Thread Troubleshooting

Troubleshoot thread contention by collecting thread dumps (jstack), counting BLOCKED threads, finding the lock owner, and diagnosing the root cause: excessive locking, undersized pool, slow downstream, or connection pool exhaustion.

Rising thread count with low CPU indicates threads are blocked on slow IO, leaked by unbounded creation, or waiting for exhausted connection pools, and the fix is timeouts and bounded resources, not more threads.

When the app feels slow but logs show no errors, the problem is hidden wait time between steps, and the fix is to segment the request timeline and use distributed tracing to find where time accumulates.

ReentrantLock waiters show as WAITING (parked by AQS), not BLOCKED, making contention invisible in basic thread dumps; use JFR jdk.ThreadPark events or async-profiler -e lock to detect and measure it.

Capture a thread dump with jstack, kill -3, or jcmd, then read each thread's state and lock ownership to find the stuck thread or the deadlock section.