How to get String from Mono<String> in reactive java

Question

I have a method which accepts Mono as a param. All I want is to get the actual String from it. Googled but didn't find answer except calling block() over Mono object but it will make a blocking call so want to avoid using block(). Please suggest other way if possible. The reason why I need this String is because inside this method I need to call another method say print() with the actual String value. I understand this is easy but I am new to reactive programming.

Code:

public String getValue(Mono<String> monoString) {
    // How to get actual String from param monoString
    // and call print(String) method
}

public void print(String str) {
    System.out.println(str);
}

Terresaterrestrial

could you explain to me what Mono is? – Crispa

check -> musigma.org/java/2016/11/21/reactor.html – Terresaterrestrial

If you use the block(Duration timeout) with zero duration - projectreactor.io/docs/core/release/api/reactor/core/publisher/… – Cicerone

Answer 1

Finally what worked for me is calling flatMap method like below:

public void getValue(Mono<String> monoString)
{
   monoString.flatMap(this::print);
}

Terresaterrestrial

Just an FYI, instead of creating your own print(String str) method and doing this::print, you can simply do System.out::print – Limpopo

Why does this exact piece of code gives me an error: "Cannot infer type argument(s) for <R> flatMap(Function<? super T,? extends Mono<? extends R>>)" ?? – Metamorphose

This gives me the error here – Cucumber

does this answer provide any extra information other than Alexey Romanov's? – Emblazonry

Answer 2

Getting a String from a Mono<String> without a blocking call isn't easy, it's impossible. By definition. If the String isn't available yet (which Mono<String> allows), you can't get it except by waiting until it comes in and that's exactly what blocking is.

Instead of "getting a String" you subscribe to the Mono and the Subscriber you pass will get the String when it becomes available (maybe immediately). E.g.

myMono.subscribe(
  value -> System.out.println(value), 
  error -> error.printStackTrace(), 
  () -> System.out.println("completed without a value")
)

will print the value or error produced by myMono (type of value is String, type of error is Throwable). At https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html you can see other variants of subscribe too.

Bloch

That's what I am looking for-> how to subscribe it? May be via lambda? – Terresaterrestrial

Yes, that's one option. See the edit. But the point is that you can't write getValue method in your question. – Bloch

Actually with string value I need to call another method... Let's say Mono<string> myMono has "abc" as actual string. With that String I need to call another method say print(String str) in that method. So how do I achieve it.. myMono.subscribe(value->print(value)) ? – Terresaterrestrial

Yes, just as I show with Console.out.println in the example. – Bloch

What is Console.out.println? Shouldn't it be System.out.println? – Cepheus

@Cepheus Yes, not sure what I confused it with. – Bloch

Answer 3

According to the doc you can do:

String getValue(Mono<String> mono) {
    return mono.block();
}

be aware of the blocking call

Alleviative

I know usage of block but as it will make a blocking call I don't want to use it. Isn't there another way? – Terresaterrestrial

Answer 4

What worked for me was the following:

monoString.subscribe(this::print);

Limpopo

Answer 5

Simplest answer is:

 String returnVal = mono.block();

Premonitory

But it blocks the call – Itinerant

Answer 6

This should work

String str = monoString.toProcessor().block();

Ludovick

I think this is now deprecated – Whang

Answer 7

Better

monoUser.map(User::getId) 

Sarto

While this might answer the authors question, it lacks some explaining words and/or links to documentation. Raw code snippets are not very helpful without some phrases around them. You may also find how to write a good answer very helpful. Please edit your answer - From Review – Dropline

What's User, monoUser and getId? None of them are in original question. So all you are trying is to solve an hypothetical query. – Terresaterrestrial

This will return a Mono back with the value of the ID --> Mono<Integer> if the id is Integer. I think the question was to return a String and not a Mono – Anodic