/*
 * Copyright (c) OSGi Alliance (2000, 2017). 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;

import java.util.Dictionary;

import org.osgi.annotation.versioning.ProviderType;

/**
 * A reference to a service.
 * 
 * <p>
 * The Framework returns {@code ServiceReference} objects from the
 * {@code BundleContext.getServiceReference} and
 * {@code BundleContext.getServiceReferences} methods.
 * <p>
 * A {@code ServiceReference} object may be shared between bundles and can be
 * used to examine the properties of the service and to get the service object.
 * <p>
 * Every service registered in the Framework has a unique
 * {@code ServiceRegistration} object and may have multiple, distinct
 * {@code ServiceReference} objects referring to it. {@code ServiceReference}
 * objects associated with a {@code ServiceRegistration} object have the same
 * {@code hashCode} and are considered equal (more specifically, their
 * {@code equals()} method will return {@code true} when compared).
 * <p>
 * If the same service object is registered multiple times,
 * {@code ServiceReference} objects associated with different
 * {@code ServiceRegistration} objects are not equal.
 * 
 * @param <S> Type of Service.
 * @see BundleContext#getServiceReference(Class)
 * @see BundleContext#getServiceReference(String)
 * @see BundleContext#getServiceReferences(Class, String)
 * @see BundleContext#getServiceReferences(String, String)
 * @see BundleContext#getService(ServiceReference)
 * @see BundleContext#getServiceObjects(ServiceReference)
 * @ThreadSafe
 * @author $Id: 1454244c30992b7a52ac3838b03bc584c3495816 $
 */
@ProviderType
public interface ServiceReference<S> extends Comparable<Object> {
	/**
	 * Returns the property value to which the specified property key is mapped
	 * in the properties {@code Dictionary} object of the service referenced by
	 * this {@code ServiceReference} object.
	 * 
	 * <p>
	 * Property keys are case-insensitive.
	 * 
	 * <p>
	 * This method must continue to return property values after the service has
	 * been unregistered. This is so references to unregistered services (for
	 * example, {@code ServiceReference} objects stored in the log) can still be
	 * interrogated.
	 * 
	 * @param key The property key.
	 * @return The property value to which the key is mapped; {@code null} if
	 *         there is no property named after the key.
	 */
	public Object getProperty(String key);

	/**
	 * Returns an array of the keys in the properties {@code Dictionary} object
	 * of the service referenced by this {@code ServiceReference} object.
	 * 
	 * <p>
	 * This method will continue to return the keys after the service has been
	 * unregistered. This is so references to unregistered services (for
	 * example, {@code ServiceReference} objects stored in the log) can still be
	 * interrogated.
	 * 
	 * <p>
	 * This method is <i>case-preserving </i>; this means that every key in the
	 * returned array must have the same case as the corresponding key in the
	 * properties {@code Dictionary} that was passed to the
	 * {@link BundleContext#registerService(String[],Object,Dictionary)} or
	 * {@link ServiceRegistration#setProperties(Dictionary)} methods.
	 * 
	 * @return An array of property keys.
	 */
	public String[] getPropertyKeys();

	/**
	 * Returns the bundle that registered the service referenced by this
	 * {@code ServiceReference} object.
	 * 
	 * <p>
	 * This method must return {@code null} when the service has been
	 * unregistered. This can be used to determine if the service has been
	 * unregistered.
	 * 
	 * @return The bundle that registered the service referenced by this
	 *         {@code ServiceReference} object; {@code null} if that service has
	 *         already been unregistered.
	 * @see BundleContext#registerService(String[],Object,Dictionary)
	 */
	public Bundle getBundle();

	/**
	 * Returns the bundles that are using the service referenced by this
	 * {@code ServiceReference} object. Specifically, this method returns the
	 * bundles whose usage count for that service is greater than zero.
	 * 
	 * @return An array of bundles whose usage count for the service referenced
	 *         by this {@code ServiceReference} object is greater than zero;
	 *         {@code null} if no bundles are currently using that service.
	 * 
	 * @since 1.1
	 */
	public Bundle[] getUsingBundles();

