {
            T elt = iter.next();
            if (filter.filter(elt)) {
                next = elt;
                return;
            }
        }

        next = null;
    }

    public boolean hasNext() {
        return next != null;
    }

    public T next() {
        if (next == null) {
            throw new NoSuchElementException();
        }
        T o = next;
        advance();
        return o;
    }

    public void remove() {
        throw new UnsupportedOperationException();
    }
}