RequestContextHolder中为何能获取HttpServletRequest

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我是靠谱客的博主火星上眼神，最近开发中收集的这篇文章主要介绍RequestContextHolder中为何能获取HttpServletRequest，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

HttpServletRequest request = ((ServletRequestAttributes) (RequestContextHolder.currentRequestAttributes())).getRequest();

可以通过RequestContextHolder随时随地获取HttpServletRequest 和 HttpServletResponse，那么参数是什么时候进行初始化的呢，又是如何进行初始化？

在RequestContextHolder初始化过程中有个非常关键的类——RequestContextFilter，通过底下RequestContextFilter源码可以看出，RequestContextFilter类继承了OncePerRequestFilter类,所以每次接收到请求处理之前，都会调用拦截器的initContextHolders方法。在initContextHolders方法中调用RequestContextHolder类中的setRequestAttributes方法

public class RequestContextFilter extends OncePerRequestFilter {
    private boolean threadContextInheritable = false;

    public RequestContextFilter() {
    }
    private void initContextHolders(HttpServletRequest request, ServletRequestAttributes requestAttributes) {
        LocaleContextHolder.setLocale(request.getLocale(), this.threadContextInheritable);
        RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Bound request context to thread: " + request);
        }

    }
    private void resetContextHolders() {
        LocaleContextHolder.resetLocaleContext();
        RequestContextHolder.resetRequestAttributes();
    }
}

RequestContextHolder中定义了俩个ThreadLoal<RequestAttributes>线程变量，setRequestAttributes方法中会给inheritableRequestAttributesHolder变量或者requestAttributesHolder 变量赋值，所以代码中可以随时获取HttpServletRequest值

public abstract class RequestContextHolder {
    private static final boolean jsfPresent = ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());
    private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal("Request attributes");
    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = new NamedInheritableThreadLocal("Request context");

    public RequestContextHolder() {
    }

public static void resetRequestAttributes() {
        requestAttributesHolder.remove();
        inheritableRequestAttributesHolder.remove();
    }

    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();
        }

    }