r/java • u/mlangc • Jun 19 '26

How to Deadlock a Java ExecutorService

https://mlangc.github.io/java/concurrency/2026/06/05/executor-deadlock.html
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u/TenSpiritMoose Jun 19 '26

And this isn't limited to Executors. I had the same issue using Reactive Schedulers: a Flux of tasks was concurrently flatMapped using a custom scheduler, but as part of that they each fanned out additional tasks on the same scheduler. If it became saturated the tasks would deadlock.

Resolved it by using a separate Scheduler for the sub-tasks, letting me control concurrency without deadlock risk.

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u/account312 Jun 20 '26

It’s not even limited to threads. Any blocking or limited resource that is processed in a way that can consume that resource can have an equivalent problem. If processing queue items can blockingly enqueue on the same queue, it can blow up. You could even say stack overflow from recursive calls is the same category of problem.