SpringMVC深入理解

Posted 2020-07-11 現実という怪物と戦う者

核心类与接口

- DispatcherServlet 前置控制器
- HandlerMapping 请求映射（到Controller）
- HandlerAdapter 请求映射（到Controller类的方法上）
- Controller 控制器
- HandlerIntercepter 拦截器
- ViewResolver 视图映射
- View 视图处理

启动过程

Spring MVC启动过程大致分为两个过程：
- ContextLoaderListener初始化，读取context-param中的contextConfigLocation指定的配置文件，创建ROOT Context，通过调用继承自ContextLoader的initWebApplicationContext方法实例化Spring IoC容器，并将此容器实例注册到ServletContext中
- ContextLoaderListener初始化完毕后，开始初始化web.xml中配置的DispatcherServlet，DispatcherServlet的初始化又包括了视图管理器、异常处理器、映射管理等等；

 1    /**
 2      * Initialize the strategy objects that this servlet uses.
 3      * <p>May be overridden in subclasses in order to initialize further strategy objects.
 4      */
 5     protected void initStrategies(ApplicationContext context) {
 6         initMultipartResolver(context);
 7         initLocaleResolver(context);
 8         initThemeResolver(context);
 9         initHandlerMappings(context);
10         initHandlerAdapters(context);
11         initHandlerExceptionResolvers(context);
12         initRequestToViewNameTranslator(context);
13         initViewResolvers(context);
14         initFlashMapManager(context);
15     }

 

ContextLoaderListener初始化的是 WebApplicationContext, 创建后可以从ServletContext中获取，WebApplicationContext是应用程序内共享的，最多只有一个，如果寻求简单也可以不初始化此容器。与之不同 DispatcherServlet可以有多个，并共享一个WebApplicationContext容器，每一个DispatcherServlet有不同的配置，控制不同的WEB访问。一般将 DAO、Service 层Bean共享的放在ContextLoaderListener配置的容器中，将WEB层的Bean放在特定的DispatcherServlet配置的容器中。

SpringMVC利用Spring的注入特性初始化资源文件，只需要调用setPropertyValues方法就可将contextConfigLocation属性设置到对应实例中，也就是以依赖注入的方式初始化属性。

时序图如下（盗图）：




请求处理流程

官网上的图



涉及到核心类与接口的过程描述：

客户端浏览器发送http请求，被`DispatcherServlet`捕获，调用关键的doDispatch方法，遍历所有注册为`Controller`的bean，为请求寻找关联映射，其中遍历查找的函数getHandler和getHandlerAdapter的源码：

 1 /**
 2      * Return the HandlerExecutionChain for this request.
 3      * <p>Tries all handler mappings in order.
 4      * @param request current HTTP request
 5      * @return the HandlerExecutionChain, or {@code null} if no handler could be found
 6      */
 7     protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
 8         for (HandlerMapping hm : this.handlerMappings) {
 9             if (logger.isTraceEnabled()) {
10                 logger.trace(
11                         "Testing handler map [" + hm + "] in DispatcherServlet with name \'" + getServletName() + "\'");
12             }
13             HandlerExecutionChain handler = hm.getHandler(request);
14             if (handler != null) {
15                 return handler;
16             }
17         }
18         return null;
19     }
20     
21     
22     /**
23      * Return the HandlerAdapter for this handler object.
24      * @param handler the handler object to find an adapter for
25      * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
26      */
27     protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
28         for (HandlerAdapter ha : this.handlerAdapters) {
29             if (logger.isTraceEnabled()) {
30                 logger.trace("Testing handler adapter [" + ha + "]");
31             }
32             if (ha.supports(handler)) {
33                 return ha;
34             }
35         }
36         throw new ServletException("No adapter for handler [" + handler +
37                 "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
38     }

 

 

找到匹配的映射后`HandlerAdapter`会依次调用preHandle、handle（返回ModelAndView）、postHandle方法，所有步骤完成后调用processDispatchResult函数处理结果，并返回View给客户端。postDispatchResult函数和其中调用的render函数源码如下：

 1 /**
 2      * Handle the result of handler selection and handler invocation, which is
 3      * either a ModelAndView or an Exception to be resolved to a ModelAndView.
 4      */
 5     private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
 6             HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
 7 
 8         boolean errorView = false;
 9 
10         if (exception != null) {
11             if (exception instanceof ModelAndViewDefiningException) {
12                 logger.debug("ModelAndViewDefiningException encountered", exception);
13                 mv = ((ModelAndViewDefiningException) exception).getModelAndView();
14             }
15             else {
16                 Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
17                 mv = processHandlerException(request, response, handler, exception);
18                 errorView = (mv != null);
19             }
20         }
21 
22         // Did the handler return a view to render?
23         if (mv != null && !mv.wasCleared()) {
24             render(mv, request, response);
25             if (errorView) {
26                 WebUtils.clearErrorRequestAttributes(request);
27             }
28         }
29         else {
30             if (logger.isDebugEnabled()) {
31                 logger.debug("Null ModelAndView returned to DispatcherServlet with name \'" + getServletName() +
32                         "\': assuming HandlerAdapter completed request handling");
33             }
34         }
35 
36         if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
37             // Concurrent handling started during a forward
38             return;
39         }
40 
41         if (mappedHandler != null) {
42             mappedHandler.triggerAfterCompletion(request, response, null);
43         }
44     }
45 
46 
47     /**
48      * Render the given ModelAndView.
49      * <p>This is the last stage in handling a request. It may involve resolving the view by name.
50      * @param mv the ModelAndView to render
51      * @param request current HTTP servlet request
52      * @param response current HTTP servlet response
53      * @throws ServletException if view is missing or cannot be resolved
54      * @throws Exception if there\'s a problem rendering the view
55      */
56     protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
57         // Determine locale for request and apply it to the response.
58         Locale locale = this.localeResolver.resolveLocale(request);
59         response.setLocale(locale);
60 
61         View view;
62         if (mv.isReference()) {
63             // We need to resolve the view name.
64             view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
65             if (view == null) {
66                 throw new ServletException("Could not resolve view with name \'" + mv.getViewName() +
67                         "\' in servlet with name \'" + getServletName() + "\'");
68             }
69         }
70         else {
71             // No need to lookup: the ModelAndView object contains the actual View object.
72             view = mv.getView();
73             if (view == null) {
74                 throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
75                         "View object in servlet with name \'" + getServletName() + "\'");
76             }
77         }
78 
79         // Delegate to the View object for rendering.
80         if (logger.isDebugEnabled()) {
81             logger.debug("Rendering view [" + view + "] in DispatcherServlet with name \'" + getServletName() + "\'");
82         }
83         try {
84             if (mv.getStatus() != null) {
85                 response.setStatus(mv.getStatus().value());
86             }
87             view.render(mv.getModelInternal(), request, response);
88         }
89         catch (Exception ex) {
90             if (logger.isDebugEnabled()) {
91                 logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name \'" +
92                         getServletName() + "\'", ex);
93             }
94             throw ex;
95         }
96     }

