RejectedExecutionHandler

1. When Rejection Happens

A running ThreadPoolExecutor rejects when its bounded execution policy cannot accept more work, or after shutdown.

Rejection is an overload signal.

2. Built-In Policies

AbortPolicy

Throws RejectedExecutionException. Explicit and often safest when callers can handle failure.

CallerRunsPolicy

Executes the task in the submitting thread, slowing producers and creating natural backpressure. Be careful if blocking the caller can deadlock or violate latency requirements.

DiscardPolicy

Silently drops the task. Only appropriate when loss is explicitly acceptable and measured.

DiscardOldestPolicy

Drops the oldest queued task then retries submission. This can violate ordering/priority assumptions and should be used deliberately.

3. Custom Policy

A production handler can emit metrics, reject with domain errors, route to another bounded path, or shed low-priority work.

Never solve overload by silently moving rejected work into another unbounded queue.