Running a JNLP application file using javaws would terminate with this error:
Caused by: java.lang.ClassNotFoundException: sun.misc.BASE64Encoder
Turns out that my Ubuntu was using OpenJDK 11 and Oracle had removed the
sun.misc.BASE64Encoder class in Java 9.
Once I installed OpenJDK 8 (Java 8) and made it the default Java, I was able to run the JNLP application without any errors.