How to keep user information after login in JavaFX Desktop Application

You can use the Java Preferences. At the first successful authentication, you need to write information about user in the Preferences like this:

Preferences userPreferences = Preferences.userRoot();
userPreferences.put(key,value);

And then take the data from the Preferences:

Preferences userPreferences = Preferences.userRoot();
String info = userPreferences.get(key,value);

You can use singleton design patter. For example:

    public final class UserSession {

    private static UserSession instance;

    private String userName;
    private Set<String> privileges;

    private UserSession(String userName, Set<String> privileges) {
        this.userName = userName;
        this.privileges = privileges;
    }

    public static UserSession getInstace(String userName, Set<String> privileges) {
        if(instance == null) {
            instance = new UserSession(userName, privileges);
        }
        return instance;
    }

    public String getUserName() {
        return userName;
    }

    public Set<String> getPrivileges() {
        return privileges;
    }

    public void cleanUserSession() {
        userName = "";// or null
        privileges = new HashSet<>();// or null
    }

    @Override
    public String toString() {
        return "UserSession{" +
                "userName='" + userName + '\'' +
                ", privileges=" + privileges +
                '}';
    }
}

and use the UserSession whenever you need. When you do login you just call: UserSession.getInstace(userName, privileges) and when you do log out: UserSession.cleanUserSession()