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Use async when the code is mostly waiting on IO such as database calls, HTTP requests, or message queues. Use threads or process pools for CPU-intensive work or code that cannot cooperate with the event loop. The key is matching the concurrency model to the bottleneck, not adding async everywhere because it seems modern.

Thread pools reuse worker threads. A ThreadLocal value can remain attached after a request ends, retaining large objects or exposing one request’s context to a later task if it is not overwritten. Clear it in a finally block or use a scoped context mechanism with well-defined propagation and cleanup.

Each platform thread consumes native resources, including stack reservation, and excessive runnable threads add scheduling and context-switching overhead. The service may hit native-thread creation errors or degrade before that point. Bound executors, measure queue depth and task latency, and apply backpressure rather than creating one thread per unbounded unit of work.

If every worker submits a task to the same bounded executor and synchronously waits for that task, no worker remains to run queued children. This is thread-pool starvation, not necessarily a monitor deadlock. Avoid blocking composition on the same pool, separate workloads, or use asynchronous completion and bounded concurrency.

Capture repeated thread dumps and identify many threads blocked on the same owner or monitor. Use a profiler or JFR lock events to quantify contention and critical-section time. Reduce lock scope, shard independent state, batch updates, or use concurrent primitives only when they preserve the invariant; verify throughput and correctness under representative load.

OutOfMemoryError identifies an allocation resource that could not be satisfied, such as Java heap, metaspace, direct buffers, or native threads. StackOverflowError usually indicates an excessively deep call chain or runaway recursion in one thread. Read the exact message, capture JVM and container metrics, and use a heap dump or thread stack evidence before changing memory flags.