写在前面

javax.servlet.ServletRequest和javax.servlet.ServletResponse都是Servlet容器中定义的接口，分别用于获取客户端请求信息和将响应消息发送给客户端。
有两种方法在Contoller方法中获取它们：

1. 直接在Controller方法参数中声明
2. 通过工具类org.springframework.web.context.request.RequestContextHolder获取

实际的对象实例类跟具体的Servlet容器有关：

• 如果Servlet容器为Tomcat，javax.servlet.ServletRequest实现类为org.apache.catalina.connector.RequestFacade，javax.servlet.ServletResponse实现类为org.apache.catalina.connector.ResponseFacade
• 如果Servlet容器为Undertow，javax.servlet.ServletRequest实现类为io.undertow.servlet.spec.HttpServletRequestImpl，javax.servlet.ServletResponse实现类为io.undertow.servlet.spec.HttpServletResponseImpl

在Controller方法参数中声明

可以直接在Controller方法参数中声明javax.servlet.ServletRequest和javax.servlet.ServletResponse对象，如下示例：

@GetMapping("/hello")
public String request(HttpServletRequest req, HttpServletResponse resp) {
    System.out.println("req: " + req);
    System.out.println("resp: " + resp);
    return "Success";
}

之所以能直接在Controller方法参数中声明javax.servlet.ServletRequest和javax.servlet.ServletResponse对象，实际上是在org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues()方法中进行了解析处理，如下：

// org.springframework.web.method.support.InvocableHandlerMethod
private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    MethodParameter[] parameters = getMethodParameters();
    Object[] args = new Object[parameters.length];
    for (int i = 0; i < parameters.length; i++) {
        MethodParameter parameter = parameters[i];
        parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
        args[i] = resolveProvidedArgument(parameter, providedArgs);
        if (args[i] != null) {
            continue;
        }
        if (this.argumentResolvers.supportsParameter(parameter)) { // 判断是否为支持解析的参数类型
            try {
                args[i] = this.argumentResolvers.resolveArgument( // 解析参数对象
                        parameter, mavContainer, request, this.dataBinderFactory);
                continue;
            }
            catch (Exception ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                }
                throw ex;
            }
        }
        if (args[i] == null) {
            throw new IllegalStateException("Could not resolve method parameter at index " +
                    parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
                    ": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
        }
    }
    return args;
}

最终通过org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver.resolveArgument()方法解析获取到具体对象。

通过RequestContextHolder工具类获取

在使用Spring Boot框架时，除了可以直接在Controller方法参数中声明javax.servlet.ServletRequest和javax.servlet.ServletResponse对象，还可以通过工具类org.springframework.web.context.request.RequestContextHolder获取，如下示例：

@GetMapping("/hello")
public String hello() {
    RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();
    if (requestAttributes != null && requestAttributes instanceof ServletRequestAttributes) {
        ServletRequestAttributes httpRequestAttributes = (ServletRequestAttributes) requestAttributes;
        HttpServletRequest req = httpRequestAttributes.getRequest();
        HttpServletResponse resp = httpRequestAttributes.getResponse();
        System.out.println("req: " + req);
        System.out.println("resp: " + resp);
    }
    return "Success";
}

之所以可以通过RequestContextHolder工具类获取javax.servlet.ServletRequest和javax.servlet.ServletResponse对象，是因为在org.springframework.web.servlet.FrameworkServlet.processRequest()方法中进行了注入。

// org.springframework.web.servlet.FrameworkServlet

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    // 省略其他代码...

    // 在工具类RequestContextHolder中注入ServletRequest和ServletResponse对象
    initContextHolders(request, localeContext, requestAttributes);

    // 省略其他代码...
}

【参考】
Spring——Web作用域：RequestContextListener
springboot 整合 ServletRequestListener监听器