前言

spring拦截器能帮我们实现验证是否登陆、验签校验请求是否合法、预先设置数据等功能，那么该如何设置拦截器以及它的原理如何呢，下面将进行简单的介绍

1.设置

public interface HandlerInterceptor {

	/**
	 * Intercept the execution of a handler. Called after HandlerMapping determined
	 * an appropriate handler object, but before HandlerAdapter invokes the handler.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can decide to abort the execution chain,
	 * typically sending a HTTP error or writing a custom response.
	 * <p><strong>Note:</strong> special considerations apply for asynchronous
	 * request processing. For more details see
	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
	 * <p>The default implementation returns {@code true}.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance evaluation
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 * @throws Exception in case of errors
	 */
	default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {

		return true;
	}

	/**
	 * Intercept the execution of a handler. Called after HandlerAdapter actually
	 * invoked the handler, but before the DispatcherServlet renders the view.
	 * Can expose additional model objects to the view via the given ModelAndView.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can post-process an execution,
	 * getting applied in inverse order of the execution chain.
	 * <p><strong>Note:</strong> special considerations apply for asynchronous
	 * request processing. For more details see
	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
	 * <p>The default implementation is empty.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started asynchronous
	 * execution, for type and/or instance examination
	 * @param modelAndView the {@code ModelAndView} that the handler returned
	 * (can also be {@code null})
	 * @throws Exception in case of errors
	 */
	default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
			@Nullable ModelAndView modelAndView) throws Exception {
	}

	/**
	 * Callback after completion of request processing, that is, after rendering
	 * the view. Will be called on any outcome of handler execution, thus allows
	 * for proper resource cleanup.
	 * <p>Note: Will only be called if this interceptor's {@code preHandle}
	 * method has successfully completed and returned {@code true}!
	 * <p>As with the {@code postHandle} method, the method will be invoked on each
	 * interceptor in the chain in reverse order, so the first interceptor will be
	 * the last to be invoked.
	 * <p><strong>Note:</strong> special considerations apply for asynchronous
	 * request processing. For more details see
	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
	 * <p>The default implementation is empty.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started asynchronous
	 * execution, for type and/or instance examination
	 * @param ex exception thrown on handler execution, if any
	 * @throws Exception in case of errors
	 */
	default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler,
			@Nullable Exception ex) throws Exception {
	}

}

自定义拦截器需要实现HandlerInteceptor接口，该接口有三个方法：

preHandle：主要在映射适配器执行handler之前调用，若返回为true则继续往下执行handler，若返回为false则直接返回不继续处理请求

postHandle：主要在适配器执行handler之后调用 

afterCompletion：在postHandle后调用可清理一些数据，若preHandle返回false那么会调用完此方法后再返回

@Component
@Slf4j(topic = "e")
public class CustomInterceptor implements HandlerInterceptor {

  @Override
  public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
      throws Exception {
    log.info("-------------拦截请求:" + request.getRequestURI() + "-------------");
    // 可以根据request设置请求头、或从请求头提取信息等等...
    return true;
  }

  @Override
  public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
      @Nullable ModelAndView modelAndView) throws Exception {
    log.info("postHandle ....");
  }

  @Override
  public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler,
      @Nullable Exception ex) throws Exception {
    log.info("afterCompletion ....");
  }
}

接着创建配置类，实现WebMvcConfigurer接口，重写addInterceptors方法将自定义拦截器添加，并且加上@EnableWebMvc注解 （springboot项目会自动配置）

@Configuration
@EnableWebMvc
public class MyMvcConfigurer implements WebMvcConfigurer {

  @Resource
  private CustomInterceptor customInterceptor;

  @Override
  public void addInterceptors(InterceptorRegistry registry) {
    registry.addInterceptor(customInterceptor)
        .addPathPatterns("/**");
  }
}

配置完之后启动项目访问某个url路径，从控制台可以看到拦截器确实生效了

2.原理

首先是@EnableWebMvc注解，spring会解析并导入DelegatingWebMvcConfiguration这个bean，继承关系如下，主要逻辑都写在父类WebMvcConfigurationSupport中

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}

WebMvcConfigurationSupport中会创建一个映射处理器RequestMappingHandlerMapping

@Bean
public RequestMappingHandlerMapping requestMappingHandlerMapping() {
	RequestMappingHandlerMapping mapping = createRequestMappingHandlerMapping();
	mapping.setOrder(0);
	// 设置拦截器到mapping
	mapping.setInterceptors(getInterceptors());
	// 设置内容协商管理器
	mapping.setContentNegotiationManager(mvcContentNegotiationManager());
	// 跨域配置
	mapping.setCorsConfigurations(getCorsConfigurations());

