談談spring中的攔截器

       在web開發中，攔截器是經常用到的功能。它可以幫我們驗證是否登陸、預先設定資料以及統計方法的執行效率等等。今天就來詳細的談一下spring中的攔截器。spring中攔截器主要分兩種，一個是HandlerInterceptor，一個是MethodInterceptor。

一，HandlerInterceptor攔截器

HandlerInterceptor是springMVC專案中的攔截器，它攔截的目標是請求的地址，比MethodInterceptor先執行。實現一個HandlerInterceptor攔截器可以直接實現HandlerInterceptor

介面，也可以繼承HandlerInterceptorAdapter類。這兩種方法殊途同歸，其實HandlerInterceptorAdapter也就是聲明瞭HandlerInterceptor介面中所有方法的預設實現，而我們在繼承他之後只需要重寫必要的方法。下面就是HandlerInterceptorAdapter的程式碼，可以看到一個方法只是預設返回true，另外兩個是空方法：

public abstract class HandlerInterceptorAdapter implements HandlerInterceptor {

	/**
	 * This implementation always returns <code>true</code>.
	 */
	public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
	    throws Exception {
		return true;
	}

	/**
	 * This implementation is empty.
	 */
	public void postHandle(
			HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
			throws Exception {
	}

	/**
	 * This implementation is empty.
	 */
	public void afterCompletion(
			HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
			throws Exception {
	}

}
 

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		int interceptorIndex = -1;

		try {
			ModelAndView mv;
			boolean errorView = false;

			try {
				processedRequest = checkMultipart(request);

				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest, false);
				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()) {
						String requestUri = urlPathHelper.getRequestUri(request);
						logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				// Apply preHandle methods of registered interceptors.
				HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
				if (interceptors != null) {
					for (int i = 0; i < interceptors.length; i++) {
						HandlerInterceptor interceptor = interceptors[i];
						if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) {
							triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
							return;
						}
						interceptorIndex = i;
					}
				}

				// Actually invoke the handler.
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				// Do we need view name translation?
				if (mv != null && !mv.hasView()) {
					mv.setViewName(getDefaultViewName(request));
				}

				// Apply postHandle methods of registered interceptors.
				if (interceptors != null) {
					for (int i = interceptors.length - 1; i >= 0; i--) {
						HandlerInterceptor interceptor = interceptors[i];
						interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
					}
				}
			}
			catch (ModelAndViewDefiningException ex) {
				logger.debug("ModelAndViewDefiningException encountered", ex);
				mv = ex.getModelAndView();
			}
			catch (Exception ex) {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				mv = processHandlerException(processedRequest, response, handler, ex);
				errorView = (mv != null);
			}

			// Did the handler return a view to render?
			if (mv != null && !mv.wasCleared()) {
				render(mv, processedRequest, 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");
				}
			}

			// Trigger after-completion for successful outcome.
			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
		}

		catch (Exception ex) {
			// Trigger after-completion for thrown exception.
			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
			throw ex;
		}
		catch (Error err) {
			ServletException ex = new NestedServletException("Handler processing failed", err);
			// Trigger after-completion for thrown exception.
			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
			throw ex;
		}

		finally {
			// Clean up any resources used by a multipart request.
			if (processedRequest != request) {
				cleanupMultipart(processedRequest);
			}
		}
	}
 

private void triggerAfterCompletion(HandlerExecutionChain mappedHandler,
			int interceptorIndex,
			HttpServletRequest request,
			HttpServletResponse response,
			Exception ex) throws Exception {

		// Apply afterCompletion methods of registered interceptors.
		if (mappedHandler != null) {
			HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
			if (interceptors != null) {
				for (int i = interceptorIndex; i >= 0; i--) {
					HandlerInterceptor interceptor = interceptors[i];
					try {
						interceptor.afterCompletion(request, response, mappedHandler.getHandler(), ex);
					}
					catch (Throwable ex2) {
						logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
					}
				}
			}
		}
	}

        根據以上的程式碼，分析一下不同攔截器及其方法的執行順序。假設有5個攔截器編號分別為12345，若一切正常則方法的執行順序是12345的preHandle，54321的postHandle，54321的afterCompletion。若編號3的攔截器的preHandle方法返回false或者丟擲了異常，接下來會執行的是21的afterCompletion方法。這裡要注意的地方是，我們在寫一個攔截器的時候要謹慎的處理preHandle中的異常，因為這裡一旦有異常丟擲就不會再受到這個攔截器的控制。12345的preHandle的方法執行過之後，若handler出現了異常或者某個攔截器的postHandle方法出現了異常，則接下來都會執行54321的afterCompletion方法，因為只要12345的preHandle方法執行完，當前攔截器的攔截器就會記錄成編號5的攔截器，而afterCompletion總是從當前的攔截器逆向的向前執行。

        另外，實現HandlerInterceptor攔截器還有一個方法，就是實現WebRequestInterceptor介面。其實它和剛才的兩種方法也是殊途同歸，最終還是被spring適配成HandlerInterceptor。有一點不同，它的preHandle方法最終只會返回true。

二，MethodInterceptor攔截器

public class MethodInvokeInterceptor implements MethodInterceptor {
    @Override
    public Object invoke(MethodInvocation methodInvocation) throws Throwable {
        System.out.println("before method invoke");
        Object object = methodInvocation.proceed();
        System.out.println("after method invoke");
        return object;
    }
}

@Component
@Aspect
public class AutoAspectJInterceptor {

    @Around("execution (* com.test.controller..*.*(..))")
    public Object around(ProceedingJoinPoint point) throws Throwable{
        System.out.println("AutoAspectJInterceptor begin around");
        Object object = point.proceed();
        System.out.println("AutoAspectJInterceptor end around");
        return object;
    }
   
}

@Component
 public class AspectJInterceptor {
    public Object around(ProceedingJoinPoint proceedingJoinPoint) throws Throwable {
        System.out.println("AspectJInterceptor around before");
        Object object = proceedingJoinPoint.proceed();
        System.out.println("AspectJInterceptor around after");
        return object;
    }
}

<!-- 自定義攔截器 ，先過mvc:interceptors-->
	<bean id="methodInvokeInterceptor" class="com.test.interceptor.MethodInvokeInterceptor"/>
	<bean id="aspectInterceptor" class="com.test.interceptor.AspectJInterceptor"/>

	<aop:config>
		<!--切入點,controlller -->
		<aop:pointcut id="pointcut_test"	  expression="execution(* com.test.controller..*.*(..))" />
		<!--在該切入點使用自定義攔截器 ,按照先後順序執行 -->
		<aop:advisor pointcut-ref="pointcut_test" advice-ref="methodInvokeInterceptor" />


		<aop:aspect ref="aspectInterceptor">
			<aop:around method="around" pointcut="execution(* com.test.controller..*.*(..))"/>
		</aop:aspect>
	</aop:config>
	<!-- 自動掃描使用了aspectj註解的類 -->
	<aop:aspectj-autoproxy/>

三，談一談區別