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java/j2seplatform should always listen on Services\Platforms\org-netbeans-api-java-Platform folder and update build.properties whenever there is a change. At the moment it does not update build.properties file and listen on the folder only if it already existed before IDE start. It will simplify testability.
The build.properties is modified only by the non default j2seplatforms. If you need this create the j2seplatform by PlatformConvertor.create () method which does what you need.