1. 通过 RequestContextHolder 对象

package com.order.utils;

import org.springframework.web.context.request.RequestAttributes;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * @author guoyiguang
 * @description houqu  Request duixiang  he  Response  duixiang
 * @date 2021/12/24$
 */
public class WebUtil {

    /** 获取request对象 **/
    public static HttpServletRequest getRequest(){
        RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        if (requestAttributes == null){
            return null;
        }
        return ((ServletRequestAttributes)requestAttributes).getRequest();
    }
    /** 获取response对象 **/
    public static HttpServletResponse getResponse(){
        RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        if (requestAttributes == null){
            return null;
        }
        return ((ServletRequestAttributes)requestAttributes).getResponse();
    }
}

理解：

	对 RequestContextHolder  的理解：
	
	RequestContextHolder  里有 ThreadLocal 对象，可以存放 线程的私有属性，如下：
	
	
	  private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal<>("Request attributes");
			

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

原理：

	Servlet 监听器（RequestContextListener implements ServletRequestListener ） 监听请求，
	HttpServletRequest  这个对象创建完之后，会把他作为 ServletRequestAttributes 构造器的参数 ，对 ServletRequestAttributes  进行实例化
	这样通过 ServletRequestAttributes 和  HttpServletRequest 对象就就行了绑定
	
	如下：（所在类： ServletRequestAttributes） 
	public ServletRequestAttributes(HttpServletRequest request) {
		Assert.notNull(request, "Request must not be null");
		this.request = request;
	}
	
	
	然后：作为线程的私有属性放到 ThreadLocal 里 
	
	inheritableRequestAttributesHolder.set(ServletRequestAttributes);
	


    最后使用：
	   
	
	    // 获取请求 对象
	    RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        if (requestAttributes == null){
            return null;
        }
        return ((ServletRequestAttributes)requestAttributes).getRequest();
		
		// 获取响应 对象
	    RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        if (requestAttributes == null){
            return null;
        }
        return ((ServletRequestAttributes)requestAttributes).getResponse();

相关的监听器：

RequestContextListener  implements ServletRequestListener extends EventListener

Thread里 有 ThreadLocalMap 类，每个Thread都会实例化一个 ThreadLocalMap 实例

set 方法如下：

// key 就是 经常用的  ThreadLocal 实例 ，value就是要设置的值 
private void set(ThreadLocal<?> key, Object value);