Web器皿启动后执行代码的几种方式 _Web前端

Web容器启动后执行代码的几种方式

1.实现Servlet监听器接口ServletContextListener

package com.yli.init;

import java.util.List;
import java.util.Map;

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

import org.springframework.web.context.WebApplicationContext;

import com.suning.teach.service.ShopService;

public class InitListener implements ServletContextListener {

    @Override
    public void contextDestroyed(ServletContextEvent context) {
        // 上下文销毁之后执行
    }

    @Override
    public void contextInitialized(ServletContextEvent context) {
        // 上下文初始化执行
        System.out.println("================>[ServletContextListener]自动加载启动开始.");
        // 读取Spring容器中的Bean[此时Bean已加载,可以使用]
        WebApplicationContext wac = WebContextUtil.getContext();
        if (wac != null) {
            ShopService shopService = wac.getBean("shopService", ShopService.class);
            System.out.println("================>shopService[" + shopService + "]");
            List<Map<String, Object>> shopList = shopService.findAllShop();
            System.out.println("================>" + shopList);
        }
        System.out.println("================>[ServletContextListener]自动加载启动结束.");
    }

}

然后在web.xml文件配置该监听器

	<listener>
		<listener-class>com.yli.init.InitListener</listener-class>
	</listener>

注意：从Spring IOC容器中读取Bean的工具类WebContextUtil

package com.yli.init;

import org.springframework.web.context.ContextLoader;
import org.springframework.web.context.WebApplicationContext;

public class WebContextUtil {
    
    private static WebApplicationContext wac;

    public static WebApplicationContext getContext(){
        if(null == wac){
            wac = ContextLoader.getCurrentWebApplicationContext();
        }
        return wac;
    }
}

2.实现Servlet的过滤器Filter

package com.yli.init;

import java.io.IOException;
import java.util.List;
import java.util.Map;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.springframework.web.context.WebApplicationContext;

import com.suning.teach.service.ShopService;

public class InitFilter implements Filter {

    @Override
    public void destroy() {

    }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException,
            ServletException {

    }

    @Override
    public void init(FilterConfig config) throws ServletException {
        System.out.println("================>[Filter]自动加载启动开始.");
        // 读取Spring容器中的Bean[此时Bean已加载,可以使用]
        WebApplicationContext wac = WebContextUtil.getContext();
        if (wac != null) {
            ShopService shopService = wac.getBean("shopService", ShopService.class);
            System.out.println("================>shopService[" + shopService + "]");
            List<Map<String, Object>> shopList = shopService.findAllShop();
            System.out.println("================>" + shopList);
        }
        System.out.println("================>[Filter]自动加载启动结束.");
    }

}

然后在web.xml文件配置过滤器即可

	<filter>
		<filter-name>InitFilter</filter-name>
		<filter-class>com.yli.init.InitFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>InitFilter</filter-name>
		<url-pattern>/</url-pattern>
	</filter-mapping>

3.编写一个Servlet,在web.xml里面配置容器启动后执行即可

import java.util.List;
import java.util.Map;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;

import org.springframework.web.context.WebApplicationContext;

import com.suning.teach.service.ShopService;

public class InitServlet extends HttpServlet {

    /**
     */
    private static final long serialVersionUID = 1L;

    @Override
    public void init(ServletConfig config) {
        try {
            super.init();
        } catch (ServletException e) {
            e.printStackTrace();
        }
        System.out.println("================>[Servlet]自动加载启动开始.");
        // 读取Spring容器中的Bean[此时Bean已加载,可以使用]
        WebApplicationContext wac = WebContextUtil.getContext();
        if (wac != null) {
            ShopService shopService = wac.getBean("shopService", ShopService.class);
            System.out.println("================>shopService[" + shopService + "]");
            List<Map<String, Object>> shopList = shopService.findAllShop();
            System.out.println("================>" + shopList);
        }
        System.out.println("================>[Servlet]自动加载启动结束.");
    }
}

然后在web.xml文件配置该Servlet的启动方式为：容器启动后执行

	<servlet>
		<servlet-name>InitServlet</servlet-name>
		<servlet-class>com.yli.init.InitServlet</servlet-class>
		<init-param>
			<param-name>username</param-name>
			<param-value>yli</param-value>
		</init-param>
                <!-- 此处指定加载顺序为2,表明还有优先级更高的Servlet要先执行 -->
		<load-on-startup>2</load-on-startup>
	</servlet>
	<servlet-mapping>
		<servlet-name>InitServlet</servlet-name>
		<url-pattern>/</url-pattern>
	</servlet-mapping>

?关于启动后执行，由load-on-startup指定：

（1）当值为0或者大于0时，表示容器在应用启动时就加载这个servlet。值越小，启动优先级越高；

（2）当是一个负数时或者没有指定时，表示该servlet被调用时才加载。

4.如果你使用Spring IOC作为Bean管理容器，那么可以指定init-method

其中init-method表示bean加载成功后，立即执行某个方法。配置如下：start为要执行的方法名称

<!-- service -->
<bean id="shopService" class="com.yli.teach.service.ShopService" init-method="start">
	<property name="shopDao" ref="shopDao" />
</bean>
	
<!-- dao -->
<bean id="shopDao" class="com.yli.teach.dao.ShopDao">
	<property name="jdbcTemplate" ref="jdbcTemplate" />
</bean>

Java代码如下

package com.yli.teach.service;

import java.util.List;
import java.util.Map;

import com.yli.teach.dao.ShopDao;

public class ShopService{

    private ShopDao shopDao;

    // set 注入
    public void setShopDao(ShopDao shopDao) {
        this.shopDao = shopDao;
    }
    
    /**
     * 
     * 〈容器启动后执行方法〉
     */
    public void start(){
        System.out.println("================>[init-method]自动加载启动开始.");
        List<Map<String, Object>> shopList = shopDao.findAllShop();
        System.out.println("================>" + shopList);
        System.out.println("================>[init-method]自动加载启动结束.");
    }
}

5.如果你使用Spring IOC作为Bean管理容器，还可以实现Spring的Bean后置处理器接口：BeanFactoryPostProcessor

表示：该Bean加载完成之后，Spring可以让开发者自定义一些事件，Java代码如下

package com.yli.init;

import java.util.List;
import java.util.Map;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;

import com.suning.teach.service.ShopService;

public class KeyWordInit implements BeanFactoryPostProcessor {

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory factory) throws BeansException {
        System.out.println("================>[BeanFactoryPostProcessor]自动加载启动开始.");
        ShopService shopService = factory.getBean("shopService", ShopService.class);
        List<Map<String, Object>> shopList = shopService.findAllShop();
        System.out.println("================>" + shopList);
        System.out.println("================>[BeanFactoryPostProcessor]自动加载启动结束.");
    }

}

这个Bean在xml文件做正常配置即可，如下

<bean id="keyWordInit" class="com.yli.init.KeyWordInit">
</bean>

=================关于以上5种方式代码执行顺序的说明===================

4===>5===>1===>2===>3

即指定init-method的Bean开始执行

接着实现Spring的Bean后置处理器最先开始执行

然后是Servlet的监听器执行

再接下来是Servlet的过滤器执行

最后才是Servlet执行