RequestContextHolder的作用是：

在Service层获取获取request和response信息

代码示例：

 ServletRequestAttributes attrs = (ServletRequestAttributes)RequestContextHolder.getRequestAttributes();
HttpServletRequest request = attrs.getRequest();

源码分析：

定义了两个ThreadLocal变量用来存储Request

 private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal("Request attributes");    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = new NamedInheritableThreadLocal("Request context");

设置方法

    public static void setRequestAttributes(@Nullable RequestAttributes attributes) {
        setRequestAttributes(attributes, false);
    }    public static void setRequestAttributes(@Nullable RequestAttributes attributes, boolean inheritable) {        if (attributes == null) {
            resetRequestAttributes();
        } else if (inheritable) {
            inheritableRequestAttributesHolder.set(attributes);
            requestAttributesHolder.remove();
        } else {
            requestAttributesHolder.set(attributes);
            inheritableRequestAttributesHolder.remove();
        }

    }

是在SpringMVC处理Servlet的类FrameworkServlet的类中，doget/dopost方法，调用processRequest方法进行初始化上下文方法中initContextHolders设置进去的

 private void initContextHolders(HttpServletRequest request, @Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {        if (localeContext != null) {
            LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
        }        if (requestAttributes != null) {
            RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
        }        if (this.logger.isTraceEnabled()) {            this.logger.trace("Bound request context to thread: " + request);
        }

    }

再看一下请求信息怎么获取

    @Nullable
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = (RequestAttributes)requestAttributesHolder.get();        if (attributes == null) {
            attributes = (RequestAttributes)inheritableRequestAttributesHolder.get();
        }        return attributes;
    }