The Universal has a log file built into its file system, and you can write to it using the scipt command textmsg(), details of which can be found in the script manual. Since you can’t natively send URScript through java, you have to send scupt using the secondary client interface, the details of which cna be found linked in this post.
You could send messges to the log file by adding secondary client communication in any relevent listener classes, for e.g. when a button is pressed.
You can then access the log history in the log_history.txt file in the home/ursim/ursim-X.X.X.XXXXX in the simulator, or if on a real robot, there is a magic file for taking a copy of the robot log file.
Hope this helps.