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Diffstat (limited to 'policy-utils/src/main/java/org/onap/policy/drools/utils/OrderedServiceImpl.java')
-rw-r--r-- | policy-utils/src/main/java/org/onap/policy/drools/utils/OrderedServiceImpl.java | 156 |
1 files changed, 156 insertions, 0 deletions
diff --git a/policy-utils/src/main/java/org/onap/policy/drools/utils/OrderedServiceImpl.java b/policy-utils/src/main/java/org/onap/policy/drools/utils/OrderedServiceImpl.java new file mode 100644 index 00000000..88a53313 --- /dev/null +++ b/policy-utils/src/main/java/org/onap/policy/drools/utils/OrderedServiceImpl.java @@ -0,0 +1,156 @@ +/*- + * ============LICENSE_START======================================================= + * policy-utils + * ================================================================================ + * Copyright (C) 2017 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.drools.utils; + +import java.util.Collections; +import java.util.Comparator; +import java.util.HashMap; +import java.util.LinkedList; +import java.util.List; +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; //ServiceLoader.load(T.class); + + /** + * 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); + } + + /** + * @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<T>(); + 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, new Comparator<T>() + { + public int compare(T o1, T o2) + { + int s1 = o1.getSequenceNumber(); + int s2 = o2.getSequenceNumber(); + int rval; + if (s1 < s2) + { + rval = -1; + } + else if (s1 > s2) + { + rval = 1; + } + else + { + rval = o1.getClass().getName().compareTo + (o2.getClass().getName()); + } + return(rval); + } + }); + + // create an unmodifiable version of this list + implementers = Collections.unmodifiableList(tmp); + logger.info("***** OrderedServiceImpl implementers:\n {}", implementers); + return(implementers); + } + + // use this to ensure that we only use one unique instance of each class + @SuppressWarnings("rawtypes") + static private HashMap<Class,OrderedService> classToSingleton = + new HashMap<>(); + + /** + * 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. + */ + static private 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); + } +} |