project reactor - How to return a Mono with a long running task inside map? -

i want transform data inside map function of mono:

long result = 0.0;  return mono.just(result).map(value -> {     // long running transformation here     // , assign result (maybe 5 seconds task)     // in our case request:      mono<result> resultobject = service.getresult();      resultobject.subscribe(new subscriber<result>() {         @override         public void onsubscribe(subscription s) {             system.out.println("subscribe: " + system.currenttimemillis());              s.request(1);         }          @override         public void onnext(result result) {             system.out.println("on next: " + system.currenttimemillis());              value = result.getvalue(); // not 0.0         }          @override         public void onerror(throwable t) {             system.out.println("error " + t);         }          @override         public void oncomplete() {              system.out.println("complete");         }     });      return value; }); 

if call 0.0 result returning before map function done. me not make sense. how else supposed transform result before returning it?

edit

i following in opinion not optimal solution:

final countdownlatch latch = new countdownlatch(1); long result = 0.0;  return mono.just(result).map(value -> {     // long running transformation here     // , assign result (maybe 5 seconds task)     // in our case request:      mono<result> resultobject = service.getresult();      resultobject.subscribe(new subscriber<result>() {         @override         public void onsubscribe(subscription s) {             system.out.println("subscribe: " + system.currenttimemillis());              s.request(1);         }          @override         public void onnext(result result) {             system.out.println("on next: " + system.currenttimemillis());              value = result.getvalue(); // not 0.0              latch.countdown();         }          @override         public void onerror(throwable t) {             system.out.println("error " + t);         }          @override         public void oncomplete() {              system.out.println("complete");         }     });      try {         latch.await();          return value;     } catch(interruptedexception e) {         e.printstacktrace();          return -1.0;     } }); 

that sounds flatmap for: if long running task asynchronous , can represented publisher<t> can triggered asynchronously flatmap.

note mono#flatmap(function) called mono#then(function) in 3.0.x.

so in 3.0.7:

mono.just(param).then(p -> service.getresult(p)); 

and in 3.1.0.m3:

mono.just(param).flatmap(p -> service.getresult(p)); 

note if don't use value (service doesn't have parameter) can provide continuation mono, using then(mono) (which valid in both 3.0.x , 3.1.x):

mono.just(paramthatgetsignored).then(service.getresult()); 

(but in case starting point of mono.just(...) isn't terribly relevant)