 

这就是一个完整的处理http请求的过程。盗图一张：

 

时序图如下（来源：http://neoremind.com/2016/02/springmvc%E7%9A%84%E4%B8%80%E4%BA%9B%E5%B8%B8%E7%94%A8%E6%9C%80%E4%BD%B3%E5%AE%9E%E8%B7%B5/）：



配置实例

这里放的是最简单的配置，可以通过这个简单的配置实例回顾一下上面的过程。

目录结构

    -SpringMVCDemo
        -src
            -me.cyan
                -WelcomeController
        -web
            -WEB-INF
                -applicationContext.xml
                -dispatcher-servlet.xml
                -web.xml
            -index.jsp
        -pom.xml
        
pom.xml
引入的包

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 3          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
 4     <modelVersion>4.0.0</modelVersion>
 5     <groupId>me.cyan</groupId>
 6     <artifactId>SpringMVCDemo</artifactId>
 7     <name>SpringMVCDemo</name>
 8     <packaging>war</packaging>
 9     <version>1.0.0</version>
10 
11     <properties>
12         <spring-version>4.2.6.RELEASE</spring-version>
13     </properties>
14 
15     <dependencies>
16         <dependency>
17             <groupId>org.springframework</groupId>
18             <artifactId>spring-context</artifactId>
19             <version>${spring-version}</version>
20         </dependency>
21         <dependency>
22             <groupId>org.springframework</groupId>
23             <artifactId>spring-core</artifactId>
24             <version>${spring-version}</version>
25         </dependency>
26         <dependency>
27             <groupId>org.springframework</groupId>
28             <artifactId>spring-web</artifactId>
29             <version>${spring-version}</version>
30         </dependency>
31         <dependency>
32             <groupId>org.springframework</groupId>
33             <artifactId>spring-beans</artifactId>
34             <version>${spring-version}</version>
35         </dependency>
36         <dependency>
37             <groupId>commons-logging</groupId>
38             <artifactId>commons-logging</artifactId>
39             <version>1.2</version>
40         </dependency>
41         <dependency>
42             <groupId>org.springframework</groupId>
43             <artifactId>spring-webmvc</artifactId>
44             <version>${spring-version}</version>
45         </dependency>
46         <dependency>
47             <groupId>org.springframework</groupId>
48             <artifactId>spring-aop</artifactId>
49             <version>${spring-version}</version>
50         </dependency>
51         <dependency>
52             <groupId>org.springframework</groupId>
53             <artifactId>spring-expression</artifactId>
54             <version>${spring-version}</version>
55         </dependency>
56     </dependencies>
57 </project>

 

web.xml

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
 3          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 4          xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
 5          version="3.1">
 6          
 7     <!--配置文件路径-->
 8     <context-param>
 9         <param-name>contextConfigLocation</param-name>
10         <param-value>/WEB-INF/applicationContext.xml</param-value>
11     </context-param>
12     <listener>
13         <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
14     </listener>
15     
16     <!--SpringMVC核心servlet-->
17     <servlet>
18         <servlet-name>dispatcher</servlet-name>
19         <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
20         <load-on-startup>1</load-on-startup>
21     </servlet>
22     <servlet-mapping>
23         <servlet-name>dispatcher</servlet-name>
24         <url-pattern>/</url-pattern>
25     </servlet-mapping>
26 </web-app>

 

 

dispatcher-servlet.xml

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <beans xmlns="http://www.springframework.org/schema/beans"
 3        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mvc="http://www.springframework.org/schema/mvc"
 4        xmlns:context="http://www.springframework.org/schema/context"
 5        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
 6 
 7     <!-- 默认的注解映射的支持 -->
 8     <mvc:annotation-driven />
 9 
10     <!-- 自动扫描的包名 -->
11     <context:component-scan base-package="me.cyan" />
12 
13 </beans>

 

 

WelcomeController

 

 1 package me.cyan;
 2 
 3 import org.springframework.stereotype.Controller;
 4 import org.springframework.web.bind.annotation.RequestMapping;
 5 import org.springframework.web.bind.annotation.ResponseBody;
 6 
 7 /**
 8  * Created by cyan on 16/5/23.
 9  */
10 
11 @Controller
12 public class welcomeController {
13 
14     @RequestMapping("/hello")
15     public @ResponseBody String sayhello(){
16         return "hello Spring MVC!";
17     }
18 }

 

 

运行结果