java/broker/src/main/java/org/apache/qpid/server/plugin/QpidServiceLoader.java

/*
 * 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.qpid.server.plugin;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ServiceLoader;

import org.apache.log4j.Logger;

/**
 * Simple facade over a {@link ServiceLoader} to instantiate all configured implementations of an interface.
 */
public class QpidServiceLoader<C>
{
    private static final Logger _logger = Logger.getLogger(QpidServiceLoader.class);

    public Iterable<C> instancesOf(Class<C> clazz)
    {
        return instancesOf(clazz, false);
    }

    /**
     * @throws RuntimeException if at least one implementation is not found.
     */
    public Iterable<C> atLeastOneInstanceOf(Class<C> clazz)
    {
        return instancesOf(clazz, true);
    }

    private Iterable<C> instancesOf(Class<C> clazz, boolean atLeastOne)
    {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        Iterator<C> serviceLoaderIterator = ServiceLoader.load(clazz, classLoader).iterator();

        // create a new list so we can log the count
        List<C> serviceImplementations = new ArrayList<C>();
        while(serviceLoaderIterator.hasNext())
        {
            serviceImplementations.add(serviceLoaderIterator.next());
        }

        if(atLeastOne && serviceImplementations.isEmpty())
        {
            throw new RuntimeException("At least one implementation of " + clazz + " expected");
        }

        if(_logger.isDebugEnabled())
        {
            _logger.debug("Found " + serviceImplementations.size() + " implementations of " + clazz);
        }

        return serviceImplementations;
    }
}