| /* |
| * 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.phoenix.util; |
| |
| import org.apache.commons.collections.IteratorUtils; |
| |
| import java.util.Iterator; |
| import java.util.List; |
| import java.util.ServiceLoader; |
| import java.util.concurrent.ConcurrentHashMap; |
| |
| /** |
| * Resolves object instances registered using the JDK 6+ {@link java.util.ServiceLoader}. |
| * |
| * |
| * @since 2.0 |
| */ |
| public class InstanceResolver { |
| private static final ConcurrentHashMap<Class, Object> RESOLVED_SINGLETONS = new ConcurrentHashMap<Class, Object>(); |
| |
| private InstanceResolver() {/* not allowed */} |
| |
| /** |
| * Resolves an instance of the specified class if it has not already been resolved. |
| * @param clazz The type of instance to resolve |
| * @param defaultInstance The instance to use if a custom instance has not been registered |
| * @return The resolved instance or the default instance provided. |
| * {@code null} if an instance is not registered and a default is not provided. |
| */ |
| @SuppressWarnings("unchecked") |
| public static <T> T getSingleton(Class<T> clazz, T defaultInstance) { |
| Object obj = RESOLVED_SINGLETONS.get(clazz); |
| if(obj != null) { |
| return (T)obj; |
| } |
| if (defaultInstance != null && !clazz.isInstance(defaultInstance)) throw new IllegalArgumentException("defaultInstance is not of type " + clazz.getName()); |
| final Object o = resolveSingleton(clazz, defaultInstance); |
| obj = RESOLVED_SINGLETONS.putIfAbsent(clazz, o); |
| if(obj == null) { |
| obj = o; |
| } |
| return (T)obj; |
| } |
| |
| /** |
| * Resolves all instances of a specified class and add it to the list of default implementations |
| * @param clazz Type of the instance to resolve |
| * @param defaultInstances {@link List} of instances that match the type clazz |
| * @param <T> Type of class passed |
| * @return {@link List} of instance of the specified class. Newly found instances will be added |
| * to the existing contents of defaultInstances |
| */ |
| @SuppressWarnings("unchecked") |
| public static <T> List get(Class<T> clazz, List<T> defaultInstances) { |
| Iterator<T> iterator = ServiceLoader.load(clazz).iterator(); |
| if (defaultInstances != null) { |
| defaultInstances.addAll(IteratorUtils.toList(iterator)); |
| } else { |
| defaultInstances = IteratorUtils.toList(iterator); |
| } |
| |
| return defaultInstances; |
| } |
| |
| private synchronized static <T> T resolveSingleton(Class<T> clazz, T defaultInstance) { |
| ServiceLoader<T> loader = ServiceLoader.load(clazz); |
| // returns the first registered instance found |
| for (T singleton : loader) { |
| return singleton; |
| } |
| return defaultInstance; |
| } |
| |
| //@VisibleForTesting |
| public static void clearSingletons() { |
| RESOLVED_SINGLETONS.clear(); |
| } |
| } |