Calling transformation function on receiver with Mono<Void> type

Reports transform operations called on Mono<Void> value inside a Reactor chain.

Mono<Void> completes without any value, that's why there is nothing to invoke the transformation function on. The quick-fix suggests replacing map() and flatMap() with then().

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

ReactorTransformationOnMonoVoid

Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Reactive Streams | Reactor

Example:

Mono.when(Mono.just(1), Mono.just(2))
                .map(v -> "New value")
                .doFinally(System.out::println);

After the quick-fix is applied:

Mono.when(Mono.just(1), Mono.just(2))
                .then(Mono.fromCallable(() -> "New value"))
                .doFinally(System.out::println);

New in 2023.1

Suppressing Inspection

You can suppress this inspection by placing the following comment marker before the code fragment where you no longer want messages from this inspection to appear:

//noinspection ReactorTransformationOnMonoVoid

More detailed instructions as well as other ways and options that you have can be found in the product documentation:

IntelliJ IDEA 2025.2

Inspection Details
By default bundled with:	IntelliJ IDEA 2025.2, Qodana for JVM 2025.2,

Last modified: 18 September 2025