WebContextBuilder是产品的制造机器，他的实现类可以自定义，不同的实现类，对应不同的产品制造方式。

WebContext 是产品

WebContextFactory对WebContextBuilder进行了封装，让购买者和产品的生产进行解耦

public class WebContextFactory
{
    public static WebContext get()
    {
        if (builder == null)
        {
            log.warn("Missing WebContextBuilder. Is DWR setup properly?");
            return null;
        }

        return builder.get();
    }


    private static WebContextBuilder builder = null;

    //此方法在容器启动时调用，所以WebContextBuilder的实现类可以自己定义，在启动时注入
    public static void attach(Container container)
    {
        WebContextFactory.builder = container.getBean(WebContextBuilder.class);
    }


    public interface WebContextBuilder
    {

        WebContext get();


        void engageThread(Container container,HttpServletRequest request,HttpServletResponse response);


        void disengageThread();
    }

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

DwrServlet 类webContextBuilder 的使用

doPost方法里使用构建器webContextBuilder负责本线程webContext对象的生产和销毁

在后面Handler方法里使用工厂WebContextFactory生产产品

public void init(ServletConfig servletConfig) throws ServletException
    {
        super.init(servletConfig);

        try
        {
            StartupUtil.logStartup(getClass().getSimpleName(),servletConfig);


            container = createContainer(servletConfig);
            //DwrServlet启动的时候赋予webContextBuilder的实现类对象
            webContextBuilder = container.getBean(WebContextBuilder.class);


            configureContainer(container,servletConfig);
        }
        catch (ServletException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            log.fatal("init Failed",ex);
            throw new ServletException(ex);
        }
        finally
        {
            if (webContextBuilder != null)
            {
                webContextBuilder.disengageThread();
            }
        }
    }
    @Override
    public void doPost(HttpServletRequest request,HttpServletResponse response) throws IOException,ServletException
    {
        try
        {
        //由webContextBuilder生产一个webContext对象，
        //此webContext对象在此线程的后面整个请求处理的过程中都能通过WebContextFactory.get()访问到
        //webContextBuilder内部使用ThreadLocal来保存webContext对象，所以只用本线程能访问
            webContextBuilder.engageThread(container,request,response);


            UrlProcessor processor = container.getBean(UrlProcessor.class);
            processor.handle(request,response);
        }
        finally
        {   //由webContextBuilder回收一个webContext产品
            webContextBuilder.disengageThread();
        }
    }