Stream returns wrong type - spring-webflux

Question

I'm trying to understand reactive style. But stuck on this example.
public class ScriptServiceImpl implements ScriptService{
private static Logger log = LoggerFactory.getLogger(ScriptServiceImpl.class);
private final ScriptEngineManager manager = new ScriptEngineManager();
private final ScriptEngine engine = manager.getEngineByName("JavaScript");
#Override
public Flux<MyFunctionResult> evaluate(MyFunction myFunction, Integer iterations){
Flux<MyFunctionResult> flux = Flux.empty();
flux.mergeWith(
Flux.range(1,iterations)
.map(counter -> {
engine.put("parametr", counter);
try {
long start = System.currentTimeMillis();
String functionResult = engine.eval(myFunction.getSource()).toString();
long timer = System.currentTimeMillis() - start;
return Mono.just(new MyFunctionResult(timer, functionResult, myFunction.getNumber(), counter));
} catch (ScriptException ex) {
return Mono.error(ex);
}
})
);
return flux;
}
}
I want to return Flux of MyFunctionResult but get Flux of Object in Flux.mergeWith section. What am i doing wrong?

Answer 1

There are multiple issues here
you don't need to wrap MyFunctionResult into Mono. map expects none-reactive return type. As result, instead of Mono.error you should just wrap checked exception into unchecked RuntimeException.
you need to return result of the flux.mergeWith and not flux. But in general for this example you don't need mergeWith
Your code could be converted into
return Flux.range(1,iterations)
.map(counter -> {
engine.put("parametr", counter);
try {
long start = System.currentTimeMillis();
String functionResult = engine.eval(myFunction.getSource()).toString();
long timer = System.currentTimeMillis() - start;
return new MyFunctionResult(timer, functionResult, myFunction.getNumber(), counter);
} catch (ScriptException ex) {
throw Exceptions.propagate(ex);
}
});
In addition, not sure about engine.eval but in case this is blocking code consider wrapping it and run on a separate scheduler How Do I Wrap a Synchronous, Blocking Call?