As mentioned in My WiFi Setup with Ubiquiti Networks UAP-AC-PRO I run the Unify Controller software on a Raspberry Pi 3. There is a ready-made package available for Debian and Ubuntu Linux, that can easily be used for this and I have been doing so for more than a year.
Just a yesterday, though, I broke things by overdoing it a bit with the removal of unneeded software from the Raspberry Pi. Through some “chain” the Unifi Controller had been removed and after re-installation it did not work anymore. Instead I saw a constant CPU utilization of an entire core by Java and also errors in
[2017-12-26 13:07:04,783] <launcher> INFO system - *** Running for the first time, creating identity ***
[2017-12-26 13:07:04,791] <launcher> INFO system - UUID: yyyyyyy-yyyy-yyyy-yyyyyy-yyyyyyy
[2017-12-26 13:07:04,817] <launcher> INFO system - ======================================================================
[2017-12-26 13:07:04,819] <launcher> INFO system - UniFi 5.6.26 (build atag_5.6.26_10236 - release) is started
[2017-12-26 13:07:04,819] <launcher> INFO system - ======================================================================
[2017-12-26 13:07:04,867] <launcher> INFO system - BASE dir:/usr/lib/unifi
[2017-12-26 13:07:05,057] <launcher> INFO system - Current System IP: xxx.xxx.xxx.xxx
[2017-12-26 13:07:05,059] <launcher> INFO system - Hostname: zzzz
[2017-12-26 13:07:05,071] <launcher> INFO system - Valid keystore is missing. Generating one ...
[2017-12-26 13:07:05,072] <launcher> INFO system - Generating Certificate[UniFi]... please wait...
[2017-12-26 13:08:33,574] <launcher> INFO system - Certificate[UniFi] generated!
[2017-12-26 13:08:53,004] <UniFi> ERROR system - [exec] error, rc=1
The last couple of lines were showing up repeatedly, so obviously the system tried to restart over and over again. When you search the Internet for this problem, you will find out that you are not alone. Most solutions address available memory and not all people succeed with the various approaches to increase it (typically by removing memory from graphics and increasing swap space).
What I realized was that most discussions were for older versions and a recurring theme was that things changed between minor versions. So something that had worked for v5.6.19 did not necessarily work for v5.6.22 and vice versa. Also, changes to how Java was dealt with were mentioned quite often. Running Java-based applications on Linux can be somewhat delicate, so I do not blame the folks at Ubiquity Networks for that.
This was when I realized that the JVM on my system had changed. Before the accidental cleanup I had used the Oracle 8 JVM that gets installed via the Debian package
oracle-java8-jdk. So I re-installed the latter and configured it as the default JVM via
sudo apt-get install oracle-java8-jdk
sudo update-alternatives --config java
This solved my problems instantly and things are up and running again.