学习笔记——Filter和Listener详解

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package com.kuang.filter;
import javax.servlet.*;
import java.io.IOException;
public class CharacterEncodingFilter implements Filter {
//初始化：web服务器启动，就已经初始化了，随时等待过滤对象出现！
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("CharacterEncodingFilter初始化");
}
//Chain：链
//1.过滤中的所有代码，在过滤特定请求的时候都会执行
//2.必须要让过滤器继续同行 filterChain.doFilter(servletRequest,servletResponse);
@Override
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，程序到这里就被拦截停止！
filterChain.doFilter(servletRequest,servletResponse);
System.out.println("CharacterEncodingFilter执行后...");
}
//销毁：web服务器关闭的时候，过滤器会销毁
@Override
public void destroy() {
System.out.println("CharacterEncodingFilter销毁");
}
}

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<filter>
<filter-name>CharacterEncodingFilter</filter-name>
<filter-class>com.kuang.filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>CharacterEncodingFilter</filter-name>
<!--只要是 /servlet 的任何请求，会经过这个过滤器-->
<url-pattern>/servlet/*</url-pattern>
</filter-mapping>

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package com.kuang.listener;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
//统计网站在线人数：统计Session
public class OnlineCountListener implements HttpSessionListener{
//创建Session监听
@Override
public void sessionCreated(HttpSessionEvent se) {
ServletContext ctx = se.getSession().getServletContext();
System.out.println(se.getSession().getId());
Integer onLineCount = (Integer) ctx.getAttribute("onLineCount");
if (onLineCount==null){
onLineCount = new Integer(1);
}else{
int count = onLineCount.intValue();
onLineCount = new Integer(count+1);
}
ctx.setAttribute("onLineCount",onLineCount);
}
//销毁Session监听
@Override
public void sessionDestroyed(HttpSessionEvent se) {
ServletContext ctx = se.getSession().getServletContext();
System.out.println(se.getSession().getId());
Integer onLineCount = (Integer) ctx.getAttribute("onLineCount");
if (onLineCount==null){
onLineCount = new Integer(0);
}else{
int count = onLineCount.intValue();
onLineCount = new Integer(count-1);
}
ctx.setAttribute("onLineCount",onLineCount);
}
}

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<!--
注册监听器
-->
<listener>
<listenerclass>com.kuang.listener.OnlineCountListener</listener-class>
</listener>

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HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;
if (request.getSession().getAttribute("USER_SESSION") == null){
response.sendRedirect("/error.jsp");
}
filterChain.doFilter(request,response);