I know that IDEA is a project-based editor, so you can't directly associate java files with it.
NetBeans is similarly project-based, though, and you can associate java files with it. If you double-click a java file (or otherwise activate the java file--such as via a modeling tool), and a NetBeans project is open that includes that java file, NetBeans is given focus and the file is opened in the editor.
If NetBeans isn't running, then the file doesn't open and the user receives a message that NetBeans isn't running. If NetBeans is running, but the file isn't in the current project, then the user is given the option to add the file(s) to the current project.
Do you think that it's possible to create a similar hook for IDEA using the OpenAPI? What part of the API should I look at?