Now I'm using the custom tags in my project with JBuilder 6. I wrote a jsp page which uses a tld file, the tld file has a common header: <?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD Jsp Tag Library 1.2//EN" "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd"> But when compiling, the JBuilder told me that: "SimplePrimeExample.jsp": org.apache.jasper.JasperException: Parse Error in the tag library descriptor: External entity not found: "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd". I surpose that this is just because I'm behind the firewall, and JBuilder cannot connect to the internet directly so cannot find the dtd file specified. I don't know if my explanation is right and if there is any solution to this case. Please help me, thanks!