spring集成web环境

1.基本三层框架

 具体的创建web工程可以查看这篇文档：https://www.cnblogs.com/GumpYan/p/13711056.html

web.xml

<?xml version="1.0" encoding="UTF-8"?>

<web-app 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_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>UserServlet</servlet-name>
        <servlet-class>com.company.web.UserServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>UserServlet</servlet-name>
        <url-pattern>/userServlet</url-pattern>
    </servlet-mapping>
</web-app>

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!--配置userDao-->
    <bean id="userDao" class="com.company.dao.impl.UserDaoImpl"></bean>

    <!--配置userService-->
    <bean id="userService" class="com.company.service.impl.UserServiceImpl">
        <property name="userDao" ref="userDao"></property>
    </bean>

</beans>

UserDao.java

package com.company.dao;

public interface UserDao {
    void save();
}

UserDaoImpl.java

package com.company.dao.impl;

import com.company.dao.UserDao;

public class UserDaoImpl implements UserDao {
    public void save() {
        System.out.println("save running...");
    }
}

　　UserService.java

package com.company.service;

public interface UserService {
    void save();
}

　　UserServlet.java

package com.company.web;

import com.company.service.UserService;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class UserServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = applicationContext.getBean(UserService.class);
        userService.save();
    }
}

　　开启Tomcat服务器

浏览器中输入：http://localhost:8080/spring_mvc_war/userServlet

 这就是web层调用service层，service层调用dao层，运行的结果

2.ApplicationContext应用上下文获取方式

应用上下文对象是通过newClasspathXmlApplicationContext(spring配置文件)方式获取的，但是每次从容器中获得Bean时都要编写newClasspathXmlApplicationContext(spring配置文件)，这样的弊端是配置文件加载多次，应用上下文对象创建多次。

在web项目中，可以使用ServletContextListener监听web应用的启动，我们可以在web应用启动时，就加载Spring的配置文件，创建应用上下文对象ApplicationContext，在将其存储到最大的域servletContext域中，这样就可以在任意位置从域中获得应用上下文ApplicationContext对象了。

2.1 添加自定义监听器

 ContextLoaderListener.java

package com.company.listener;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ContextLoaderListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        // 将Spring的应用上下文存储到ServletContext域中
        ServletContext servletContext = servletContextEvent.getServletContext();
        servletContext.setAttribute("app", applicationContext);
    }

    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }
}

在web.xml中添加监听器

<?xml version="1.0" encoding="UTF-8"?>

<web-app 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_4_0.xsd"
         version="4.0">

    <!--配置监听器-->
    <listener>
        <listener-class>com.company.listener.ContextLoaderListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>UserServlet</servlet-name>
        <servlet-class>com.company.web.UserServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>UserServlet</servlet-name>
        <url-pattern>/userServlet</url-pattern>
    </servlet-mapping>
</web-app>

修改UserServlet.java中的，将原来从applicationContext.xml中获取对象，改成从应用上下文中获取

package com.company.web;

import org.springframework.context.ApplicationContext;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class UserServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
        ServletContext servletContext = this.getServletContext();
        ApplicationContext app = (ApplicationContext) servletContext.getAttribute("app");
        System.out.println("spring容器创建完毕....");
    }
}

　　运行结果：

启动服务器：

 服务器刚启动，还没有访问，spring就创建完毕了，等于我在访问时，不用创建了，直接从容器中拿对象就可以了

 对上面的代码优化

 将applicationContext.xml放到配置文件web.xml中引用

listener中添加WebApplicationContextUtils.java

package com.company.listener;

import org.springframework.context.ApplicationContext;

import javax.servlet.ServletContext;

public class WebAppliactionContextUtils {
    public static ApplicationContext getWebApplicationContext (ServletContext servletContext) {
        return (ApplicationContext) servletContext.getAttribute("app");

    }
}

　　ContextLoaderListener.java

UserServlet.java中修改

 完整版web.xml

<?xml version="1.0" encoding="UTF-8"?>

<web-app 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_4_0.xsd"
         version="4.0">

    <!--全局初始化参数-->
    <context-param>
        <param-name>contextConfigLoaction</param-name>
        <param-value>applicationContext.xml</param-value>
    </context-param>

    <!--配置监听器-->
    <listener>
        <listener-class>com.company.listener.ContextLoaderListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>UserServlet</servlet-name>
        <servlet-class>com.company.web.UserServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>UserServlet</servlet-name>
        <url-pattern>/userServlet</url-pattern>
    </servlet-mapping>
</web-app>

3.Spring提供获取应用上下文的工具

上面的分析不用手动实现，Spring提供了一个监听器ContextLoaderListener就是对上述功能的封装，该监听器内部加载Spring配置文件，创建应用上下文对象，并存储到ServletContext域中，提供了一个客户端工具WebApplicationContextUtils供使用者获得应用上下文对象。

所以我们只需要做两件事：

（1）在web.xml中配置ContextLoaderListener监听器（导入spring-web坐标）

（2）使用WebApplicationContextUtils获得应用上下文对象ApplicationContext

 第一步主要是在web.xml中操作，在此之前需要在pom.xml中导入

<?xml version="1.0" encoding="UTF-8"?>

<web-app 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_4_0.xsd"
         version="4.0">

    <!--全局初始化参数-->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:applicationContext.xml</param-value>
    </context-param>

    <!--配置监听器-->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>UserServlet</servlet-name>
        <servlet-class>com.company.web.UserServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>UserServlet</servlet-name>
        <url-pattern>/userServlet</url-pattern>
    </servlet-mapping>
</web-app>

第二步主要是在UserServlet.java中操作

 之前自定义的listener就可以不要了