last time it was: install tcpdump on java server doing nasty things. save pcap. Analyze with Wireshark, find out the JAVA app was doing insane things against the Oracle DB even after vendo said they had "optimized" database access. Th good thing is the same technique applies to REST apis, sockets, load balancer problems, other databases. You probably can do a better job using each specific stack debug facilities, but a .pcap and wireshark is the swiss army of debugging.
Also: strace for reading what anything in Linux is trying to do when failing.
Also: strace for reading what anything in Linux is trying to do when failing.