that seems error-prone to me, since the issue is caused by an incorrect initialization order. Perhaps we should use anonymous class to ensure the initialization order. for example:

        this.expirationReaper = new ShutdownableThread("ExpirationReaper-" + brokerId + "-" + purgatoryName, false) {
            @Override
            public void doWork() {
                try {
                    advanceClock(200L);
                } catch (InterruptedException ie) {
                    throw new RuntimeException(ie);
                }
            }
        };

Hmmm, is it really required as expirationReaper.start(); is post initialization of other objects?

Apologies for unclear comment. My point was that the expirationReaper initialization must happen after brokerId and purgatoryName are initialized. The dependency is implicit, which means we might inadvertently re-introduce a bug during future refactoring. Using an anonymous class could eliminate the happen-before constraint as it can directly reference the input arguments.

Alternatively, we could add comments to explicitly remind developers that expirationReaper must be initialized after brokerId and purgatoryName

Make sense. I have added the comment for now. Though the other suggestion was also valid.