过滤器与监听器

Filter

一个 Filter 程序就是一个 Java 类，这个类必须实现 Filter 接口。javax.servlet.Filter 接口中定义了三个方法：init、doFilter、destory。

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

//控制uri访问权限
@WebFilter(filterName = "UrlFilter",urlPatterns = {"/admin.jsp"})
public class UrlFilter implements Filter {
    public void destroy() {
        System.out.println("过滤结束。。");
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        HttpServletResponse response = (HttpServletResponse)resp;
        HttpServletRequest  request = ( HttpServletRequest)req;
        //向下转型
        String user = (String)request.getSession().getAttribute("user");
        request.getRequestURI();
        //如果用户名为空，返回原页面
        if(user==null){
            response.sendRedirect("/index.jsp");
        }
        //不为空  就放行
        chain.doFilter(req, resp);
    }
    public void init(FilterConfig config) throws ServletException {
        System.out.println("开始过滤");
    }
}

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import java.io.IOException;
//过滤字符    /*代表根目录所有的都过滤  多个urlPatterns = "{"",""}"
@WebFilter(filterName = "MyFilter",urlPatterns = "/*")
public class MyFilter implements Filter {
    public void destroy() {
        System.out.println("过滤结束。。");
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("UTF-8");
        System.out.println("执行过滤");
        chain.doFilter(req, resp);
    }

    public void init(FilterConfig config) throws ServletException {
        System.out.println("开始过滤");
    }
}

//在web.xml配置filter与应射   (servlet3.0以上，tomact版本支持的话，这种方式可以用注解代替)
<filter>
    <filter-name>FirstFilter</filter-name>
    <filter-class>FirstFilter</filter-class>
    <init-param>
    <param-name>encoding</param-name>
    <param-value>GB2312</param-value>
    </init-param>
</filter>
    
<filter-mapping>
    <filter-name>FirstFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Servlet3.0提供注解(如以上两个例子)，这样我们在web应用中使用监听器过滤器等时，不再需要在web.xml文件中配置监听器的相关描述信息了。

执行顺序 ：如果在web.xml配置就按照xml的顺序来。如果在filter中注解配置，按照字符集编码的首字母顺序来执行

Listener

https://blog.csdn.net/menghuanzhiming/article/details/79042182