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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 boilerplate code between the web browser and your Java code. This version 4.0.2 works with Jakarta Servlet 4.0.2.

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package org.directwebremoting.impl;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.Container;
import org.directwebremoting.ServerContext;
import org.directwebremoting.WebContextFactory;
import org.directwebremoting.ServerContextFactory.ServerContextBuilder;

/**
 * A ServerContextBuilder that creates DefaultServerContexts.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DefaultServerContextBuilder implements ServerContextBuilder
{
    /* (non-Javadoc)
     * @see org.directwebremoting.extend.Builder#get()
     */
    public ServerContext get()
    {
        ServerContext serverContext = WebContextFactory.get();
        if (serverContext == null)
        {
            // If not see if there is a singleton
            serverContext = StartupUtil.getSingletonServerContext();
            if (serverContext == null)
            {
                log.fatal("Error initializing ServerContext because this is not a DWR thread and there is more than one DWR servlet in the current classloader.");
                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 get() yourself.");
                throw new IllegalStateException("No singleton ServerContext see logs for possible causes and solutions.");
            }
        }

        return serverContext;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.Builder#get(org.directwebremoting.ServerContext)
     */
    public ServerContext get(ServerContext context)
    {
        return context;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.Builder#attach(org.directwebremoting.ServerContext)
     */
    public ServerContext attach(Container container)
    {
        try
        {
            return container.newInstance(DefaultServerContext.class);
        }
        catch (RuntimeException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            throw new RuntimeException(ex);
        }
    }

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