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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.spring;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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

import org.directwebremoting.Container;
import org.directwebremoting.WebContextFactory.WebContextBuilder;
import org.directwebremoting.extend.Configurator;
import org.directwebremoting.impl.StartupUtil;
import org.directwebremoting.servlet.DwrServlet;
import org.directwebremoting.spring.namespace.ConfigurationParser;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

/**
 * The servlet that handles all calls to DWR. 

 * It retrieves its configuration from the Spring IoC container. This is done in two ways:
 * 
 *   Use the Spring namespace. When using the Spring namespace for DWR, the configuration for DWR is
 *       automatically picked up by this servlet.
 *   Explicitly specify which configurations to pick up. When explicitly defining the DWR configuration in
 *       Spring yourself, you can explicitly specify them in the init parameters.
 * 
 * Same as with the DwrServlet, you can specify a debug init parameter on this servlet
 * to put DWR in debug mode (allowing access to the very handy debug pages).
 *
 * @see org.directwebremoting.servlet.DwrServlet
 *
 * @author Bram Smeets
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DwrSpringServlet extends DwrServlet
{
    /* (non-Javadoc)
     * @see org.directwebremoting.servlet.DwrServlet#createContainer(javax.servlet.ServletConfig)
     */
    @Override
    protected SpringContainer createContainer(ServletConfig servletConfig)
    {
        ApplicationContext appContext = getApplicationContext(servletConfig.getServletContext());

        SpringContainer springContainer = new SpringContainer();
        springContainer.setBeanFactory(appContext);
        StartupUtil.setupDefaultContainer(springContainer, servletConfig);
        return springContainer;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.servlet.DwrServlet#configureContainer(org.directwebremoting.Container, javax.servlet.ServletConfig)
     */
    @Override
    protected void configureContainer(Container container, ServletConfig servletConfig) throws ServletException, IOException
    {
        // retrieve the configurators from Spring (loaded by the ContextLoaderListener)
        try
        {
            ApplicationContext appContext = getApplicationContext(servletConfig.getServletContext());
            configurators.add((Configurator) appContext.getBean(ConfigurationParser.DEFAULT_SPRING_CONFIGURATOR_ID));
        }
        catch (NoSuchBeanDefinitionException ex)
        {
            throw new ServletException("No DWR configuration was found in your application context, make sure to define one", ex);
        }

        try
        {
            if (includeDefaultConfig)
            {
                StartupUtil.configureFromSystemDwrXml(container);
            }

            StartupUtil.configureFromInitParams(container, servletConfig);
            StartupUtil.configure(container, configurators);
        }
        catch (IOException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            throw new ServletException(ex);
        }
    }

    /**
     * Setter for use by the Spring IoC container to tell us what Configurators
     * exist for us to configure ourselves.
     * @param configurators
     */
    public void setConfigurators(List configurators)
    {
        this.configurators = configurators;
    }

    /**
     * Do we prefix the list of Configurators with a default to read the system
     * dwr.xml file?
     * @param includeDefaultConfig the includeDefaultConfig to set
     */
    public void setIncludeDefaultConfig(boolean includeDefaultConfig)
    {
        this.includeDefaultConfig = includeDefaultConfig;
    }

    /**
     * Use provided application context rather than the default.
     * @param applicationContext
     */
    public void setApplicationContext(ApplicationContext applicationContext)
    {
        this.applicationContext = applicationContext;
    }

    /**
     * Allow easy override when retrieving the application context.
     * @param servletContext
     * @return the default application context.
     */
    protected ApplicationContext getApplicationContext(ServletContext servletContext)
    {
        return this.applicationContext == null
                ? WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext)
                : this.applicationContext;
    }

    /**
     * The application context that has the configuration. If null,
     * the default application context will be used.
     */
    private ApplicationContext applicationContext = null;

    /**
     * Do we prefix the list of Configurators with a default to read the system
     * dwr.xml file?
     */
    private boolean includeDefaultConfig = true;

    /**
     * What Configurators exist for us to configure ourselves.
     */
    private List configurators = new ArrayList();

    /**
     * The WebContext that keeps http objects local to a thread
     */
    protected WebContextBuilder webContextBuilder = null;
}