Replacing Default Schedulers

As we described in the Threading and Schedulers section, Reactor Core comes with several Scheduler implementations. While you can always create new instances through the new* factory methods, each Scheduler flavor also has a default singleton instance that is accessible through the direct factory method (such as Schedulers.boundedElastic() versus Schedulers.newBoundedElastic(…​)).

These default instances are the ones used by operators that need a Scheduler to work when you do not explicitly specify one. For example, Flux#delayElements(Duration) uses the Schedulers.parallel() instance.

In some cases, however, you might need to change these default instances with something else in a cross-cutting way, without having to make sure every single operator you call has your specific Scheduler as a parameter. An example is measuring the time every single scheduled task takes by wrapping the real schedulers, for instrumentation purposes. In other words, you might want to change the default Schedulers.

Changing the default schedulers is possible through the Schedulers.Factory class. By default, a Factory creates all the standard Scheduler through similarly named methods. You can override each of these with your custom implementation.

Additionally, the factory exposes one additional customization method: decorateExecutorService. It is invoked during the creation of every Reactor Core Scheduler that is backed by a ScheduledExecutorService (even non-default instances, such as those created by calls to Schedulers.newParallel()).

This lets you tune the ScheduledExecutorService to be used: The default one is exposed as a Supplier and, depending on the type of Scheduler being configured, you can choose to entirely bypass that supplier and return your own instance or you can get() the default instance and wrap it.

Finally, there is a last customizable hook in Schedulers: onHandleError. This hook is invoked whenever a Runnable task submitted to a Scheduler throws an Exception (note that if there is an UncaughtExceptionHandler set for the Thread that ran the task, both the handler and the hook are invoked).