理解SpringMVC在JavaWeb应用的基本原理

SpringMVC在JavaWeb中应用很广，处理前台、接请求并返回页面、数据，本文尝试理解SpringMVC在web应用中的工作流程。

假定我们建立测试项目WGS、使用Tomcat8.0作为我们的调试服务器，我们将web应用程序WGS部署到Tomcat的webapps目录，如图：

Tomcat目录下有其内置的其它应用模块，每个应用的下的目录结构相似，下面是WGS的应用结构

WEB-INF下是应用程序的核心代码，其中web.xml文件是整个Web应用的部署文件和工程加载文件(web.xml不是Web应用必须的配置文件，但在基于jsp开发的动态网站应用中，web.xml几乎无处不在)。

其实Tomcat的配置文件夹下也有web.xml，这是配置所有Web应用的公共配置，每个Web应用拥有自己的web.xml进行个性化配置。

web.xml of Tomcat

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<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">

	<!-- The default servlet for all web applications, that serves static     -->
	<!-- resources.  It processes all requests that are not mapped to other   -->
	<!-- servlets with servlet mappings (defined either here or in your own   -->
	<!-- web.xml file).  This servlet supports the following initialization   -->
	<!-- parameters (default values are in square brackets):                  -->
	<servlet>
	    <servlet-name>default</servlet-name>
	    <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
	    <init-param>
	        <param-name>debug</param-name>
	        <param-value>0</param-value>
	    </init-param>
	    <init-param>
	        <param-name>listings</param-name>
	        <param-value>false</param-value>
	    </init-param>
	    <load-on-startup>1</load-on-startup>
	</servlet>
	<!-- The mapping for the default servlet -->
	<servlet-mapping>
	    <servlet-name>default</servlet-name>
	    <url-pattern>/</url-pattern>
	</servlet-mapping>

	<!-- The JSP page compiler and execution servlet, which is the mechanism  -->
	<!-- used by Tomcat to support JSP pages.  Traditionally, this servlet    -->
	<!-- is mapped to the URL pattern "*.jsp".  This servlet supports the     -->
	<!-- following initialization parameters (default values are in square    -->
	<!-- brackets):                                                           -->
	<servlet>
	    <servlet-name>jsp</servlet-name>
	    <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
	    <init-param>
	        <param-name>fork</param-name>
	        <param-value>false</param-value>
	    </init-param>
	    <init-param>
	        <param-name>xpoweredBy</param-name>
	        <param-value>false</param-value>
	    </init-param>
	    <load-on-startup>3</load-on-startup>
	</servlet>
	<!-- The mappings for the JSP servlet -->
	<servlet-mapping>
	    <servlet-name>jsp</servlet-name>
	    <url-pattern>*.jsp</url-pattern>
	    <url-pattern>*.jspx</url-pattern>
	</servlet-mapping>

	<!-- ==================== Default Session Configuration ================= -->
  	<!-- You can set the default session timeout (in minutes) for all newly   -->
  	<!-- created sessions by modifying the value below.                       -->
	<session-config>
		<session-timeout>30</session-timeout>
	</session-config>

	<!-- ===================== Default MIME Type Mappings =================== -->
	<!-- When serving static resources, Tomcat will automatically generate    -->
	<!-- a "Content-Type" header based on the resource's filename extension,  -->
	<!-- based on these mappings.  Additional mappings can be added here (to  -->
	<!-- apply to all web applications), or in your own application's web.xml -->
	<!-- deployment descriptor.                                               -->
	<!-- Note: Extensions are always matched in a case-insensitive manner.    -->
	<mime-mapping>
	    <extension>zmm</extension>
	    <mime-type>application/vnd.handheld-entertainment+xml</mime-type>
	</mime-mapping>
	.
	.
	.
	<mime-mapping>
	    <extension>123</extension>
	    <mime-type>application/vnd.lotus-1-2-3</mime-type>
	</mime-mapping>

	<!-- ==================== Default Welcome File List ===================== -->
	<welcome-file-list>
	    <welcome-file>index.html</welcome-file>
	    <welcome-file>index.htm</welcome-file>
	    <welcome-file>index.jsp</welcome-file>
	</welcome-file-list>

</web-app>

  • ** Servlet default **
    默认Servlet，/ 过滤所有Web应用的请求，可以处理所有应用的静态资源，也处理在其他应用中未匹配到请求，所以优先级比较低。

  • ** Servlet jsp **
    JspServlet，*.jsp处理.jsp后缀文件,编译、执行JSP，浏览器不能显示动态网页(基于动态网站技术的页面，e.g jsp,php,.net)，JspServlet将jsp编译后生成html，返回给客户端。

  • ** session-timeout **
    session的存活时间。

  • ** mime-mapping **
    mine-mapping通过匹配文件后缀判断mine类型，123即为文件扩展名，服务器通过mine-mapping告诉浏览器返回内容的mine类型。

  • ** Welcome List **
    welcome-file-list配置Web应用的欢迎页，如果Web应用没有在自己的web.xml中配置此项，index.html指定其默认页面为index.html。

注：动态网页不是指有动态效果的页面，而是基于动态网站技术开发的页面，如jsp,php,asp等，动态网页和静态网页可通过下图来区分。

web.xml of WGS

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<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5">

