Filter过滤器（权限过滤&乱码过滤）

• Filter

  • 过滤器: 对用户请求进行预处理
  • 对Web应用进行资源控制
  • 请求与结果都需要通过过滤器
  • 流程图:
    

• 实现Filter步骤

  • 实现Filter接口
  • 配置Filter
  • 实现接口&配置示例:

package com.eric.filter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class HelloFilter implements Filter {
    @Override
    public void init(FilterConfig config) throws ServletException {
        Filter.super.init(config);
    }
    @Override
    //实现过滤操作
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
            throws IOException, ServletException {
        //执行Servlet前
        System.out.println("before servlet");
        System.out.println("hello filter");

        //调用下一个Filter
        chain.doFilter(req, resp);

        //执行Servlet后
        System.out.println("after servlet");
    }
    @Override
    public void destroy() {
        Filter.super.destroy();
    }
}

<!-- 配置filter -->
<filter>
  <filter-name>helloFilter</filter-name>
  <filter-class>com.eric.filter.HelloFilter</filter-class>
</filter>
<filter-mapping>
  <filter-name>helloFilter</filter-name>
  <url-pattern>/*</url-pattern> <!-- 设置过滤内容 -->
</filter-mapping>

• Filter生命周期

  • Filter在服务器启动时，进行实例化和初始化，由服务器管理

• 权限过滤

package com.eric.filter;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class AuthorizationFilter implements Filter{
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    /* 
     * 检查用户是否登陆
     * 登陆，跳转到下一个Filter
     * 未登陆，跳转到登陆页面
     */
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        /*
         * 登陆失败需要让login.jsp通过Filter
         * 避免陷入重定向循环
         * 难点: 需要使部分页面，避免被过滤器过滤掉
         * login.jsp / login
         */
        //获得请求的页面信息
//      String path = req.getRequestURL().substring(req.getRequestURL().lastIndexOf("/") + 1);
//      System.out.println(path);
//      
//      if("login.jsp".equals(path) || "login".equals(path)) {
//          //避免login.jsp再次被过滤，陷入死循环
//          chain.doFilter(request, response);
//          return ;
//      }
//      Object obj = req.getSession().getAttribute("user");
//      if(obj == null) {
//          resp.sendRedirect("login.jsp");
//          return ;
//      }
//      chain.doFilter(request, response);

        /* 优化后的代码
         * 从session中读取用户信息，如果没有信息，则说明用户没有登陆，有一下两种情况需要处理
         * 第一种：没有登陆就去请求其他资源（这些资源要求用户登陆）
         * 直接跳回登陆页面
         * 第二种：没有登陆，现在去登陆: login/login.jsp
         * 让其进行登陆验证
         * 如果从session中获取到用户信息，说明用户已经登陆，不需要过滤
         */
        Object obj = req.getSession().getAttribute("user");
        if(obj == null) {
            String path = req.getRequestURL().substring(req.getRequestURL().lastIndexOf("/") + 1);
            //未登陆，切访问path不是login
            if(!("login.jsp".equals(path) || "login".equals(path))) {
                resp.sendRedirect("login.jsp");
                return ;
            }
        }
        chain.doFilter(request, response);
    }
    @Override
    public void destroy() {
    }   
}

• 乱码过滤

package com.eric.filter;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

public class CharacterEncodingFilter implements Filter {
    String encoding = null;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        encoding = filterConfig.getInitParameter("encoding");

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        if(encoding == null || encoding == "") {
            encoding = "utf-8";
        }

        request.setCharacterEncoding(encoding); // 只能解决post请求
        response.setCharacterEncoding(encoding);
        chain.doFilter(new HttpServletRequestWrapper((HttpServletRequest) request) {
            @Override
            //重新封装请求对象
            public String getParameter(String name) {
                if("get".equals(this.getMethod().toLowerCase())) {
                    //如果方法为get提交方式，则转换编码格式
                    String value = super.getParameter(name);
                    try {
                        //转换编码格式
                        return new String(value.getBytes("iso-8859-1"), encoding);
                    } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                    }
                }
                return super.getParameter(name);
            }
        } , response);
    }

    @Override
    public void destroy() {
    }
}