Java Reactor : unhandled exception from map(), but is handled from a fromCallable()

Mono.fromCallable :- Uses Callable as the parameter and creates a new MonoCallable object which extends Mono.So callable is a FunctionalInterface which has a method call() so this call method throws an exception.

For Example :- This won't throw an exception

Callable<JsonNode> callableObj = () -> new ObjectMapper().readTree(jsonString);

but when we call the method call() this will throw an exception.

callableObj.call();

So when we call this we are creating a MonoCallable over here,hence it is not throwing an exception.Since it is of the type Callable.

Mono.fromCallable(() -> {
    return new ObjectMapper().readTree(jsonString);
})

So when we apply a downstream call on this such as map or flatMap it will call the call() method. So if an exception occours it will be wrapped into MonoError and we don't have to explicitly handle.

This is the reason why readTree was not throwing error in Mono.fromCallable.

For further reference you can check callable here :- javadoc for callable where you can check that until you call call() it won't throw an exception.

Whereas in this case :-

Mono.just(jsonString)
.map(jsonString1 -> {
    return new ObjectMapper().readTree(jsonString1);
})
.onErrorResume(IOException.class, error -> {
    return Mono.error(new InvalidContentException("error with the JSON. " + error));
})

It is returning MonoMap which extends Mono in the .map call hence we have to handle it.

The reason is that Mono.fromCallable is taking one Callable interface which is able to throw one exception. However, map takes Function interface which does not throw any exception. But the code inside

.map(jsonString1 -> {
return new ObjectMapper().readTree(jsonString1);})

might throw one checked exception. So the static syntax checking will report error. If map throws one runtime exception, It will not report any error. So one approach is to

try {
    //code that might throw checked exception

} catch(IOException e) 
{
  throw new RuntimeException(e)
}