Spring-WEB原理

SpringWeb原理 问题:在Spring未集成controller层的时候我们需要在每个方法中实例化容器才可以拿到对应的bean

例如:

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ApplicationContext context=new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = context.getBean(UserService.class);
        userService.getUser();
 }

这样导致的问题是频繁创建容器导致的性能低下，那么该如何解决这个问题呢？ 在原生的servlet中，提供了接口ServletContextListener，我们可以将applicationContext容器放在web的最大作用域servletContext中,我们就可以在任意位置拿到spring上下文容器（Spring源码思想）

public class ContextLoaderListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ApplicationContext applicationContext=new ClassPathXmlApplicationContext("applicationContext.xml");
        ServletContext servletContext = servletContextEvent.getServletContext();
        servletContext.setAttribute("app",applicationContext);
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }
}

	@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = req.getServletContext();
        ApplicationContext app =(ApplicationContext) servletContext.getAttribute("app");
        UserService service = app.getBean(UserService.class);
        service.getUser();
    }

代码仍然存在的问题:

spring核心配置文件名称是由用户定义的，名称不确定applicationContext上下文在servletContext中的名称具有耦合性，用户在不查看源码的情况下无法知晓名称，不方便使用 解决方案 1.将spring核心配置文件名称提取到web.xml中

    applicationContext
    applicationContext.xml

这些参数会在servlet作为初始化参数优先加载到servletContext中，通过initParam获取

	@Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ServletContext servletContext = servletContextEvent.getServletContext();
        String context = servletContext.getInitParameter("applicationContext");
        ApplicationContext applicationContext=new ClassPathXmlApplicationContext(context);
        servletContext.setAttribute("app",applicationContext);
    }

2.通过工具类的方式获取spring上下文对象,这样用户就无需知道在servletContext中applicationContext的名称是什么

package com.vmware.util;

import org.springframework.context.ApplicationContext;
import javax.servlet.ServletContext;

public class WebApplicationContextUtils {
    public static ApplicationContext getContext(ServletContext context) {
        return (ApplicationContext) context.getAttribute("app");
    }
}

使用:

	@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = req.getServletContext();
        ApplicationContext applicationContext = WebApplicationContextUtils.getContext(servletContext);
        UserService userService = applicationContext.getBean(UserService.class);
        userService.getUser();
    }

使用spring-web自带的监听器与上下文工具类

1.导入依赖

     org.springframework
     spring-web
     5.3.15

2.注册监听器,将applicationContext对象加载到servletContext全局上下文中

     org.springframework.web.context.ContextLoaderListener

3.配置spring核心配置文件名称

    contextConfigLocation
    classpath:applicationContext.xml

4.使用spring提供的WebApplicationContextUtils获取spring容器

package com.vmware.controller;


import com.vmware.service.UserService;
import org.springframework.context.ApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class UserControllerSpring extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = req.getServletContext();
        ApplicationContext applicationContext = WebApplicationContextUtils.getWebApplicationContext(context);
        UserService userService = applicationContext.getBean(UserService.class);
        userService.getUser();
    }
}