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As of JDK 11, the javafx.* modules are no longer included as part of the JDK. These modules They are now distributed separately as a standalone, unbundled release of OpenJFX. They are available either as as maven artifacts for use with maven/gradle or as a standalone SDK that includes jmod files for use with jlink.
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OpenJFX 11 Early Access Builds - provided by Gluon (also available on jdk.java.net)
Overview
Content Tools
ThemeBuilder