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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.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * We need to keep container specific logic out of BasePollHandler, and other
 * parts of DWR. Each container will need an implementation of this interface,
 * which will generally just return configurations of generic classes.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public interface ContainerAbstraction
{
    /**
     * We ask ContainerAbstractions to identify if the environment given is
     * 'theirs'. Are they fit for this environment?
     * @param servletConfig Access to {@link javax.servlet.ServletContext} etc.
     * @return true if this implementation should be used
     */
    boolean isNativeEnvironment(ServletConfig servletConfig);

    /**
     * The setup process allows the ContainerAbstraction to select a special
     * ServerLoadMonitor implementation
     * @return The ServerLoadMonitor implementation to go with this container
     */
    Class getServerLoadMonitorImplementation();

    /**
     * Some async-servlet implementations (Jetty) restart requests, when we
     * might have already completed dealing with them. If this method returns
     * true then PollHandler will assume that we're done and will bail out
     * before we even get started.
     * @param request The request that we might be finished with
     * @return true if the request is completed
     */
    boolean handleResumedRequest(HttpServletRequest request);

    /**
     * Create a sleeper that is appropriate for the given servlet container
     * @param request The Sleeper will probably need to know about the request
     * @param response The Sleeper will probably need to know about the response
     * @param scriptSession The Sleeper will probably need to know about the scriptSession
     * @param conduit The Sleeper will probably need to know about the conduit
     * @return A method of sending threads to sleep.
     */
    Sleeper createSleeper(HttpServletRequest request, HttpServletResponse response, RealScriptSession scriptSession, ScriptConduit conduit) throws IOException;
}