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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.

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/*
 * Copyright 2005 Joe Walker
 *
 * 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.directwebremoting;

import org.directwebremoting.event.MessageListener;

/**
 * A server-side hub that can publish data across reverse ajax and sync with
 * a number of other hubs, including: JMS, the OpenAjax Alliance hub, and
 * potentially others.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public interface Hub
{
    /**
     * Called to create a subscription so that future publishes to a similarly
     * named topic alert the MessageListener that a publish has happened.
     * Warning
     * Currently the topic is a simple string pattern match. This more closely
     * resembles JMS rather than the OpenAjax hub because we're taking the
     * simplistic approach for now. I'm sure there will be some nasty problems
     * that fall out of this.
     * The OpenAjax hub allows subscriber data that is passed back to the
     * subscriber when the event happens. Since listeners are an ideal place for
     * this data it isn't supported here. The OAA hub also allows for filters,
     * but I'm thinking that this can be easily supported by the listener. What
     * am I missing?
     * @param topicName The topic to subscribe to.
     * @param listener The object to notify of matching calls to publish()
     */
    void subscribe(String topicName, MessageListener listener);

    /**
     * Reverse the action of {@link #subscribe(String, MessageListener)}
     * @param topicName The topic to subscribe to.
     * @param listener The object to notify of matching calls to publish()
     */
    boolean unsubscribe(String topicName, MessageListener listener);

    /**
     * Publish some data to a certain topic.
     * @param topicName The topic to subscribe to.
     * @param message The data to publish
     */
    void publish(String topicName, Object message);
}