framework/src/main/java/org/osgi/framework/hooks/service/ListenerHook.java

/*
 * Copyright (c) OSGi Alliance (2008, 2018). 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.
 */

package org.osgi.framework.hooks.service;

import java.util.Collection;
import org.osgi.annotation.versioning.ConsumerType;
import org.osgi.annotation.versioning.ProviderType;
import org.osgi.framework.BundleContext;

/**
 * OSGi Framework Service Listener Hook Service.
 * 
 * <p>
 * Bundles registering this service will be called during service listener
 * addition and removal.
 * 
 * @ThreadSafe
 * @author $Id: b17e35a5cab84256d78ed9b749f9829d690a752d $
 */
@ConsumerType
public interface ListenerHook {
	/**
	 * Added listeners hook method. This method is called to provide the hook
	 * implementation with information on newly added service listeners. This
	 * method will be called as service listeners are added while this hook is
	 * registered. Also, immediately after registration of this hook, this
	 * method will be called to provide the current collection of service
	 * listeners which had been added prior to the hook being registered.
	 * 
	 * @param listeners A collection of {@link ListenerInfo}s for newly added
	 *        service listeners which are now listening to service events.
	 *        Attempting to add to or remove from the collection will result in
	 *        an {@code UnsupportedOperationException}. The collection is not
	 *        synchronized.
	 */
	void added(Collection<ListenerInfo> listeners);

	/**
	 * Removed listeners hook method. This method is called to provide the hook
	 * implementation with information on newly removed service listeners. This
	 * method will be called as service listeners are removed while this hook is
	 * registered.
	 * 
	 * @param listeners A collection of {@link ListenerInfo}s for newly removed
	 *        service listeners which are no longer listening to service events.
	 *        Attempting to add to or remove from the collection will result in
	 *        an {@code UnsupportedOperationException}. The collection is not
	 *        synchronized.
	 */
	void removed(Collection<ListenerInfo> listeners);

	/**
	 * Information about a Service Listener. This interface describes the bundle
	 * which added the Service Listener and the filter with which it was added.
	 * 
	 * @ThreadSafe
	 */
	@ProviderType
	public interface ListenerInfo {
		/**
		 * Return the context of the bundle which added the listener.
		 * 
		 * @return The context of the bundle which added the listener.
		 */
		BundleContext getBundleContext();

		/**
		 * Return the filter string with which the listener was added.
		 * 
		 * @return The filter string with which the listener was added. This may
		 *         be {@code null} if the listener was added without a filter.
		 */
		String getFilter();

		/**
		 * Return the state of the listener for this addition and removal life
		 * cycle. Initially this method will return {@code false} indicating the
		 * listener has been added but has not been removed. After the listener
		 * has been removed, this method must always return {@code true}.
		 * 
		 * <p>
		 * There is an extremely rare case in which removed notification to
		 * {@link ListenerHook}s can be made before added notification if two
		 * threads are racing to add and remove the same service listener.
		 * Because {@link ListenerHook}s are called synchronously during service
		 * listener addition and removal, the Framework cannot guarantee
		 * in-order delivery of added and removed notification for a given
		 * service listener. This method can be used to detect this rare
		 * occurrence.
		 * 
		 * @return {@code false} if the listener has not been removed,
		 *         {@code true} otherwise.
		 */
		boolean isRemoved();

		/**
		 * Compares this {@code ListenerInfo} to another {@code ListenerInfo}.
		 * Two {@code ListenerInfo}s are equals if they refer to the same
		 * listener for a given addition and removal life cycle. If the same
		 * listener is added again, it must have a different
		 * {@code ListenerInfo} which is not equal to this {@code ListenerInfo}.
		 * 
		 * @param obj The object to compare against this {@code ListenerInfo}.
		 * @return {@code true} if the other object is a {@code ListenerInfo}
		 *         object and both objects refer to the same listener for a
		 *         given addition and removal life cycle.
		 */
		@Override
		boolean equals(Object obj);

		/**
		 * Returns the hash code for this {@code ListenerInfo}.
		 * 
		 * @return The hash code of this {@code ListenerInfo}.
		 */
		@Override
		int hashCode();
	}
}