NOTE
6.33 Thread.sleep
Thread.sleep timing semantics and the important difference between sleeping and waiting on an object monitor.
This is a historical learning note and may contain outdated or incomplete understanding.
1. What sleep Guarantees
Thread.sleep(1000);This does not guarantee that the next Java instruction runs exactly one second later. It requests that the current thread remain non-runnable for at least roughly the requested duration; after that, actual execution still depends on timer granularity and scheduling.
For elapsed-time measurement, use a monotonic clock such as System.nanoTime() rather than wall-clock timestamps.
2. sleep Does Not Release Monitors
If a thread calls sleep while it owns a synchronized monitor, it keeps owning that monitor while sleeping.
This is fundamentally different from Object.wait().
| Operation | Thread.sleep |
Object.wait |
|---|---|---|
| Called on | static Thread method |
monitor object |
| Must own monitor | no | yes |
| Releases that monitor | no | yes |
| Typical wake-up | timeout | notification, timeout, or interruption |
| Interrupted | throws InterruptedException |
throws InterruptedException |
3. Interruption
When sleep throws InterruptedException, the interrupted status is cleared as part of throwing the exception. Code that cannot fully handle the interruption should commonly restore the status with Thread.currentThread().interrupt() and propagate/exit according to its cancellation policy.