springmvc拦截器入门及其执行顺序源码分析

springmvc拦截器是偶尔会用到的一个功能，本案例来演示一个较简单的springmvc拦截器的使用，并通过源码来分析拦截器的执行顺序的控制。
具体操作步骤为：
1、maven项目引入spring依赖
2、配置web.xml中的DispatcherServlet
3、准备两个拦截器，并在springmvc配置文件中进行配置管理
4、准备业务类，该类转发到一个JSP页面，并在页面做后台打印
5、测试发送请求到业务类，查看执行顺序
6、源码分析
7、总结以及代码附件
————————————————————————————————————————————————————————————
下面开始开发！
1、maven项目引入spring依赖

[XML] 纯文本查看复制代码

<dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.0.2.RELEASE</version>
        </dependency>
 
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>5.0.2.RELEASE</version>
        </dependency>
 
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>5.0.2.RELEASE</version>
        </dependency>
 
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
            <scope>provided</scope>
        </dependency>
 
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.0</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

2、配置web.xml中的DispatcherServlet

[XML] 纯文本查看复制代码

<!-- 前端控制器（加载classpath:springmvc.xml 服务器启动创建servlet） -->
    <servlet>
        <servlet-name>dispatcherServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!-- 配置初始化参数，创建完DispatcherServlet对象，加载springmvc.xml配置文件 -->
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:springmvc.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcherServlet</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

3、准备两个拦截器，并在springmvc配置文件中进行配置管理
两个拦截器代码如下：

[Java] 纯文本查看复制代码

public class MyInterceptor1 implements HandlerInterceptor {
     
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        System.out.println("==1-1====前置拦截器1 执行======");
        return true; //ture表示放行
    }
 
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) {
        System.out.println("==1-2=====后置拦截器1 执行======");
    }
 
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
        System.out.println("==1-3======最终拦截器1 执行======");
    }
}

[Java] 纯文本查看复制代码

public class MyInterceptor2 implements HandlerInterceptor {
 
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        System.out.println("==2-1====前置拦截器2 执行======");
        return true; //ture表示放行
    }
 
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) {
        System.out.println("==2-2=====后置拦截器2 执行======");
    }
 
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
        System.out.println("==2-3======最终拦截器2 执行======");
    }
}

springmvc配置如下(此处只挑选拦截器配置，具体代码参见附件)：

[XML] 纯文本查看复制代码

<!--配置拦截器-->
    <mvc:interceptors>
        <!--配置拦截器-->
        <mvc:interceptor>
            <mvc:mapping path="/**" />
            <bean class="com.itheima.interceptor.MyInterceptor1" />
        </mvc:interceptor>
 
        <mvc:interceptor>
            <mvc:mapping path="/**" />
            <bean class="com.itheima.interceptor.MyInterceptor2" />
        </mvc:interceptor>
 
    </mvc:interceptors>

注意配置顺序拦截器1先于拦截器2，并且都拦截所有的方法
4、准备业务类，该类转发到一个JSP页面，并在页面做后台打印

[Java] 纯文本查看复制代码

@Controller
public class BizController {
    @RequestMapping("testBiz")
    public String showUserInfo(Integer userId, Model model){
        System.out.println(">>>>>业务代码执行-查询用户ID为:"+ userId);
        User user = new User(userId);
        user.setName("宙斯");
        model.addAttribute("userInfo",user);
        return "user_detail";
    }
}

响应的页面如下：

[HTML] 纯文本查看复制代码

<html>
<head>
    <title>detail</title>
</head>
<body>
    用户详情：
    ${userInfo.id}:${userInfo.name}
 
    <%System.out.print(">>>>>jsp页面的输出为：");%>
    <%System.out.println(((User)request.getAttribute("userInfo")).getName());%>
</body>
</html>

5、测试发送请求到业务类，查看执行顺序
启动项目后，在浏览器中访问/testBiz?userId=1，会在后台打印如下内容：

[Java] 纯文本查看复制代码

==1-1====前置拦截器1 执行======
==2-1====前置拦截器2 执行======
>>>>>业务代码执行-查询用户ID为:1
==2-2=====后置拦截器2 执行======
==1-2=====后置拦截器1 执行======
>>>>>jsp页面的输出为：宙斯
==2-3======最终拦截器2 执行======
==1-3======最终拦截器1 执行======

