Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

deadlock

Threads each waiting for a lock another of them holds, so none can ever proceed.

A deadlock needs mutual exclusion, hold and wait, no preemption and circular wait; the JVM never breaks one between monitors, but a thread dump reports it.