/*
 * Copyright (c) OSGi Alliance (2000, 2010). 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.service.log;

import java.util.Enumeration;

/**
 * Provides methods to retrieve {@code LogEntry} objects from the log.
 * <p>
 * There are two ways to retrieve {@code LogEntry} objects:
 * <ul>
 * <li>The primary way to retrieve {@code LogEntry} objects is to register a
 * {@code LogListener} object whose {@code LogListener.logged} method will
 * be called for each entry added to the log.
 * <li>To retrieve past {@code LogEntry} objects, the {@code getLog}
 * method can be called which will return an {@code Enumeration} of all
 * {@code LogEntry} objects in the log.
 * 
 * @ThreadSafe
 * @version $Id: bb22587248982a76202b77d03550515b3205f935 $
 * @see LogEntry
 * @see LogListener
 * @see LogListener#logged(LogEntry)
 */
public interface LogReaderService {
	/**
	 * Subscribes to {@code LogEntry} objects.
	 * 
	 * <p>
	 * This method registers a {@code LogListener} object with the Log Reader
	 * Service. The {@code LogListener.logged(LogEntry)} method will be
	 * called for each {@code LogEntry} object placed into the log.
	 * 
	 * <p>
	 * When a bundle which registers a {@code LogListener} object is stopped
	 * or otherwise releases the Log Reader Service, the Log Reader Service must
	 * remove all of the bundle's listeners.
	 * 
	 * <p>
	 * If this Log Reader Service's list of listeners already contains a
	 * listener {@code l} such that {@code (l==listener)}, this method
	 * does nothing.
	 * 
	 * @param listener A {@code LogListener} object to register; the
	 *        {@code LogListener} object is used to receive {@code LogEntry}
	 *        objects.
	 * @see LogListener
	 * @see LogEntry
	 * @see LogListener#logged(LogEntry)
	 */
	public void addLogListener(LogListener listener);

	/**
	 * Unsubscribes to {@code LogEntry} objects.
	 * 
	 * <p>
	 * This method unregisters a {@code LogListener} object from the Log
	 * Reader Service.
	 * 
	 * <p>
	 * If {@code listener} is not contained in this Log Reader Service's list
	 * of listeners, this method does nothing.
	 * 
	 * @param listener A {@code LogListener} object to unregister.
	 * @see LogListener
	 */
	public void removeLogListener(LogListener listener);

	/**
	 * Returns an {@code Enumeration} of all {@code LogEntry} objects in
	 * the log.
	 * 
	 * <p>
	 * Each element of the enumeration is a {@code LogEntry} object, ordered
	 * with the most recent entry first. Whether the enumeration is of all
	 * {@code LogEntry} objects since the Log Service was started or some
	 * recent past is implementation-specific. Also implementation-specific is
	 * whether informational and debug {@code LogEntry} objects are included
	 * in the enumeration.
	 * @return An {@code Enumeration} of all {@code LogEntry} objects in
	 * the log.
	 */
	public Enumeration getLog();
}