Caused by: java.awt.HeadlessException

The error happens as Integrating SpringBoot application with JavaFX application; 

what does headless mean? 

Headless software (e.g. "headless Java" or "headless Linux",) is software capable of working on a device without a graphical user interface. ... 

solution: 

SpringApplicationBuilder builder = new SpringApplicationBuilder(DemoApplication.class);
builder.headless(false);
applicationContext = builder.run();