I don't know if this has an impact on the demo applets and so on but I remember there has been some discussions on these forums about applet signing. If I read the link correctly the default security in JDK will exclude self signed certificates.
https://blogs.oracle.com/java-platform-group/entry/new_security_requirements_for_rias* Code signatures from a trusted authority. All code for Applets and Web Start applications must be signed.
* Permissions – Introduced in 7u25, and required as of 7u51. Indicates if the RIA should run within the sandbox or require full-permissions.
* Codebase – Introduced in 7u25 and optional/encouraged as of 7u51. Points to the known location of the hosted code (e.g. intranet.example.com).