	<display-name>WGS</display-name>

	<!-- Context Param -->
	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>
			classpath*:/applicationContext.xml,
			classpath*:/applicationContext-shiro.xml
		</param-value>
	</context-param>
	<context-param>
		<param-name>spring.profiles.default</param-name>
		<param-value>production</param-value>
	</context-param>
	
	<!-- Context Loader Listener  -->
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

	<!-- Encoding Config -->
	<filter>
		<filter-name>encodingFilter</filter-name>
		<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
		<init-param>
			<param-name>encoding</param-name>
			<param-value>UTF-8</param-value>
		</init-param>
		<init-param>
			<param-name>forceEncoding</param-name>
			<param-value>true</param-value>
		</init-param>
	</filter>
	<filter-mapping>
		<filter-name>encodingFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>

	<!-- JPA Config -->
	<filter>
		<filter-name>openEntityManagerInViewFilter</filter-name>
		<filter-class>org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>openEntityManagerInViewFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>	  
	
	<!-- Shiro Config -->
	<filter>
		<filter-name>shiroFilter</filter-name>
		<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
		<init-param>
			<param-name>targetFilterLifecycle</param-name>
			<param-value>true</param-value>
		</init-param>
	</filter>
	<filter-mapping>
		<filter-name>shiroFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>

	<!-- Spring MVC Config -->
	<servlet>
		<servlet-name>springServlet</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>/WEB-INF/spring-mvc.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<servlet-mapping>
		<servlet-name>springServlet</servlet-name>
		<url-pattern>/</url-pattern>
	</servlet-mapping>

	<error-page>
		<error-code>404</error-code>
		<location>/WEB-INF/views/error/404.jsp</location>
	</error-page>

</web-app>

如上，Web应用web.xml中主要有几个标签，context-param、listener、filter、filter-mapping、servlet、servlet-mapping，Tomcat初始化顺序为context-param、listener、filter、servlet，初始化流程如下：

  • Tomcat读取web.xml中的context-param、listener两类节点
  • Tomcat创建ServletContext，即上Web应用下文，将context-param对应的键值交给ServletContext，接着创建listener
  • Tomcat实例化listener对应的类(ContextLoaderListener)，根据listener中的方法加载context-param的值，也就是去加载applicationContext.xml和applicationContext-shiro.xml，以及根据spring.profiles.default加载生产环境下的属性文件。这些配置文件描述了如何加载应用程序。
  • Tomcat初始化过滤器Filter
  • Tomcat初始化Servlet，servlet-mapping定义过滤规则，load-on-startup指定启动顺序，大于1则此Servlet启动优先级最高。

springServlet就是应用程序的Servlet，使用的是org.springframework.web.servlet.DispatcherServlet这个类，Tomcat实例化此类来处理web请求，具体实例化配置在 /WEB-INF/spring-mvc.xml 中定义，此配置文件如下：

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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
	xmlns:mvc="http://www.springframework.org/schema/mvc"
	xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd
		http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
		http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd">

	<!-- 自动扫描且只扫描 @Controller -->
	<context:component-scan base-package="com.smarts"
		use-default-filters="false">
		<context:include-filter type="annotation"
			expression="org.springframework.stereotype.Controller" />
		<context:include-filter type="annotation"
			expression="org.springframework.web.bind.annotation.ControllerAdvice" />
	</context:component-scan>

	<mvc:annotation-driven>
		<mvc:message-converters register-defaults="true">
			<!-- 将StringHttpMessageConverter的默认编码设为UTF-8 -->
			<bean class="org.springframework.http.converter.StringHttpMessageConverter">
				<constructor-arg value="UTF-8" />
			</bean>
		</mvc:message-converters>
	</mvc:annotation-driven>

	<!-- 定义JSP文件的位置 -->
	<bean
		class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<property name="prefix" value="/WEB-INF/views/" />
		<property name="suffix" value=".jsp" />
	</bean>

	<!-- 容器默认的DefaultServletHandler处理 所有静态内容与无RequestMapping处理的URL -->
	<mvc:default-servlet-handler />

	<!-- 定义无需Controller的url<->view直接映射 -->
	<mvc:view-controller path="/" view-name="redirect:/manage/" />
	
	<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
	   <property name="defaultEncoding" value="utf-8"></property>
	</bean>
	
	<!-- 替换异常处理方式。指定的异常转到指定页面；否则作为json返回 -->
	<bean id="exceptionResolver"
		class="com.smarts.exception.CustomSimpleMappingExceptionResolver">
		<property name="exceptionMappings">
			<props>
				<prop key="com.smarts.exception.WebException">error/jsonerror</prop>
				<prop key="java.lang.NullPointerException">error/jsonerror</prop>
				<prop key="java.lang.exception">error/500</prop>

			</props>
		</property>
	</bean>
</beans>

如上，是我们熟悉的SpringMVC的配置文件，通过@Controller注解来管理请求处理类，并定义返回jsp文件的位置。

（完）

Spring
#javaweb #spring
理解SpringMVC在JavaWeb应用的基本原理
https://blackist.org/2018-11-06-javaweb-springmvc-dispatcher/
作者
董猿外
发布于
2018年11月6日
许可协议