6、源码分析
通过打印结果会发现，执行顺序为：拦截器1的前置>拦截器2的前置>业务代码>拦截器2后置>拦截器1后置>拦截器2最终>拦截器1最终
我们可以通过源码来分析一下为何是该打印顺序。
最关键的为DispatcherServlet的核心方法——doDispatch。
当浏览器发送/testBiz?userId=1的请求时，会经过DispatcherServlet的doDispatch方法，省略掉部分代码，贴出如下（注释有说明）：

[Java] 纯文本查看复制代码

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        //...
        try {
            try {
                ModelAndView mv = null;
                Object dispatchException = null;
 
                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    //1.获取执行链
                    mappedHandler = this.getHandler(processedRequest);
                    if (mappedHandler == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }
                    //2.获取处理器适配器
                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
 
                    //...
                    //3.执行前置拦截器
                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }
                    //4.执行业务handler
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                    this.applyDefaultViewName(processedRequest, mv);
                    //5.执行后置拦截器
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }
                //6.处理页面响应，并执行最终拦截器
                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
            }
 
        }finally {
            //...
        }
    }

其中，第一步中"获取执行链"，执行链内容可以截图查看：
<ignore_js_op>

可以看到我们自定义的两个拦截器按顺序保存。
在doDispatch方法中，我们注释的第3、5、6步骤是对拦截器的执行处理，现在分别来查看第3、5、6步骤的源码。
第三步骤源码如下：

[Java] 纯文本查看复制代码

//3.执行前置拦截器中的详细代码
    boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        //获得本次请求对应的所有拦截器
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            //按照拦截器顺序依次执行每个拦截器的preHandle方法.
            //并且，interceptorIndex值会一次 + 1 (该值是给后面的最终拦截器使用的)
            for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) {
                HandlerInterceptor interceptor = interceptors[/color][i][color=black];
                //只要每个拦截器不返回false,则继续执行，否则执行最终拦截器
                if (!interceptor.preHandle(request, response, this.handler)) {
                    this.triggerAfterCompletion(request, response, (Exception)null);
                    return false;
                }
            }
        }
        //最终返回true
        return true;
    }

我们可以看到拦截器的preHandler方法是按拦截器顺序执行的。紧接着贴出第5步的后置拦截器源码：

[Java] 纯文本查看复制代码

//5.执行后置拦截器
    void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {
        //获得本次请求对应的所有拦截器
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            //按倒叙执行每个拦截器的postHandle方法——所以我们看到先执行的拦截器2的postHandle，再执行拦截器1的postHandle
            for(int i = interceptors.length - 1; i >= 0; --i) {
                HandlerInterceptor interceptor = interceptors[/color][color=black];
                interceptor.postHandle(request, response, this.handler, mv);
            }
        }
    }

会发现，后置处理是按照拦截器顺序倒叙处理的。最后，我们贴出执行最终拦截器方法的代码：

[Java] 纯文本查看复制代码

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {
    //...
 
    if (mv != null && !mv.wasCleared()) {
        //处理响应
        this.render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    } else if (this.logger.isDebugEnabled()) {
        this.logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + this.getServletName() + "': assuming HandlerAdapter completed request handling");
    }
 
    if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        if (mappedHandler != null) {
            //6、执行拦截器的最终方法们
            mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
        }
 
    }
}

其中，有一个render()方法，该方法会直接处理完response。再后则是触发triggerAfterCompletion方法：

[Java] 纯文本查看复制代码

//6、执行拦截器的[align=left][color=#b00000]最终[/color][/align]方法
    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) throws Exception {
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            //倒叙执行每个拦截器(interceptorIndex为前置拦截器动态计算)的afterCompletion方法
            for(int i = this.interceptorIndex; i >= 0; --i) {
                HandlerInterceptor interceptor = interceptors[/color][/i][color=black][i];
 
                try {
                    interceptor.afterCompletion(request, response, this.handler, ex);
                } catch (Throwable var8) {
                    logger.error("HandlerInterceptor.afterCompletion threw exception", var8);
                }
            }
        }
    }