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/*
* ============LICENSE_START=======================================================
* utils
* ================================================================================
* Copyright (C) 2019-2020 AT&T Intellectual Property. All rights reserved.
* ================================================================================
* 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.
* ============LICENSE_END=========================================================
*/
package org.onap.policy.common.utils.services;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.ServiceLoader;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* This class is a template for building a sorted list of service instances,
* which are discovered and created using 'ServiceLoader'.
*/
public class OrderedServiceImpl<T extends OrderedService> {
// logger
private static Logger logger = LoggerFactory.getLogger(OrderedServiceImpl.class);
// sorted list of instances implementing the service
private List<T> implementers = null;
// 'ServiceLoader' that is used to discover and create the services
private ServiceLoader<T> serviceLoader = null;
// use this to ensure that we only use one unique instance of each class
private static Map<Class<?>, OrderedService> classToSingleton = new HashMap<>();
/**
* Constructor - create the 'ServiceLoader' instance.
*
* @param clazz the class object associated with 'T' (I supposed it could
* be a subclass, but I'm not sure this is useful)
*/
public OrderedServiceImpl(Class<T> clazz) {
// This constructor wouldn't be needed if 'T.class' was legal
serviceLoader = ServiceLoader.load(clazz);
}
/**
* Get List of implementers.
*
* @return the sorted list of services implementing interface 'T' discovered
* by 'ServiceLoader'.
*/
public synchronized List<T> getList() {
if (implementers == null) {
rebuildList();
}
return implementers;
}
/**
* This method is called by 'getList', but could also be called directly if
* we were running with a 'ClassLoader' that supported the dynamic addition
* of JAR files. In this case, it could be invoked in order to discover any
* new services implementing interface 'T'. This is probably a relatively
* expensive operation in terms of CPU and elapsed time, so it is best if it
* isn't invoked too frequently.
*
* @return the sorted list of services implementing interface 'T' discovered
* by 'ServiceLoader'.
*/
@SuppressWarnings("unchecked")
public synchronized List<T> rebuildList() {
// build a list of all of the current implementors
List<T> tmp = new LinkedList<>();
for (T service : serviceLoader) {
tmp.add((T) getSingleton(service));
}
// Sort the list according to sequence number, and then alphabetically
// according to full class name.
Collections.sort(tmp, (o1, o2) -> {
int s1 = o1.getSequenceNumber();
int s2 = o2.getSequenceNumber();
if (s1 < s2) {
return -1;
} else if (s1 > s2) {
return 1;
} else {
return o1.getClass().getName().compareTo(o2.getClass().getName());
}
});
// create an unmodifiable version of this list
implementers = Collections.unmodifiableList(tmp);
logger.info("***** OrderedServiceImpl implementers:\n {}", implementers);
return implementers;
}
/**
* If a service implements multiple APIs managed by 'ServiceLoader', a
* separate instance is created for each API. This method ensures that
* the first instance is used in all of the lists.
*
* @param service this is the object created by ServiceLoader
* @return the object to use in place of 'service'. If 'service' is the first
* object of this class created by ServiceLoader, it is returned. If not,
* the object of this class that was initially created is returned
* instead.
*/
private static synchronized OrderedService getSingleton(OrderedService service) {
// see if we already have an instance of this class
OrderedService rval = classToSingleton.get(service.getClass());
if (rval == null) {
// No previous instance of this class exists -- use the supplied
// instance, and place it in the table.
rval = service;
classToSingleton.put(service.getClass(), service);
}
return rval;
}
}
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