Files which you have written entirely yourself for your custom modules are not covered under the SPL, and so need not be disclosed as source code. Remember to carefully check over the Binary Code Licenses for third-party binaries included in the NetBeans IDE to be sure that any product you distribute is in compliance with these licenses. (Generally it should suffice to simply ask customers to accept the same licenses they would have to accept if they downloaded the extra binary files from the web site.)

Source: NetBeans User FAQ