How to get String from Mono 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);
}

If you use the block(Duration timeout) with zero duration - https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html#block-java.time.Duration- — indranil32, Nov 08 '17 at 12:50

Alexey Romanov · Answer 1 · 2022-05-10T14:34:20.463

61

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.

edited May 10 '22 at 14:34

answered Nov 08 '17 at 13:09

Alexey Romanov

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That's what I am looking for-> how to subscribe it? May be via lambda? – nanosoft Nov 08 '17 at 13:10
Yes, that's one option. See the edit. But the point is that you can't write `getValue` method in your question. – Alexey Romanov Nov 08 '17 at 13:24
Actually with string value I need to call another method... Let's say Mono 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)) ? – nanosoft Nov 08 '17 at 14:27
Yes, just as I show with `Console.out.println` in the example. – Alexey Romanov Nov 08 '17 at 14:28
What is Console.out.println? Shouldn't it be System.out.println? – Kingamere May 10 '22 at 14:07
1

@Kingamere Yes, not sure what I confused it with. – Alexey Romanov May 10 '22 at 14:34

ΦXocę 웃 Пepeúpa ツ · Answer 2 · 2017-11-08T12:51:03.960

11

According to the doc you can do:

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

be aware of the blocking call

edited Nov 08 '17 at 12:51

answered Nov 08 '17 at 12:44

ΦXocę 웃 Пepeúpa ツ

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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? – nanosoft Nov 08 '17 at 12:56

score 9 · Accepted Answer · edited Mar 22 '18 at 18:16

9

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

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

edited Mar 22 '18 at 18:16

AjCodez

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answered Nov 09 '17 at 04:52

nanosoft

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Just an FYI, instead of creating your own `print(String str)` method and doing `this::print`, you can simply do `System.out::print` – AjCodez Mar 22 '18 at 17:03
Why does this exact piece of code gives me an error: "Cannot infer type argument(s) for flatMap(Function super T,? extends Mono extends R>>)" ?? – Ray Zhang Jun 13 '19 at 04:11
This gives me the error [here](https://stackoverflow.com/questions/66492668/to-take-a-string-from-a-fluxstring-throws-type-mismatch-cannot-convert-from-d) – rupweb Mar 05 '21 at 12:52
1

does this answer provide any extra information other than Alexey Romanov's? – Lei Yang Apr 28 '21 at 07:52

score 3 · Answer 4 · answered Mar 22 '18 at 17:01

3

What worked for me was the following:

monoString.subscribe(this::print);

answered Mar 22 '18 at 17:01

AjCodez

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score 3 · Answer 5 · answered Jun 02 '20 at 16:48

3

Simplest answer is:

 String returnVal = mono.block();

answered Jun 02 '20 at 16:48

KayV

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But it blocks the call – Spartan May 30 '21 at 10:59

score 1 · Answer 6 · answered Mar 01 '21 at 18:53

1

This should work

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

answered Mar 01 '21 at 18:53

Vijay

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I think this is now deprecated – PatPanda Mar 10 '21 at 22:38

score 0 · Answer 7 · answered Nov 30 '18 at 19:46

0

Better

monoUser.map(User::getId)

answered Nov 30 '18 at 19:46

Armen Arzumanyan

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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](https://stackoverflow.com/help/how-to-answer) very helpful. Please edit your answer - [From Review](https://stackoverflow.com/review/low-quality-posts/21564390) – Nick Dec 01 '18 at 05:08
2

What's User, monoUser and getId? None of them are in original question. So all you are trying is to solve an hypothetical query. – nanosoft Dec 03 '18 at 09:46
This will return a Mono back with the value of the ID --> Mono if the id is Integer. I think the question was to return a String and not a Mono – Ahmed Aziz Oct 06 '21 at 13:34

How to get String from Mono in reactive java

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