filter的用法

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class TimerFilter implements Filter {
  private FilterConfig config = null;
  public void init(FilterConfig config) throws ServletException {
    this.config = config;
  }
  public void destroy() {
    config = null;
  }
  public void doFilter(ServletRequest request, ServletResponse response,
                     FilterChain chain) throws IOException, ServletException {
    long before = System.currentTimeMillis();
    chain.doFilter(request, response);
    long after = System.currentTimeMillis();
    String name = "";
    if (request instanceof HttpServletRequest) {
      name = ((HttpServletRequest)request).getRequestURI();
    }
    config.getServletContext().log(name + ": " + (after - before) + "ms");
  }
}