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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.impl;

import javax.servlet.ServletContext;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.Hub;
import org.directwebremoting.ServerContext;
import org.directwebremoting.WebContextFactory;
import org.directwebremoting.HubFactory.HubBuilder;

/**
 * A HubBuilder that creates DefaultHubs.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DefaultHubBuilder implements HubBuilder
{
    /* (non-Javadoc)
     * @see HubBuilder#set(javax.servlet.ServletContext)
     */
    public void set(ServletContext context)
    {
        try
        {
            Hub hub = new DefaultHub();
            context.setAttribute(ATTRIBUTE_SERVER_CONTEXT, hub);
        }
        catch (Exception ex)
        {
            log.fatal("Failed to create an ExecutionContext", ex);
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.HubBuilder#get()
     */
    public Hub get()
    {
        ServerContext serverContext;

        // Try to get the context from the thread
        serverContext = WebContextFactory.get();
        if (serverContext == null)
        {
            // If not see if there is a singleton
            serverContext = ContainerUtil.getSingletonServerContext();
            if (serverContext == null)
            {
                log.fatal("Error initializing Hub because singleton ServerContext == null.");
                log.fatal("This probably means that either DWR has not been properly initialized (in which case you should delay the current action until it has)");
                log.fatal("or that there is more than 1 DWR servlet is configured in this classloader, in which case you should provide a ServletContext to the Hub yourself.");
                throw new IllegalStateException("No singleton ServerContext see logs for possible causes and solutions.");
            }
        }

        ServletContext servletContext = serverContext.getServletContext();
        Hub reply = (Hub) servletContext.getAttribute(ATTRIBUTE_SERVER_CONTEXT);
        if (reply == null)
        {
            log.warn("HubFactory.get() returns null when DWR has not been initialized in the given ServletContext");
        }

        return reply;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.HubBuilder#get()
     */
    public Hub get(ServletContext servletContext)
    {
        if (servletContext == null)
        {
            throw new NullPointerException("servletContext");
        }

        Hub reply = (Hub) servletContext.getAttribute(ATTRIBUTE_SERVER_CONTEXT);
        if (reply == null)
        {
            log.warn("HubFactory.get(ServletContext) returns null when DWR has not been initialized in the given ServletContext");
        }

        return reply;
    }

    /**
     * The attribute under which we publish the Hub
     */
    private static final String ATTRIBUTE_SERVER_CONTEXT = "org.directwebremoting.impl.Hub";

    /**
     * The log stream
     */
    private static final Log log = LogFactory.getLog(DefaultHubBuilder.class);
}