Filter&Listen

最新推荐文章于 2025-09-27 10:53:54 发布

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博客介绍了过滤器和监听器相关内容。过滤器可在访问服务器资源时拦截请求，如处理中文乱码、登录验证等，开发步骤包括编写过滤器（实现接口、方法及配置拦截路径）；监听器需实现监听器接口并在 web.xml 中注册。

过滤器 Filter

当访问服务器的资源时，过滤器可以将请求拦截下来，完成开发者所达成的效果。

eg：处理中文乱码、登录验证…

开发步骤

编写过滤器

a.实现接口 implements Filter（import javax.servlet.Filter; ！！！！）

b.实现方法

c.配置拦截路径

eg：

解决中文乱码

Filter.class

public class FilterDemo implements Filter {
    //初始化
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("初始化...");
    }

    //Chain : 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;chatset=UTF-8");

        //让请求继续走，若不写则程序到此结束-->放行 
        filterChain.doFilter(servletRequest,servletResponse);
    }

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

Servler.class

public class ServletDemo extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //未加setCharact...方法回乱码
        resp.getWriter().write("你好哇！世界！");
    }

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

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"
         metadata-complete="true">



    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.rz.servlet.ServletDemo</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

    <!--filter的设置-->
    <filter>
        <filter-name>CharacterFilter</filter-name>
        <filter-class>com.rz.servlet.FilterDemo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>

监听器

实现监听器接口

Listen.class

//统计在线人数=统计session
public class ListenDemo implements HttpSessionListener {
    //创建session监听
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        Integer num = (Integer) servletContext.getAttribute("num");
        if (num == null){
            num = new Integer(1);
        }else {
            int count = num.intValue();
            num = new Integer(count+1);
        }

        servletContext.setAttribute("num",num);
    }

    //销毁session监听
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        Integer num = (Integer) servletContext.getAttribute("num");
        if (num == null){
            num = new Integer(0);
        }else {
            int count = num.intValue();
            num = new Integer(count-1);
        }

        servletContext.setAttribute("num",num);
    }
}

注册监听器

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"
         metadata-complete="true">


    <!--注册监听器-->
    <listener>
        <listener-class>com.rz.servlet.ListenDemo</listener-class>
    </listener>
</web-app>

index.jsp

 <%@ page contentType="text/html;charset=UTF-8" language="java" %>

 <html>
 <head>
     <title>JSP</title>
 </head>
 <body>

    <p>当前有 <i><%=this.getServletConfig().getServletContext().getAttribute("num")%></i> 人在线</p>


 </body>
 </html>