Which JDK tools do you use for concurrency problems?

Print statements
Correct: jcmd and jstack for dumps, JFR in production, async-profiler for lock contention
A debugger, stepping through each thread
VisualVM only

The toolbox worth naming:

jcmd / jstack: thread dumps; jcmd Thread.dump_to_file -format=json for virtual threads.
JFR: jcmd JFR.start settings=profile. Events for monitor contention (jdk.JavaMonitorEnter), park duration, thread starts and virtual thread pinning, with low enough overhead for production.
async-profiler: -e lock for lock contention and wall-clock mode, which is the one that shows where threads wait rather than where they burn CPU.
jcstress: for memory model questions, where an ordinary test cannot reproduce the effect at all.
Thread MXBean: findDeadlockedThreads() programmatically, for a health check endpoint.

A debugger is the tool that least often helps, because stopping at a breakpoint changes the interleaving you are trying to observe.

diagnostics tooling