介绍
HandlerInterceptor 中文名拦截器，其实 Interceptor 也是拦截器的意思，这里加个 Handler 示意这只是一个接口，具体的实现须要自定义。
- 机会：客户端拜访服务端的 controller
- 具体作用点：拜访前，拜访后
- 作用形式：对 request 或 response 进行解决，加日志，包装返回值，增加申请头属性。
应用形式
以通用返回值为例。
1. 自定义拦截器
实现 HandlerInterceptor 并注册为 Bean
// 通用返回值拦截器
@Component
public class CommonResultInterceptor implements HandlerInterceptor {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {if (handler instanceof HandlerMethod) {final HandlerMethod handlerMethod = (HandlerMethod) handler;
final Class<?> clazz = handlerMethod.getBeanType();
final Method method = handlerMethod.getMethod();
if (clazz.isAnnotationPresent(CommonResult.class)) {request.setAttribute(RESPONSE_RESULT, clazz.getAnnotation(CommonResult.class));
} else if (method.isAnnotationPresent(CommonResult.class)) {request.setAttribute(RESPONSE_RESULT, method.getAnnotation(CommonResult.class));
}
}
return true;
}
}2. 增加拦截器
@Configuration
public class WebAppConfig implements WebMvcConfigurer {
@Override
public void addInterceptors(InterceptorRegistry registry) {CommonResultInterceptor interceptor = new CommonResultInterceptor();
registry.addInterceptor(interceptor);
WebMvcConfigurer.super.addInterceptors(registry);
}
}3. 解决返回值
解决的形式有很多种，例如再加拦截器等等，这里我应用 springboot 的 ResponseBodyAdvice 接口
@ControllerAdvice
public class CommonResultHandler implements ResponseBodyAdvice<Object> {
/**
* 判断是否要执行 beforeBodyWrite 办法
*
* @param methodParameter
* @param aClass
* @return true 为执行，false 不执行，有注解标记的时候解决返回值
*/
@Override
public boolean supports(MethodParameter methodParameter, Class<? extends HttpMessageConverter<?>> aClass) {ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
HttpServletRequest request = attributes.getRequest();
CommonResult commonResult = (CommonResult) request.getAttribute(CRM_RESPONSE_RESULT);
return Objects.nonNull(commonResult);
}
/**
* 对返回值做包装解决
*
* @return ResultBody
*/
@Override
public Object beforeBodyWrite(Object body, MethodParameter methodParameter, MediaType mediaType,
Class<? extends HttpMessageConverter<?>> aClass,
ServerHttpRequest request,
ServerHttpResponse response) {if (body instanceof ResultBody) {return body;}
// 当返回类型是 String 时，用的是 StringHttpMessageConverter 转换器，无奈转换为 Json 格局
// 必须在办法体上标注 RequestMapping(produces = "application/json; charset=UTF-8")
if (body instanceof String) {String str = JSON.toJSONString(ResultBody.ok(body));
return str;
}
return ResultBody.ok(body);
}
}作用原理
1. 申请首先会达到 org.springframework.web.servlet.DispatcherServlet 下的 doDispatch 办法
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}
}
// 调用已注册拦截器的 prehandle 办法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}
// 真正激活 handler，也就是对应 controller 的办法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {return;}
applyDefaultViewName(processedRequest, mv);
// 调用已注册拦截器的 posthandle 办法
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 {if (asyncManager.isConcurrentHandlingStarted()) {
// 调用 posthandle 和 aftercompletion 办法
if (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {cleanupMultipart(processedRequest);
}
}
}
}2. 接着，咱们来看看 preHandle 办法的实现。
/**
* Apply preHandle methods of registered interceptors.
* @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.
*/
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];
// 执行 prehandle
if (!interceptor.preHandle(request, response, this.handler)) {triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
}
return true;
}3. 逻辑很简略，就是获取拦截器数组，而后调用 preHandle 办法，那么接下来看看 getInterceptors 办法
/**
* Return the array of interceptors to apply (in the given order).
*/
@Nullable
public HandlerInterceptor[] getInterceptors() {if (this.interceptors == null && this.interceptorList != null) {this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[0]);
}
return this.interceptors;
}4. 这里是间接调用 this.interceptors，那么要害就在于HandlerExecutionChain 的初始化怎么实现的
doDispatch办法中，有以下片段
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {noHandlerFound(processedRequest, response);
return;
}这里 getHandler 正式获取了这个 request 对应的 handler
/**
* Return the HandlerExecutionChain for this request.
* <p>Tries all handler mappings in order.
* @param request current HTTP request
* @return the HandlerExecutionChain, or {@code null} if no handler could be found
*/
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) {for (HandlerMapping mapping : this.handlerMappings) {HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {return handler;}
}
}
return null;
}显然，在这里获取了 request 对应的所有的 HandlerExecutionChain，察看到这是通过handlerMappings 拿到HandlerMapping，再查问 Handler。
handlerMappings的作用是通过 urlpath 匹配对应的 controller，HandlerMapping 就通过申请失去 HandlerExecutionChain, 至于HandlerMapping 的源码剖析请参考其余文章。