写在前面：
代码实现（可直接运行）：filter01
文章目录
一. 过滤器（Filter）[重要 !]
1.1 Filter 的作用
Filter：过滤器 ，用来过滤网站的数据；
- 处理中文乱码
- 登录验证….
1.2 Filter 开发步骤（解决中文乱码）
- 导包
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>4.0.1</version>
</dependency>
<!-- https://mvnrepository.com/artifact/javax.servlet.jsp/javax.servlet.jsp-api -->
<dependency>
<groupId>javax.servlet.jsp</groupId>
<artifactId>javax.servlet.jsp-api</artifactId>
<version>2.3.3</version>
</dependency>
<dependency>
<groupId>javax.servlet.jsp.jstl</groupId>
<artifactId>jstl-api</artifactId>
<version>1.2</version>
</dependency>
<dependency>
<groupId>taglibs</groupId>
<artifactId>standard</artifactId>
<version>1.1.2</version>
</dependency>
<!-- 连接数据库 -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.47</version>
</dependency>
</dependencies>
- 编写过滤器
public class CharacterEncodingFilter implements Filter {
//初始化：web服务器启动，就以及初始化了，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
chain.doFilter(request,response);
*/
@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....");
filterChain.doFilter(servletRequest,servletResponse); //让我们的请求继续走，如果不写，程序到这里就被拦截停止！
System.out.println("CharacterEncodingFilter执行后....");
}
// 销毁：web服务器关闭的时候，过滤会销毁
@Override
public void destroy() {
System.out.println("CharacterEncodingFilter销毁");
}
}
- 在 web.xml 中配置 Filter
<!-- 注册filter -->
<filter>
<filter-name>filter</filter-name>
<filter-class>com.filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>filter</filter-name>
<!-- 只要 /servlet/ * 的任何请求，都会经过过滤器 -->
<url-pattern>/*</url-pattern>
</filter-mapping>
二. 监听器（Listener）
实现一个监听器的接口；（有N种）
实现一个功能：统计网站在线人数（统计 session）
- index.jsp
<%@ page contentType="text/html; charset=UTF-8" language="java" %>
<html>
<body>
<h2>Hello World!</h2>
<h1>当前有<span style="color: blue"><%=this.getServletConfig().getServletContext().getAttribute("OnlineCount")%></span>人在线。</h1>
</body>
</html>
- OnlineCountListener
public class OnlineCountListener implements HttpSessionListener {
// 创建 Session 监听
//一旦创建 Session 就会触发一次这个事件！
@Override
public void sessionCreated(HttpSessionEvent se) {
ServletContext ctx = se.getSession().getServletContext();
System.out.println(se.getSession().getId());
Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount"); // 取对象
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
//一旦销毁 Session 就会触发一次这个事件！
@Override
public void sessionDestroyed(HttpSessionEvent se) {
ServletContext ctx = se.getSession().getServletContext();
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);
}
}
- 在 web.xml 中注册监听器
<!--注册监听器-->
<listener>
<listener-class>com.kuang.listener.OnlineCountListener</listener-class>
</listener>
三. 过滤器、监听器的常见应用
3.1 监听器：GUI 编程中经常使用
public class TestPanel {
public static void main(String[] args) {
Frame frame = new Frame("中秋节快乐"); //新建一个窗体
Panel panel = new Panel(null); //面板
frame.setLayout(null); //设置窗体的布局
frame.setBounds(300,300,500,500);
frame.setBackground(new Color(0,0,255)); //设置背景颜色
panel.setBounds(50,50,300,300);
panel.setBackground(new Color(0,255,0)); //设置背景颜色
frame.add(panel);
frame.setVisible(true);
//监听事件，监听关闭事件
frame.addWindowListener(new WindowAdapter() {
@Override
public void windowOpened(WindowEvent e) {
System.out.println("打开");
}
@Override
public void windowClosed(WindowEvent e) {
System.out.println("关闭ed");
}
@Override
public void windowClosing(WindowEvent e) {
System.out.println("关闭ing");
System.exit(0);
}
});
}
}
3.2 登录页面
功能：
- 用户登录之后才能进入主页；
- 用户注销后就不能进入主页了；
主要代码如下：
- LoginServlet
public class LoginServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 获取前端请求的参数
String username = req.getParameter("username");
if(username.equals("admin")){
// 登录成功
req.getSession().setAttribute("USER_SESSION",req.getSession().getId());
resp.sendRedirect("/sys/success.jsp"); // 重定向
}else{
// 登录失败
resp.sendRedirect("/sys/error.jsp"); // 重定向
}
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
- LogoutServlet
public class LogoutServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
Object user_session = req.getSession().getAttribute("USER_SESSION");
if (user_session != null){
req.getSession().removeAttribute("USER_SESSION");
resp.sendRedirect("/Login.jsp");
}
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
- 添加监听器 SysFilter：
public class SysFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) servletRequest;
HttpServletResponse response = (HttpServletResponse) servletResponse;
if (request.getSession().getAttribute(Constant.USER_SESSION)==null){
response.sendRedirect("/sys/error.jsp");
}
filterChain.doFilter(request,response);
}
@Override
public void destroy() {
}
}