【记录版】SpringBoot框架中Request请求获取方式及源码解读

SpringBoot + HttpServletRequest

背景： 我们现在有很多WEB项目基于SpringBoot开发，其中接口管理是必不可少的，接口中Request对象是核心内容，我们对参数、属性、header等等元数据的获取和配置都离不开它，常用的Filter、拦截器等操作类也都有它的身影。本篇在此基础上，从源码角度讨论获取Request的三种方式：

方式一、接口方法或全局异常处理方法参数注入
通常在开发简单POST接口时，请求参数或Header等信息都可以用*@RequestBody、@RequestParam、@PathVariable或@RequestHeader*注解直接取值，部分场景下当接口方法参数名和请求体参数名值和类型一致时，也可以赋值，如下所示：

当然最原始的也可用Request实例对象，即在此接口上配置HttpServletRequest参数获取，这是最简单也是最直接的方式。（PS: 接口方法也可设置HttpSession参数控制session逻辑）
源码一、方法参数解析入口类

package org.springframework.web.method.support;
public class InvocableHandlerMethod extends HandlerMethod {
    @Nullable
    public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    	// 解析方法所有参数，并将其传给最终的反射方法对象执行invoke操作
        Object[] args = this.getMethodArgumentValues(request, mavContainer, providedArgs);
        return this.doInvoke(args);
    }
}

源码二、方法参数解析类（选一：RequestResponseBodyMethodProcessor）

// 方法参数解析接口
public interface HandlerMethodArgumentResolver {
    boolean supportsParameter(MethodParameter parameter);

    @Nullable
    Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;
}

package org.springframework.web.servlet.mvc.method.annotation;
public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {
	......省略......
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        parameter = parameter.nestedIfOptional();
        // 通过转换器将Request中请求流数据转换为对应的实体
        Object arg = this.readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
        String name = Conventions.getVariableNameForParameter(parameter);
        if (binderFactory != null) {
            WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
            if (arg != null) {
            	// 参数校验
                this.validateIfApplicable(binder, parameter);
                if (binder.getBindingResult().hasErrors() && this.isBindExceptionRequired(binder, parameter)) {
                    throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
                }
            }

            if (mavContainer != null) {
                mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
            }
        }

        return this.adaptArgumentIfNecessary(arg, parameter);
    }
	
	// 参数校验
	protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) {
		// 获取参数所有注解，下面将判断是否包含校验类注解
        Annotation[] annotations = parameter.getParameterAnnotations();
        Annotation[] var4 = annotations;
        int var5 = annotations.length;

        for(int var6 = 0; var6 < var5; ++var6) {
            Annotation ann = var4[var6];
            // 此处获取@Valid等校验注解，如果有，binder执行校验并获取校验结果
            Object[] validationHints = ValidationAnnotationUtils.determineValidationHints(ann);
            if (validationHints != null) {
                binder.validate(validationHints);
                break;
            }
        }

    }
......省略......
}

HandlerMethodArgumentResolver默认有31种，覆盖绝大部分类型参数的解析，使我们从Request的API的操作中解放出来，从而关注业务逻辑的开发和管理，以下仅列出核心解析器：
1、RequestResponseBodyMethodProcessor // 支持@RequestBody注解参数解析
2、ServletRequestMethodArgumentResolver //支持ServletRequest、MultipartRequest及HttpSession等参数解析
3、RequestHeaderMethodArgumentResolver // 支持Header参数解析

方法二：方法体内通过RequestContextHolder全局持久化类获取

HttpServletRequest holderRequest = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();

在SpringBoot项目中，有两处地方进行了上下文保存动作：
1、OrderedRequestContextFilter extends RequestContextFilter implements OrderedFilter 设置

public class RequestContextFilter extends OncePerRequestFilter {
	// 是否线程间传递，适用于后端存在异步线程池场景，需手工开启
    private boolean threadContextInheritable = false;
    
    public void setThreadContextInheritable(boolean threadContextInheritable) {
        this.threadContextInheritable = threadContextInheritable;
    }
    
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
    	// 请求与响应封装体
        ServletRequestAttributes attributes = new ServletRequestAttributes(request, response);
        // 内存保存
        this.initContextHolders(request, attributes);

        try {
            filterChain.doFilter(request, response);
        } finally {
            // 清除
            LocaleContextHolder.resetLocaleContext();
        	RequestContextHolder.resetRequestAttributes();
            attributes.requestCompleted();
        }
    }
	
	// 请求&响应上下文具体保存逻辑-核心为ThreadLocal
    private void initContextHolders(HttpServletRequest request, ServletRequestAttributes requestAttributes) {
        LocaleContextHolder.setLocale(request.getLocale(), this.threadContextInheritable);
        RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
    }
}

OrderedRequestContextFilter是通过SpringBoot自动配置，只要是Web项目都不用手工设置，自动配置源码：

@AutoConfiguration(
    after = {DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class, ValidationAutoConfiguration.class}
)
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@ConditionalOnClass({Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class})
@ConditionalOnMissingBean({WebMvcConfigurationSupport.class})
@AutoConfigureOrder(-2147483638)
public class WebMvcAutoConfiguration {
    @Configuration(
        proxyBeanMethods = false
    )
    @Import({EnableWebMvcConfiguration.class})
    @EnableConfigurationProperties({WebMvcProperties.class, WebProperties.class})
    @Order(0)
    public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer, ServletContextAware {
        @Bean
        @ConditionalOnMissingBean({RequestContextListener.class, RequestContextFilter.class})
        @ConditionalOnMissingFilterBean({RequestContextFilter.class})
        public static RequestContextFilter requestContextFilter() {
            return new OrderedRequestContextFilter();
        }
    }
}

