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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.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;

import org.directwebremoting.Container;
import org.directwebremoting.ServerContextFactory;
import org.directwebremoting.WebContextFactory;
import org.directwebremoting.ServerContextFactory.ServerContextBuilder;
import org.directwebremoting.WebContextFactory.WebContextBuilder;
import org.directwebremoting.util.FakeServletConfig;
import org.directwebremoting.util.FakeServletContext;
import org.directwebremoting.util.Logger;
import org.directwebremoting.util.ServletLoggingOutput;
import org.directwebremoting.util.VersionUtil;

/**
 * Some utilities to help get DWR up and running
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class StartupUtil
{
    /**
     * A way to setup DWR outside of any Containers.
     * This method can also serve as a template for in container code wanting
     * to get DWR setup. Callers of this method should clean up after themselves
     * by calling {@link #outOfContainerDestroy(Container)}
     * @return A new initialized container.
     * @throws ServletException If the setup fails.
     */
    public Container outOfContainerInit() throws ServletException
    {
        try
        {
            ServletConfig servletConfig = new FakeServletConfig("test", new FakeServletContext());
            ServletContext servletContext = servletConfig.getServletContext();

            StartupUtil.setupLogging(servletConfig, null);
            StartupUtil.logStartup(servletConfig);

            DefaultContainer container = ContainerUtil.createDefaultContainer(servletConfig);
            ContainerUtil.setupDefaultContainer(container, servletConfig);

            WebContextBuilder webContextBuilder = StartupUtil.initWebContext(servletConfig, servletContext, container);
            StartupUtil.initServerContext(servletConfig, servletContext, container);

            ContainerUtil.prepareForWebContextFilter(servletContext, servletConfig, container, webContextBuilder, null);
            ContainerUtil.configureContainerFully(container, servletConfig);
            ContainerUtil.publishContainer(container, servletConfig);

            return container;
        }
        catch (ServletException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            throw new ServletException(ex);
        }
    }

    /**
     * Clean up the current thread when {@link #outOfContainerInit()} has been
     * called.
     * @param container The container created by {@link #outOfContainerInit()}.
     */
    public void outOfContainerDestroy(Container container)
    {
        ServletLoggingOutput.unsetExecutionContext();

        WebContextBuilder webContextBuilder = (WebContextBuilder) container.getBean(WebContextBuilder.class.getName());
        if (webContextBuilder != null)
        {
            webContextBuilder.unset();
        }
    }

    /**
     * Some logging so we have a good clue what we are working with.
     * @param config The servlet config
     */
    public static void logStartup(ServletConfig config)
    {
        log.info("DWR Version " + VersionUtil.getVersion() + " starting.");
        log.info("- Servlet Engine: " + config.getServletContext().getServerInfo());
        log.info("- Java Version:   " + System.getProperty("java.version"));
        log.info("- Java Vendor:    " + System.getProperty("java.vendor"));
    }

    /**
     * Get the {@link WebContextFactory.WebContextBuilder} out of the
     * {@link Container}, configure it (call WebContextBuilder#set()) and use it
     * to configure the {@link WebContextFactory}.
     * @param servletConfig The servlet configuration
     * @param servletContext The servlet context
     * @param servlet The servlet that we are running under
     * @param container The container to save in the ServletContext
     * @return a new WebContextBuilder
     */
    public static WebContextBuilder initWebContext(ServletConfig servletConfig, ServletContext servletContext, Container container)
    {
        WebContextBuilder webContextBuilder = (WebContextBuilder) container.getBean(WebContextBuilder.class.getName());
        WebContextFactory.setWebContextBuilder(webContextBuilder);
        webContextBuilder.set(null, null, servletConfig, servletContext, container);

        return webContextBuilder;
    }

    /**
     * Get the {@link ServerContextFactory.ServerContextBuilder} out of the
     * {@link Container}, configure it and use it to configure the
     * {@link ServerContextFactory}
     * @param servletConfig The servlet configuration
     * @param servletContext The servlet context
     * @param container The container to save in the ServletContext
     * @return The newly created ServerContextBuilder
     */
    public static ServerContextBuilder initServerContext(ServletConfig servletConfig, ServletContext servletContext, Container container)
    {
        ServerContextBuilder serverContextBuilder = (ServerContextBuilder) container.getBean(ServerContextBuilder.class.getName());
        ServerContextFactory.setServerContextBuilder(serverContextBuilder);
        serverContextBuilder.set(servletConfig, servletContext, container);

        return serverContextBuilder;
    }

    /**
     * We have some special logging classes to maintain an optional dependence
     * on commons-logging. This sets the servlet for when this is not available.
     * @param servletConfig The servlet configuration
     * @param servlet The servlet that we are running under
     */
    public static void setupLogging(ServletConfig servletConfig, HttpServlet servlet)
    {
        ServletLoggingOutput.setExecutionContext(servlet);
        String logLevel = servletConfig.getInitParameter(ContainerUtil.INIT_LOGLEVEL);
        if (logLevel != null)
        {
            ServletLoggingOutput.setLevel(logLevel);
        }
    }

    /**
     * The log stream
     */
    private static final Logger log = Logger.getLogger(StartupUtil.class);
}