Spring Reactor: Mono.zip fails on empty Mono

In case of String it is quite easy to define a default value for the empty case which solves the issue nicely as described in Brian's answer. However, for other custom types it might be difficult to create an empty object for one reason or another. An alternative for these cases is to use Optional. This solution has some heavy boilerplate, though.

Mono<Optional<String>> m1 = Mono.just("A").map(Optional::of).defaultIfEmpty(Optional.empty());
Mono<Optional<String>> m2 = Mono.just("B").map(Optional::of).defaultIfEmpty(Optional.empty());
Mono<Optional<String>> m3 = Mono.<String>empty().map(Optional::of).defaultIfEmpty(Optional.empty());

Mono<String> combined = Mono.zip(strings -> {
    StringBuffer sb = new StringBuffer();
    for (Object string : strings) {
        ((Optional<String>) string).ifPresent(sb::append);
    }
    return sb.toString();
}, m1, m2, m3);
System.out.println("Combined " + combined.block());

The zip operator doesn't behave like this. It would be in fact counter-intuitive: your code is expecting a Tuple of 3 elements and you're only getting two?!?

In this case, you're in control and only you can decide what's a good default value if none is provided (remember, null values are forbidden by the reactive streams specification).

Mono<String> m1 = Mono.just("A");
Mono<String> m2 = Mono.just("B");
Mono<String> m3 = Mono.empty().defaultIfEmpty("");

Mono<String> combined = Mono.when(m1, m2, m3).map(t -> {
    StringBuffer sb = new StringBuffer();
    sb.append(t.getT1());
    sb.append(t.getT2());
    sb.append(t.getT3());
    return sb.toString();
});

Edit

You seem to be confused by the nature of a Publisher type, see:

if one of the Monos is an empty Mono, zip fails without an error

and

So if I was to try and zip Mono's and for some reason one is empty, the zip would fail and I cannot seem to put in any code to safeguard against that

An empty Mono isn't a failure case: it's just that no value is emitted and it is completed successfully. You can verify that by changing the code sample:

    combined.subscribe(
            s -> System.out.println("element: " + s), // doesn't execute
            s -> System.out.println("error: " + s), // doesn't execute
            () -> { System.out.println("complete!"); // prints
    });

So depending on your requirements, you can:

• apply a defaultIfEmpty operator on those 3 Mono instances, if there are convenient default values you can rely on
• apply a defaultIfEmpty operator on the combined Mono, with a default value or even transform that into an error message with combined.switchIfEmpty(Mono.error(...))