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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.extend;

import java.util.Collection;

import javax.servlet.http.HttpSession;

import org.directwebremoting.ScriptSession;
import org.directwebremoting.event.ScriptSessionListener;

/**
 * A ScriptSessionManager looks after a number of sessions (keyed using a
 * Javascript variable)
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public interface ScriptSessionManager
{
    /**
     * Get a list of all the currently known ScriptSessions by id.
     * Note that the list of known sessions is continually changing so it is
     * possible that the list will be out of date by the time it is used. For
     * this reason you should check that getScriptSession(String id) returns
     * something non null.
     * @return An iterator over the currently known sessions, by id
     */
    Collection getAllScriptSessions();

    /**
     * Lookup all the windows associated with a given browser
     * @param httpSessionId The browser id to lookup
     * @return A list of script sessions for each open window
     */
    Collection getScriptSessionsByHttpSessionId(String httpSessionId);

    /**
     * Lookup the ScriptSession corresponding to a ScriptSession id.
     * @param scriptSessionId The id to lookup
     * @return The corresponding ScriptSession object, or null.
     */
    ScriptSession getScriptSessionById(String scriptSessionId);

    /**
     * For a given script session id, return the existing ScriptSession object
     * or create a new if needed.
     * @param id The id to get a ScriptSession object for
     * @param url The URL including 'http://', up to (but not including) '?' or '#' (or null if not known)
     * @param httpSession The session to associate the ScriptSession with (if any)
     * @return A ScriptSession to match the ID.
     */
    RealScriptSession getOrCreateScriptSession(String id, String url, HttpSession httpSession);

    /**
     * Accessor for the time (in milliseconds) when unused ScriptSessions will expire
     * @return the scriptSessionTimeout
     */
    long getScriptSessionTimeout();

    /**
     * Maintain the list of {@link ScriptSessionListener}s
     * @param li the ScriptSessionListener to add
     */
    void addScriptSessionListener(ScriptSessionListener li);

    /**
     * Maintain the list of {@link ScriptSessionListener}s
     * @param li the ScriptSessionListener to remove
     */
    void removeScriptSessionListener(ScriptSessionListener li);

    /**
     * Some implementations of ScriptSessionManager need to add custom code into
     * engine.js to register ScriptSessions with the server.
     */
    String getInitCode();

    /**
     * The default length of time a session can go unused before it
     * automatically becomes invalid and is recycled.
     * The default is 5mins
     */
    static final long DEFAULT_TIMEOUT_MILLIS = 5L * 60L * 1000L;
}