错误提示：cannot resolve symbol import javax.servlet.FilterChain;

Question

这些包不是JDK自带的吗，有好多例子都用这些包了啊。如：import java.io.IOException;import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class IPMonitorFilter implements Filter {private FilterConfig config = null;public void init(FilterConfig config) throws ServletException {
this.config = config;
}public void destroy( ) {
this.config = null;
}public void doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
throws IOException, ServletException
{
if(config == null) {
return;
}String legalIP = this.config.getInitParameter("LegalIP");String thisIP = request.getRemoteAddr( );if(legalIP.equals(thisIP)) {
chain.doFilter(request, response);
} else {
response.setContentType("text/html");
PrintWriter out = response.getWriter( );
out.write("You are not allowed to connect " +
"to this URL at the moment. ");
}}
}