[Review the SSM framework series] 4 - Spring integrated web environment (three-tier structure and configuration listener)

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Three-tier architecture environment construction

In the previous explanation of the Spring core configuration file and data source configuration, mainly in the dao layer and the service layer, now we also integrate the web layer environment into it.

Current project structure

Build the web layer

1. Introduce servlet and jsp dependencies

<!-- 引入Servlet和JSP依赖 -->
<dependency>
   <groupId>javax.servlet</groupId>
   <artifactId>javax.servlet-api</artifactId>
   <version>3.1.0</version>
   <scope>provided</scope>
</dependency>
<dependency>
   <groupId>javax.servlet.jsp</groupId>
   <artifactId>javax.servlet.jsp-api</artifactId>
   <version>2.2.1</version>
   <scope>provided</scope>
</dependency>
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2. Create the UserServlet class

@WebServlet("/userServlet")
public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ApplicationContext application = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = (UserService) application.getBean("userService");
        userService.save();
    }
}
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3. Configure Tomcat and start the test

Start Tomcat and access /UserServlet, the console prints as follows, and the basic environment of the three-tier architecture is successfully built.

Three-tier architecture basic project structure

Set the listener to get applicationContext.xml

Now there is a problem: every time you get a bean from the container, you need to ApplicationContext application = new ClassPathXmlApplicationContext("applicationContext.xml"). When the configuration file is loaded multiple times, many ApplicationContext objects will be created, resulting in a waste of resources.

In the Web project, we use the characteristics of the listener to create a configuration file loading listener, load the Spring configuration file when the application starts, and create an ApplicationContext object; when it is to be used, it can be obtained directly from the domain. , to achieve the effect of one load and use everywhere.

Configure the location of the core configuration file in web.xml

  <context-param>
    <param-name>applicationContextLocation</param-name>
    <param-value>classpath:applicationContext.xml</param-value>
  </context-param>
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Create a ContextLoaderListener class

public class ContextLoaderListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ServletContextEvent sce;
        ServletContext servletContext = servletContextEvent.getServletContext();

        // 读取web.xml中配置的Spring核心配置文件的位置
        String applicationContextLocation = servletContext.getInitParameter("applicationContextLocation");

        // 创建ApplicationContext上下文对象
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext(applicationContextLocation);

        // 将ApplicationContext对象存到域中
        servletContext.setAttribute("applicationContext",applicationContext);

        System.out.println("创建ApplicationContext对象成功：" + applicationContext);
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }
}
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Configure the ContextLoaderListener listener in web.xml

<listener>
  <listener-class>com.wang.listener.ContextLoaderListener</listener-class>
</listener>
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Start Tomcat, check the console print, and find that the ApplicationContext object is created when Tomcat starts:

Modify UserServlet and use listener to get ApplicationContext

@WebServlet("/userServlet")
public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        ApplicationContext application = new ClassPathXmlApplicationContext("applicationContext.xml");
        ApplicationContext application = (ApplicationContext) req.getServletContext().getAttribute("applicationContext");
        UserService userService = (UserService) application.getBean("userService");
        userService.save();
    }
}
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Integrating ContextLoaderListener listeners with Spring

The above way of customizing the listener realizes that the Spring configuration file is loaded when the application starts, and the ApplicationContext object is created. However, this process is still relatively complicated. You can use the tools provided by Spring to obtain the application context to directly obtain the context object.

Introduce spring-web package

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>5.0.5.RELEASE</version>
</dependency>
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Modify web.xml to configure Spring's ContextLoaderListener listener

<listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
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Get Application Context Object Using Spring Tools

@WebServlet("/userServlet")
public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        ApplicationContext application = new ClassPathXmlApplicationContext("applicationContext.xml");
//        ApplicationContext application = (ApplicationContext) req.getServletContext().getAttribute("applicationContext");
        ApplicationContext application = WebApplicationContextUtils.getWebApplicationContext(req.getServletContext());
        UserService userService = (UserService) application.getBean("userService");
        userService.save();
    }
}
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