JavaWeb - 过滤器之乱码

package com.imooc.sm.global;
import javax.servlet.*;
import java.io.IOException;
public class EncodingFilter implements Filter {
    private String encoding ="UTF-8";
    public void init(FilterConfig filterConfig) throws ServletException {
        if(filterConfig.getInitParameter("ENCODING")!=null)
            encoding = filterConfig.getInitParameter("ENCODING");
    }
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding(encoding);
        servletResponse.setCharacterEncoding(encoding);
        filterChain.doFilter(servletRequest,servletResponse);
    }
    public void destroy() {
        encoding=null;
    }
}

<filter>
  <filter-name>Encoding</filter-name>
  <filter-class>com.imooc.sm.global.EncodingFilter</filter-class>
  <init-param>
    <param-name>ENCODING</param-name>
    <param-value>UTF-8</param-value>
  </init-param>
</filter>
<filter-mapping>
  <filter-name>Encoding</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>