Legacy software is often used longer than planned. Running older Java software on high-resolution displays with more than 1080p is quite hard by default.
To fix this, manually setting scaling via command-line pararameters is sufficient:
$ java -Dsun.java2d.uiScale=3.5 -jar app.jar
In this case, 3.5x or 350% scaling is set. Alternatively you can also set the environment variable GDK_SCALE:
$ GDK_SCALE=3.5 java -jar app.jar
This should make applications human readable again: