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);
}

Answer 1

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.

Answer 2

According to the doc you can do:

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

be aware of the blocking call

Answer 3

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

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

Answer 4

What worked for me was the following:

monoString.subscribe(this::print);

Answer 5

Simplest answer is:

 String returnVal = mono.block();

Answer 6

Better

monoUser.map(User::getId)