How to extract value from MonoNext (and convert to byte[]) - mono

Question

Map<String,Mono<byte[]>> map = new HashMap<>();
List<User> userList = new ArrayList<>();
map.entrySet().stream().forEach(entry -> {
if (entry.getValue() == null) {
log.info("Data not found for key {} ", entry.getKey());
} else {
entry.getValue().log().map(value -> {
try {
return User.parseFrom(value);
} catch (InvalidProtocolBufferException e) {
e.printStackTrace();
}
return null;
}).log().subscribe(p -> userList.add(p));
}
here entry.getValue() => MonoNext
parseFrom(accepts byte[])
I am new to reactive programming world, How to resolve this MonoNext to values it actually have, tried using flatMap instead but that also didnot work
Any suggestion appreciated !! Thanks in advance !!

Answer 1

MonoNext (an internal Reactor implementation of Mono) emits the value asynchronously, which means that it might not have yet the value when evaluated in your code. The only way to retrieve the value is to subscribe to it (either manually or as part of a Reactor pipeline using flatMap and others) and wait until the Mono emits its item.
Here is what your code would look like if placed in a Reactor pipeline using flatMap:
Map<String, Mono<byte[]>> map = new HashMap<>();
List<User> userList = Flux.fromIterable(map.entrySet())
.filter(entry -> entry.getValue() != null)
.doOnDiscard(Map.Entry.class, entry -> log.info("Data not found for key {} ", entry.getKey()))
.flatMap(entry -> entry.getValue()
.log()
.map(User::parseFrom)
.onErrorResume(error -> Mono.fromRunnable(error::printStackTrace)))
.collectList()
.block();
Note that the block operator will wait until all items are retrieved. If you want to stay asynchronous, you can remove the block and return a Mono<List<User>>, or also remove the collectList to return a Flux<User>.