二、DispatcherServlet执行时设置

public class DispatcherServlet extends FrameworkServlet {
    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);
        }
    }
}

三、定义HttpServletRequest类实例变量，通过Spring容器注入，如@Autowired注解修饰

这种方式比较少见，但既然能够使用，说明此时我们看到的request对象，不再是前面方法里获取的Request对象，而是披了一层羊皮的代理对象，这样每个接口获取时才能找到自己的上下文，那它是怎么实现的呢？

熟悉Spring框架的话，就会知道容器在实例化外部控制器实例时，会在BeanPostProcessor等处优先处理需要注入的实例，当需要注入实例到Request变量时，BeanFactory会自动处理：

// Bean实例化时会执行
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
 // beanName为外部类名称，requiredType为HttpServletRequest.class
 protected Map<String, Object> findAutowireCandidates(@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
        String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this, requiredType, true, descriptor.isEager());
        Map<String, Object> result = CollectionUtils.newLinkedHashMap(candidateNames.length);
        // BeanFactory所有缓存的resolvableDependencies，具体见如上截图中8个依赖
        Iterator var6 = this.resolvableDependencies.entrySet().iterator();

        while(var6.hasNext()) {
            Map.Entry<Class<?>, Object> classObjectEntry = (Map.Entry)var6.next();
            Class<?> autowiringType = (Class)classObjectEntry.getKey();
            if (autowiringType.isAssignableFrom(requiredType)) {
                Object autowiringValue = classObjectEntry.getValue();
                // 此处根据变量类型，获取对应的ObjectFactory工厂，再生成对应的Proxy
                autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
                if (requiredType.isInstance(autowiringValue)) {
                    result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
                    break;
                }
            }
        }
        ..........		
	}
}

abstract class AutowireUtils {
	public static Object resolveAutowiringValue(Object autowiringValue, Class<?> requiredType) {
    	if (autowiringValue instanceof ObjectFactory && !requiredType.isInstance(autowiringValue)) {
        	ObjectFactory<?> factory = (ObjectFactory)autowiringValue;
        	if (!(autowiringValue instanceof Serializable) || !requiredType.isInterface()) {
        	    return factory.getObject();
        	}
			
			// 此处使用JDK动态代理
        	autowiringValue = Proxy.newProxyInstance(requiredType.getClassLoader(), new Class[]{requiredType}, new ObjectFactoryDelegatingInvocationHandler(factory));
    		}

   	 		return autowiringValue;
		}
	}
	// 私有静态内部类封装InvocationHandler具体的反射调用内容，配合JDK动态代理生成代理类
	private static class ObjectFactoryDelegatingInvocationHandler implements InvocationHandler, Serializable {
        private final ObjectFactory<?> objectFactory;

        ObjectFactoryDelegatingInvocationHandler(ObjectFactory<?> objectFactory) {
            this.objectFactory = objectFactory;
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            switch (method.getName()) {
                case "equals":
                    return proxy == args[0];
                case "hashCode":
                    return System.identityHashCode(proxy);
                case "toString":
                    return this.objectFactory.toString();
                default:
                    try {
                        return method.invoke(this.objectFactory.getObject(), args);
                    } catch (InvocationTargetException var6) {
                        throw var6.getTargetException();
                    }
            }
        }
    }
}

ObjectFactoryDelegatingInvocationHandler是处理封装类，具体逻辑为ObjectFactory实现类提供

public abstract class WebApplicationContextUtils {
    public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, @Nullable ServletContext sc) {
        beanFactory.registerScope("request", new RequestScope());
        beanFactory.registerScope("session", new SessionScope());
        if (sc != null) {
            ServletContextScope appScope = new ServletContextScope(sc);
            beanFactory.registerScope("application", appScope);
            sc.setAttribute(ServletContextScope.class.getName(), appScope);
        }
		// 此处注册Request执行的工厂类
        beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
        beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
        beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
        beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
        if (jsfPresent) {
            WebApplicationContextUtils.FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
        }
    }
}

Request代理类真实执行逻辑：

public abstract class WebApplicationContextUtils {
    private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {
        // ObjectFactory核心就是getObject方法，封装实例创建的过程
        public ServletRequest getObject() {
        	// 实际获取的还是RequestContextHolder缓存的Request对象
            return WebApplicationContextUtils.currentRequestAttributes().getRequest();
        }
    }
}

总结：
1、一般需求场景，直接在接口方法上标注HttpServletRequest对象即可
2、如果需要在任意场景使用HttpServletRequest对象，方法二和方法三都可，方法三使用更加简洁
3、方法二和方法三底层都依赖ThreadLocal，如果后台使用异步线程池，需要手工设置inheritable属性为true
4、从方法一中，我们可以学到接口方法可以定义多个我们需要的上下文参数，如HttpSession；从方法三中我们可以学习到全局使用Request、Response、Session等8个对象。
5、方法三虽然涉及Bean生命周期、JDK动态代理等底层知识，但实际内容还是围绕RequestContextHolder做的封装，使用时更加方便，全局场景推荐使用此方式。