拦截器的应用场景是比较多的，比如单点登录时判断用户是否已登录，是则允许用户在不同子系统间的跳转。那么怎么实现呢，直接上代码：
spring-mvc.xml配置
<mvc:interceptors> <mvc:interceptor> <mvc:mapping path="/**" /> <mvc:exclude-mapping path="index.html" /> <mvc:exclude-mapping path="/login**" /> <mvc:exclude-mapping path="/register**" /> <mvc:exclude-mapping path="/getVerifyCode**" /> <mvc:exclude-mapping path="/getMethod**" /> <bean class="com.wulinfeng.test.testpilling.util.InterceptorUtil" /> </mvc:interceptor> </mvc:interceptors>
上面的配置是拦截所有url请求，但是index.html，login开头的url等配置了例外的除外。具体拦截器实现类：
package com.wulinfeng.test.testpilling.util;
import javax.servlet.http.Cookie; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.commons.lang.StringUtils; import org.apache.logging.log4j.LogManager; import org.apache.logging.log4j.Logger; import org.springframework.web.servlet.HandlerInterceptor; import org.springframework.web.servlet.ModelAndView; /** * 登陆拦截 * * @author wulinfeng * @version C10 2017年10月11日 * @since SDP V300R003C10 */ public class InterceptorUtil implements HandlerInterceptor { /** 日志对象 */ private static Logger logger = LogManager.getLogger(InterceptorUtil.class.getName()); /** 是否启用cookie */ private static final String IS_COOKIE = PropertiesConfigUtil.getProperty("iscookie"); @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { logger.debug("InterceptorUtil.doFilter requesturl: " + request.getRequestURL()); String tokenId = null; if (IS_COOKIE.equals("1")) { Cookie[] cookies = request.getCookies(); if (cookies != null) { for (Cookie cookie : cookies) { if (cookie == null) { continue; } if (cookie.getName().equals("tokenId")) { tokenId = cookie.getValue(); break; } } } } else { if (request.getSession() != null) { tokenId = (String)request.getSession().getAttribute("tokenId"); } } if (StringUtils.isEmpty(tokenId)) { response.sendRedirect("/login.html"); } return true; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { // TODO Auto-generated method stub } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { // TODO Auto-generated method stub } }
上面实现了HandlerInterceptor接口的3个方法，分别是调用前、调用后和完成后，具体对应DispatcherServlet类流程实现方法doDispatch里标红的这3处地方：
/** * Process the actual dispatching to the handler. * <p>The handler will be obtained by applying the servlet's HandlerMappings in order. * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters * to find the first that supports the handler class. * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers * themselves to decide which methods are acceptable. * @param request current HTTP request * @param response current HTTP response * @throws Exception in case of any kind of processing failure */ 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 || mappedHandler.getHandler() == 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 (logger.isDebugEnabled()) { logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified); } if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err); } finally { if (asyncManager.isConcurrentHandlingStarted()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { // Clean up any resources used by a multipart request. if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } }
/** * Handle the result of handler selection and handler invocation, which is * either a ModelAndView or an Exception to be resolved to a ModelAndView. */ private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception { boolean errorView = false; if (exception != null) { if (exception instanceof ModelAndViewDefiningException) { logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException) exception).getModelAndView(); } else { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isDebugEnabled()) { logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() + "': assuming HandlerAdapter completed request handling"); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { mappedHandler.triggerAfterCompletion(request, response, null); } }
我们进去这3处地方看看：
/** * 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]; if (!interceptor.preHandle(request, response, this.handler)) { triggerAfterCompletion(request, response, null); return false; } this.interceptorIndex = i; } } return true; }
/** * Apply postHandle methods of registered interceptors. */ void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception { HandlerInterceptor[] interceptors = getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for (int i = interceptors.length - 1; i >= 0; i--) { HandlerInterceptor interceptor = interceptors[i]; interceptor.postHandle(request, response, this.handler, mv); } } }
/** * Trigger afterCompletion callbacks on the mapped HandlerInterceptors. * Will just invoke afterCompletion for all interceptors whose preHandle invocation * has successfully completed and returned true. */ void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex) throws Exception { HandlerInterceptor[] interceptors = getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for (int i = this.interceptorIndex; i >= 0; i--) { HandlerInterceptor interceptor = interceptors[i]; try { interceptor.afterCompletion(request, response, this.handler, ex); } catch (Throwable ex2) { logger.error("HandlerInterceptor.afterCompletion threw exception", ex2); } } } }
从上面可以看到拦截时都发生了什么。调用Controller的方法前，我们去拦截器实现类InterceptorUtil执行preHandle逻辑，根据返回的真假值决定流程是否继续往下走，如果返回真，那么流程继续，进入Controller方法里面，如果返回false，那么执行triggerAfterCompletion，退出流程。调用Controller方法后，拦截器进入InterceptorUtil执行postHandle，然后进入后处理的triggerAfterCompletion。