	// 路径匹配设置
	PathMatchConfigurer configurer = getPathMatchConfigurer();

	Boolean useSuffixPatternMatch = configurer.isUseSuffixPatternMatch();
	if (useSuffixPatternMatch != null) {
		mapping.setUseSuffixPatternMatch(useSuffixPatternMatch);
	}
	Boolean useRegisteredSuffixPatternMatch = configurer.isUseRegisteredSuffixPatternMatch();
	if (useRegisteredSuffixPatternMatch != null) {
		mapping.setUseRegisteredSuffixPatternMatch(useRegisteredSuffixPatternMatch);
	}
	Boolean useTrailingSlashMatch = configurer.isUseTrailingSlashMatch();
	if (useTrailingSlashMatch != null) {
		mapping.setUseTrailingSlashMatch(useTrailingSlashMatch);
	}

	UrlPathHelper pathHelper = configurer.getUrlPathHelper();
	if (pathHelper != null) {
		mapping.setUrlPathHelper(pathHelper);
	}
	PathMatcher pathMatcher = configurer.getPathMatcher();
	if (pathMatcher != null) {
		mapping.setPathMatcher(pathMatcher);
	}

	return mapping;
}


#获取拦截器
protected final Object[] getInterceptors() {
	if (this.interceptors == null) {
		InterceptorRegistry registry = new InterceptorRegistry();
		// 调用DelegatingWebMvcConfiguration.addInterceptors 添加自定义的拦截器
		addInterceptors(registry);
		registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService()));
		registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider()));
		// 获取拦截器并根据order排序，若有匹配路径则封装成MappedInterceptor
		this.interceptors = registry.getInterceptors();
	}
	return this.interceptors.toArray();
}

注意这一行代码mapping.setInterceptors(getInterceptors());  getInterceptors方法会调用子类DelegatingWebMvcConfiguration的addInterceptors方法，接着会调用委托类即我们自定义配置类MyMvcConfigurer类的addInterceptors方法，将自定义的拦截器添加到拦截器注册类中，而后通过拦截器注册类获取到拦截器列表，最后将拦截器添加到映射处理器handlerMapping中，供后续使用。

最后看下请求处理的DispatcherServlet#doDispatch方法 （为了看的更清楚一点删掉了一些代码）

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
	HttpServletRequest processedRequest = request;
	// 处理程序执行链
	HandlerExecutionChain mappedHandler = null;

	try {
		ModelAndView mv = null;
		Exception dispatchException = null;

		try {
			// Determine handler for the current request.
			// 遍历handlerMapping获取能处理request的处理器，mappedHandler里封装着之前我们定义的拦截器供后续调用
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// Determine handler adapter for the current request.
			// 确定处理当前请求的处理适配器 RequestMappingHandlerAdapter
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			// 执行handler之前应用拦截器执行拦截器的后置方法 返回为false表示请求不合理直接返回了
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

			// Actually invoke the handler.
			// 真正执行这个HandlerMethod
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
            
			applyDefaultViewName(processedRequest, mv);
			// 执行拦截器的后置方法
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		catch (Throwable err) {
			// As of 4.3, we're processing Errors thrown from handler methods as well,
			// making them available for @ExceptionHandler methods and other scenarios.
			dispatchException = new NestedServletException("Handler dispatch failed", err);
		}
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) {
		triggerAfterCompletion(processedRequest, response, mappedHandler,
				new NestedServletException("Handler processing failed", err));
	}
	finally {

	}
}


#mappedHandler.applyPreHandle
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
	HandlerInterceptor[] interceptors = getInterceptors();
	if (!ObjectUtils.isEmpty(interceptors)) {
		for (int i = 0; i < interceptors.length; i++) {
			HandlerInterceptor interceptor = interceptors[i];
			// 前置处理为false时
			if (!interceptor.preHandle(request, response, this.handler)) {
				// 触发拦截器的afterCompletion方法
				triggerAfterCompletion(request, response, null);
				return false;
			}
			this.interceptorIndex = i;
		}
	}
	return true;
}

可以看到再真正执行handler之前会调用mappedHandler.applyPreHandle 方法，遍历拦截器执行preHandle方法，若返回false则根据先前执行过的拦截器顺序倒序执行afterCompletion方法，都通过的话后续执行handler获取请求结果，再接着执行拦截器的postHandle方法最后执行afterCompletion方法。