Mono.subscriberContext() returning old context

I have a class PublishContext with a method context() as follows :

public static Mono<Object> context(){

    return Mono.empty().subscriberContext( context -> {
        Context context1 = context.put("key", "hello");
        System.out.println((String) context1.get("key"));
        return context1;
    });
}

In above code, context object is Context0@744 and context1 is Context@747 which is understandable since context is immutable and always returns a new context.

In my main class, I have the following code :

public static void main(String[] args) {

    Mono<Object> objectMono = PublishContext.context();
    objectMono.subscribe();

    Mono<Object> objectMono1 = Mono.subscriberContext().flatMap(context ->       {
        System.out.println((String) context.get("key"));
        return Mono.empty();
    });
    objectMono1.subscribe();
}

Here, I am getting context as Context0@744, i.e Old context and hence, getting “context is empty” exception. Can someone please explain this behaviour? Also, How can I access context0@747 which I am returning from my context() method?

Answer

• A) the Context write needs to be part of the same chain of operators
• B) it needs to be “downstream” of the part where you need to read it

As such, a “self-contained” sequence where you have the source (Mono.empty()), the Context write (.subscriberContext(...)) AND the final step of subscribing (objectMono.subscribe()) doesn’t make much sense.

Maybe try turning context() into a decorating method?

public static <T> Mono<T> addContext(Mono<T> source){
    return source.subscriberContext( context -> {
        Context context1 = context.put("key", "Hello");
        System.out.println((String) context1.get("key"));
        return context1;
    });
}

Then apply and read it:

public static void main(String[] args) throws Exception {
    Mono<String> mono = Mono.just("foo")
        .flatMap(value -> Mono.subscriberContext()
            .map(ctx -> value + ctx.getOrDefault("key", "UNKNOWN"))
        );
    Mono<String> monoWithCtxWrite = addContext(mono);

    mono.subscribe(System.out::println);
    monoWithCtxWrite.subscribe(System.out::println);
}

This prints (provided the main doesn’t exit too early):

fooUNKNOWN
fooHello