过滤器和监听器的使用

Filter

• xml配置：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>javaFilterStudying</artifactId>
    <version>1.0-SNAPSHOT</version>


    <dependencies>
        <!--Servlet 依赖-->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
        </dependency>
        <!--JSP 依赖-->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>
        <!--JSTL表达式的依赖-->
        <dependency>
            <groupId>javax.servlet.jsp.jstl</groupId>
            <artifactId>jstl-api</artifactId>
            <version>1.2</version>
        </dependency>
        <!--standard标签库-->
        <dependency>
            <groupId>taglibs</groupId>
            <artifactId>standard</artifactId>
            <version>1.1.2</version>
        </dependency>
        <!--数据库依赖-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
    </dependencies>
</project>

• 编写过滤器

package com.filter;

import javax.servlet.*;
import java.io.IOException;

/**
 * @author 125827
 */
public class CharacterEncodingFilter implements Filter {

    // 初始化
    public void init(FilterConfig filterConfig) {
        System.out.println("characterEncodingFilter初始化");
    }
    //filter Chain过滤链
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("UTF-8");
        servletResponse.setCharacterEncoding("UTF-8");
        servletResponse.setContentType("text/html;charset=UTF-8");

        System.out.println("CharacterEncodingFilter执行前....");
        //让我们的请求继续走，如果不写，科序到这里就被拦截停止!
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("CharacterEncodingFilter执行后....");

    }
    // 销毁
    public void destroy() {
        System.out.println("characterEncodingFilter销毁");
    }
}

配置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>showServlet</servlet-name>
        <servlet-class>com.servlet.ShowServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>showServlet</servlet-name>
        <url-pattern>/servlet/show</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>showServlet</servlet-name>
        <url-pattern>/show</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>
</web-app>

• servlet类

package com.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * @author 125827
 */
public class ShowServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().write("你好，世界!");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

监听器

实现一个监听器的接口;(有N种)

1、编写一个监听器实现

​ 监听器的接口.….

package com.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

/**
 * @author 125827
 * @ 统计网站在线人数
 */
public class OnlineListener implements HttpSessionListener {
    // 创建session监听:看你的一举一动
    //一旦创建Session就会触发一次这个事件
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
        System.out.println( httpSessionEvent.getSession().getId());
        Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
        if(onlineCount == null ){
            onlineCount = new Integer(1);
        }else{
            int count = onlineCount.intValue();
            onlineCount = new Integer(count + 1);
        }
        servletContext.setAttribute("OnlineCount",onlineCount);

    }
    //销毁session监助
    //一旦销毁Session就会触发一次这个事件!
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
        Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
        if(onlineCount == null ){
            onlineCount = new Integer(0);
        }else{
            int count = onlineCount.intValue();
            onlineCount = new Integer(count - 1);
        }
        servletContext.setAttribute("OnlineCount",onlineCount);
    }
    /* <!--session销毁;
		1.手动销毁	getsession().invalidate( );
		2．自动销毁-->
        <session-config>
            <session-timeout>1</session-timeout>
        </session-config>
    */
}

2、注册监听器(xml)

    <!--注册监听器-->
    <listener>
        <listener-class>com.listener.OnlineListener</listener-class>
    </listener>

3、看情况是否使用