In fact javafx JRE comes in a jar.
The reason why you are a new java project, call javafx class, will complain, perhaps because you set jdk1.8, the java-Installed JRE-Execution .... eclipse set in, javaSE 1.8, the right is not checked.
Right or project -build path-libraries selected jre click edit selected workspace default jre, the error will not ....
This could also be a reason not javafx popular bar ....
Also: There javafx-src packages in the jar jdk directory, javafx is open source.