ApplicationContext应用上下文获取方式
初步设想代码示例：
<!--pom.xml配置文件-->
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>5.0.5.RELEASE</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.0.1</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>
</dependencies>
public class UserServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
UserService userService = app.getBean(UserService.class);
userService.save();
}
}
<!--web.xml配置文件-->
<servlet>
<servlet-name>UserServlet</servlet-name>
<servlet-class>com.itheima.web.UserServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>UserServlet</servlet-name>
<url-pattern>/userServlet</url-pattern>
</servlet-mapping>
弊端：应用上下文对象是通过new ClasspathXmlApplicationContext(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new ClasspathXmlApplicationContext(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。
改进设想代码示例：
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。
<!--web.xml配置文件-->
<!--配置监听器-->
<listener>
<listener-class>com.study.listener.MyContextLoaderListener</listener-class>
</listener>
public class MyContextLoaderListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent servletContextEvent) {
ServletContext servletContext = servletContextEvent.getServletContext();
ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
//将Spring的应用上下文对象存储到ServletContext域中
servletContext.setAttribute("app",app);
System.out.println("spring容器创建完毕....");
}
public void contextDestroyed(ServletContextEvent servletContextEvent) {
}
}
public class UserServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
ApplicationContext app = (ApplicationContext) servletContext.getAttribute("app");
UserService userService = app.getBean(UserService.class);
userService.save();
}
}
优化设想代码示例：
applicationContext.xml从web.xml配置文件文件中获取；ApplicationContext对象从工具类中直接获取；
<!--web.xml配置文件-->
<!--全局初始化参数-->
<context-param>
<param-name>myContextConfigLocation</param-name>
<param-value>applicationContext.xml</param-value>
</context-param>
public class MyContextLoaderListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent servletContextEvent) {
ServletContext servletContext = servletContextEvent.getServletContext();
//读取web.xml中的全局参数
String contextConfigLocation = servletContext.getInitParameter("myContextConfigLocation");
ApplicationContext app = new ClassPathXmlApplicationContext(contextConfigLocation);
//将Spring的应用上下文对象存储到ServletContext域中
servletContext.setAttribute("app",app);
System.out.println("spring容器创建完毕....");
}
public void contextDestroyed(ServletContextEvent servletContextEvent) {
}
}
public class MyWebApplicationContextUtils {
public static ApplicationContext getWebApplicationContext(ServletContext servletContext){
return (ApplicationContext) servletContext.getAttribute("app");
}
}
public class UserServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
ApplicationContext app = MyWebApplicationContextUtils.getWebApplicationContext(servletContext);
UserService userService = app.getBean(UserService.class);
userService.save();
}
}
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。
所以我们需要做的只有两件事：
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
导入Spring集成web的坐标
<!--pom.xml配置文件-->
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>5.0.5.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
<version>5.0.5.RELEASE</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.0.1</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>
</dependencies>
配置ContextLoaderListener监听器
<!--web.xml配置文件-->
<!--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>
通过工具获得应用上下文对象
public class UserServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
ApplicationContext app = WebApplicationContextUtils.getWebApplicationContext(servletContext);
UserService userService = app.getBean(UserService.class);
userService.save();
}
}