源码跟踪-springmvc(二)：LocaleContextHolder和RequestContextHolder

FrameworkServlet.processRequest（删减版）

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // (1)
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        // (2)
        initContextHolders(request, localeContext, requestAttributes);

            doService(request, response);

            // (3)
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }
    }

两个ContextHolder做的事情是一样的：设置和重置

1. 获取以前的值
2. 设置新值
   new SimpleLocaleContext(request.getLocale())和new ServletRequestAttributes(request, response)
3. 设置回原来的值

这里我们产生了一些问题

1. previousLocaleContext和previousAttributes从哪里来的？
2. 为什么把ContextHolder中的对象换成新的，doService后又要重置ContextHolder？
3. requestAttributes.requestCompleted()做了些什么？
4. 为什么ContextHolder要用ThreadLocal？为什么有NamedThreadLocal和NamedInheritableThreadLocal，它们有啥区别？
   //todo