Usually we face Java issues in accessing oracle forms, especially when we are accessing the instance for the first time. This is because of not using instance compatible JRE plugin.
You can download instance compatible JRE plugin using below URL:
http://< hostname >:< port >/OA_HTML/oaj2se.exe
Example
The oaj2se.exe can usually be downloaded manually by adding ‘/OA_HTML/oaj2se.exe’ to the base url of the environment. For example; if the base url of the environment was ‘https://server1.example.com:4443’, you could run ‘https://server1.example.com:4443/OA_HTML/oaj2se.exe’.