Filter 拦截器

拦截器概念

在HttpServletRequest 到达Servlet 之前，拦截客户的HttpServletRequest 。
根据需要检查HttpServletRequest ，也可以修改HttpServletRequest 头和数据。

编写拦截器

package com.filter;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
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;
/**
 * @author 19745
 */
public class FirstFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        request.setCharacterEncoding("UTF-8");
//        String ip = request.getRemoteAddr();
//        String url = request.getRequestURL().toString();
//        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
//        Date d = new Date();
//        String date = sdf.format(d);
//
//        System.out.printf("%s %s 访问了 %s%n", date, ip, url);
        filterChain.doFilter(request, response);
    }
    @Override
    public void init(FilterConfig arg0) throws ServletException {
    }
}

配置web.xml

<filter>
    <filter-name>FirstFilter</filter-name>
    <filter-class>com.filter.FirstFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>FirstFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

url-pattern内的/* 代表拦截所有资源

中文处理

在通过Servlet获取中文参数 的章节中知道，可以通过

request.setCharacterEncoding(“UTF-8”);

正确获取UTF-8编码的中文。但是如果有很多servlet都需要获取中文，那么就必须在每个Servlet中增加这段代码。
有一个简便的办法，那就是通过Filter过滤器进行中文处理 ，那么所有的Servlet都不需要单独处理了。

HttpServletRequest request = (HttpServletRequest) servletRequest;
HttpServletResponse response = (HttpServletResponse) servletResponse;
request.setCharacterEncoding("UTF-8");

注意，需要重启tomcat服务。

在拦截器中添加request中文处理

取消拦截器和servlert中的request的中文处理

登录验证

如果session中没有保存username，强制跳转回登录界面。

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        String uri = request.getRequestURI();
        System.out.println("uri=" + uri);
        if (uri.endsWith("login.html") || uri.endsWith("login")) {
            chain.doFilter(request, response);
            return;
        }
        String userName = (String) request.getSession().getAttribute("userName");
        if (null == userName) {
            response.sendRedirect("login.html");
            return;
        }
        chain.doFilter(request, response);
    }