Take a Java Thread Dump and Heap Dump with jstack and jcmd

Find the process

systemctl show -p MainPID --value myapp        # PID of a systemd service
pgrep -af java                                 # all Java processes

The JDK tools must run as the same user as the JVM, and with a JDK, not a JRE. Use sudo -u appuser. They are in the openjdk-21-jdk-headless package.

Thread dump

A thread dump shows what every thread is doing right now. Take three, ten seconds apart, so you can see what is stuck and what only looks busy.

for i in 1 2 3; do sudo -u appuser jcmd <pid> Thread.print -l > /tmp/threads-$i.txt; sleep 10; done

If jcmd cannot attach, sudo kill -3 <pid> prints the dump into the application's standard output (the journal or catalina.out). It does not stop the process.

Reading it

Same stack in all three dumps for many threads, often waiting for a lock or a database connection: look for BLOCKED or WAITING and the line waiting to lock <0x...>, then find which thread holds that lock.
Many threads at getConnection in the pool: the connection pool is exhausted, either connections are leaked or queries are slow.
High CPU: find the hot thread. Run top -H -p <pid>, take the thread ID with the highest CPU, convert it to hex (printf '%x\n' TID) and search the dump for nid=0x....
Found one Java-level deadlock at the bottom: jcmd prints it for you.

Upload the dumps to a thread dump analyzer, or read them yourself with grep. Remove customer data before sharing a dump publicly: stacks can include request details.

Heap dump

sudo -u appuser jcmd <pid> GC.heap_dump /var/tmp/heap.hprof
ls -lh /var/tmp/heap.hprof

The file is about the size of the live heap, and taking it pauses the application while it writes. Check df -h first. Compress and copy it off the server:

gzip /var/tmp/heap.hprof
scp deploy@server:/var/tmp/heap.hprof.gz .

Open it in Eclipse Memory Analyzer on your computer. Then delete it from the server, because a heap dump contains passwords, tokens and personal data in plain text. Handle it like a database backup. To get a dump automatically at the moment of failure, use -XX:+HeapDumpOnOutOfMemoryError, see fix OutOfMemoryError.

Other quick checks

sudo -u appuser jcmd <pid> GC.heap_info
sudo -u appuser jcmd <pid> VM.flags
sudo -u appuser jstat -gcutil <pid> 2s 10

These show the current heap use, the effective JVM flags and GC activity, without stopping anything.

Last updated October 29, 2026.

Take a thread dump and a heap dump