Investigate possibility to use a lazy threadpool for Quartz backed scheduler

A recently merged PR https://github.com/quarkusio/quarkus/pull/4554 adds code which doesn’t start the Scheduler if there’s no @Scheduled annotations detected. A side-effect of this is that the Schduler if injected and used programmatically to start timers, will no longer function.

The reason why the start() of the scheduler has been skipped, when no @Scheduled is present is to prevent Quartz from eagerly starting a threadpool with certain (configured) number of threads. Details here https://github.com/quarkusio/quarkus/issues/4243. Looking at the implementation of the default and the only functional thread pool that comes in Quartz (org.quartz.simpl.SimpleThreadPool), it creates a fixed set of threads eagerly and doesn’t let users configure the pool to be lazily initialized. This enhancement request is to investigate if it’s possible to use Quarkus internal thread pool as an implementation of org.quartz.spi.ThreadPool

[mkouba]

Just one more comment - the handler passed to the Vertx.setTimer() method is executed using the event loop thread and so the code should not block. If you need to perform some blocking/long-running operation use the Vertx.executeBlocking() method inside the handler:

@Inject Vertx vertx;
// …
vertx.setTimer(delay, id -> {
   vertx.executeBlocking(p -> {
      // ...
    }, p -> {});
});

See https://vertx.io/docs/vertx-core/java/#blocking_code for more information.

[mkouba]

That means that the Scheduler extension API only supports declarative approach via the @Scheduled annotation.

for cron-like tasks

Not really. You can use @Scheduled(every="10s") where the value is parsed with Duration#parse(). The value is always rounded to full second.

For programmatic approach you could try Vertx.setTimer(long, Handler<Long>) and Vertx.setPeriodic(long, Handler<Long>) methods for non-blocking stuff (see https://vertx.io/docs/vertx-core/java/#_executing_periodic_and_delayed_actions). We will probaly add support for an injectable java.util.concurrent.ScheduledExecutorService for blocking tasks.