过滤非法字符

package com.abc.zp.filter;

import java.io.*;
import javax.servlet.*;

public class CharFilter implements Filter
{
public void init(FilterConfig filterConfig)
          throws ServletException
{
//System.out.println("过滤器初始化*****");
}
public void doFilter(ServletRequest request,
              ServletResponse response,
              FilterChain chain)
              throws IOException,
                     ServletException
{
String content = request.getParameter("content");
if(content!=null)
{
if(content.indexOf("AAA")==-1)
{
chain.doFilter(request,response) ;
}
else
{
System.out.println("有非法文字");
}

}
else
{
chain.doFilter(request,response) ;
}
}
public void destroy()
{
System.out.println("过滤器销毁******");
}
}