java.awt.HeadlessException - 未显示Applet

I am using Tomcat 5.5.23, JDK 1.5 on HP Unix. We have an application which when invoked form tomcat starts an applet. It was working fine till JDK 1.4. But now we have moved to JDK 1.5 and the applet does not start. The exception thrown is - java.awt.HeadlessException: No X11 DISPLAY variable was set, but this program performed an operation which requires it.I am using Tomcat 5.5.23, JDK 1.5 on HP Unix. W