Shawyeok commented on issue #18269:
URL: https://github.com/apache/pulsar/issues/18269#issuecomment-1297145269
Update:
> `metadata-store-zk-session-watcher` thread can always acquire lock (I
don't understand why, a jvm bug?)
`synchronized` lock is unfair, a thread has big chance acquire lock if it
just release it, see example below:
```java
class Scratch {
private static final CountDownLatch latch = new CountDownLatch(1);
public static void main(String[] args) throws InterruptedException {
ScheduledExecutorService scheduler =
Executors.newSingleThreadScheduledExecutor();
Scratch s = new Scratch();
scheduler.scheduleAtFixedRate(s::checkState, 2, 2, TimeUnit.SECONDS);
latch.await();
while (true) {
s.process();
}
}
private void process() {
checkState();
}
private synchronized void checkState() {
System.out.println(Thread.currentThread().getName() + " checking
state");
if (latch.getCount() > 0) {
latch.countDown();
}
CompletableFuture<Void> future = new CompletableFuture<>();
try {
future.get(2, TimeUnit.SECONDS);
} catch (TimeoutException ignore) {
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " finished");
}
}
```
Output:
```
pool-1-thread-1 checking state
pool-1-thread-1 finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
main checking state
main finished
...
```
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