过滤器的用法？

对客户端的请求参数统一编码 对未登录用户进行拦截

参考解答：

以对请求参数统一编码过滤器为例：

package org.yihang.common.filter;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

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 EncodingFilter implements Filter {

    private String charset = "UTF-8";

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String charset = filterConfig.getInitParameter("charset");
        if (charset != null) {
            this.charset = charset;
        }
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;        
        req.setCharacterEncoding(charset);
        chain.doFilter(request, response);
    }

    @Override
    public void destroy() {
    }

}

在web.xml配置：

<filter>
    <filter-name>encoding</filter-name>
    <filter-class>org.yihang.common.filter.EncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>encoding</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>