	/**
	 * Tests if the bundle that registered the service referenced by this
	 * {@code ServiceReference} and the specified bundle use the same source for
	 * the package of the specified class name.
	 * <p>
	 * This method performs the following checks:
	 * <ol>
	 * <li>Get the package name from the specified class name.</li>
	 * <li>For the bundle that registered the service referenced by this
	 * {@code ServiceReference} (registrant bundle); find the source for the
	 * package. If no source is found then return {@code true} if the registrant
	 * bundle is equal to the specified bundle; otherwise return {@code false}.</li>
	 * <li>If the package source of the registrant bundle is equal to the
	 * package source of the specified bundle then return {@code true};
	 * otherwise return {@code false}.</li>
	 * </ol>
	 * 
	 * @param bundle The {@code Bundle} object to check.
	 * @param className The class name to check.
	 * @return {@code true} if the bundle which registered the service
	 *         referenced by this {@code ServiceReference} and the specified
	 *         bundle use the same source for the package of the specified class
	 *         name. Otherwise {@code false} is returned.
	 * @throws IllegalArgumentException If the specified {@code Bundle} was not
	 *         created by the same framework instance as this
	 *         {@code ServiceReference}.
	 * @since 1.3
	 */
	public boolean isAssignableTo(Bundle bundle, String className);

	/**
	 * Compares this {@code ServiceReference} with the specified
	 * {@code ServiceReference} for order.
	 * 
	 * <p>
	 * If this {@code ServiceReference} and the specified
	 * {@code ServiceReference} have the same {@link Constants#SERVICE_ID
	 * service id} they are equal. This {@code ServiceReference} is less than
	 * the specified {@code ServiceReference} if it has a lower
	 * {@link Constants#SERVICE_RANKING service ranking} and greater if it has a
	 * higher service ranking. Otherwise, if this {@code ServiceReference} and
	 * the specified {@code ServiceReference} have the same
	 * {@link Constants#SERVICE_RANKING service ranking}, this
	 * {@code ServiceReference} is less than the specified
	 * {@code ServiceReference} if it has a higher {@link Constants#SERVICE_ID
	 * service id} and greater if it has a lower service id.
	 * 
	 * @param reference The {@code ServiceReference} to be compared.
	 * @return Returns a negative integer, zero, or a positive integer if this
	 *         {@code ServiceReference} is less than, equal to, or greater than
	 *         the specified {@code ServiceReference}.
	 * @throws IllegalArgumentException If the specified
	 *         {@code ServiceReference} was not created by the same framework
	 *         instance as this {@code ServiceReference}.
	 * @since 1.4
	 */
	@Override
	public int compareTo(Object reference);

	/**
	 * Returns a copy of the properties of the service referenced by this
	 * {@code ServiceReference} object.
	 * <p>
	 * This method will continue to return the properties after the service has
	 * been unregistered. This is so references to unregistered services (for
	 * example, {@code ServiceReference} objects stored in the log) can still be
	 * interrogated.
	 * <p>
	 * The returned {@code Dictionary} object:
	 * <ul>
	 * <li>Must map property values by using property keys in a
	 * <i>case-insensitive manner</i>.</li>
	 * <li>Must return property keys is a <i>case-preserving</i> manner. This
	 * means that the keys must have the same case as the corresponding key in
	 * the properties {@code Dictionary} that was passed to the
	 * {@link BundleContext#registerService(String[],Object,Dictionary)} or
	 * {@link ServiceRegistration#setProperties(Dictionary)} methods.</li>
	 * <li>Is the property of the caller and can be modified by the caller but
	 * any changes are not reflected in the properties of the service.
	 * {@link ServiceRegistration#setProperties(Dictionary)} must be called to
	 * modify the properties of the service.</li>
	 * </ul>
	 * 
	 * @return A copy of the properties of the service referenced by this
	 *         {@code ServiceReference} object
	 * @since 1.9
	 */
	public Dictionary<String,Object> getProperties();
}