Project reactor - Mono zip with void

May 7, 2022

`Mono<Void>`

In reactor, sometime we want to give certain operation return type like Mono<Void>, like we don’t need any information from the operation, as long as the operation succeed. In this sense, we are using Void as Unit type

awkward case of Unit type in java

a unit type is a type that allows only one value.

https://en.wikipedia.org/wiki/Unit_type

In Java, the unit type is Void and its only value is null.

https://docs.oracle.com/javase/8/docs/api/java/lang/Void.html

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.

So Void supposes to be uninstantiable, but in practices, people use it as unit type along with null

implication in project reactor

In project reactor, there is Mono.zip

Aggregate given monos into a new Mono that will be fulfilled when all of the given Monos have produced an item, aggregating their values according to the provided combinator function. An error or empty completion of any source will cause other sources to be cancelled and the resulting Mono to immediately error or complete, respectively.

this method doesn’t work well with Mono<Void>

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

public class UnitTest {

  @DisplayName("prove Mono void is not zip able, otherwise pipeline should have one element")
  @Test
  void voidIsNotzipable() {
    StepVerifier.create(
            Mono.zip(
                    Mono.<Void>fromCallable(
                        () -> {
                          return null;
                        }),
                    Mono.just(2))
                .map(tuples -> 2))
        .verifyComplete();

    StepVerifier.create(Mono.zip(Mono.just(2).then(), Mono.just(2)).map(tuples -> 2))
        .verifyComplete();
  }
}

a simple workaround

we define our own unit type

import java.io.Serializable;

/** There is only one value of type Unit, () Void with null doesn't play well with Mono.zip */
public class Unit implements Serializable {
  private Unit() {}

  public static final Unit INSTANCE = new Unit();
}

    StepVerifier.create(
            Mono.zip(Mono.just(Unit.INSTANCE), Mono.just(Unit.INSTANCE)).map(tuples -> 2))
        .assertNext(val -> Assertions.assertThat(val).isEqualTo(2))
        .verifyComplete();