Spring / Spring Webflux Interview questions
Difference between Mono.just() and Mono.defer().
The key difference between Mono.just() and Mono.defer() lies in when the value or the Mono pipeline is generated.
Mono.just() creates a Mono eagerly at the time of code assembly, capturing the value immediately. The same value is emitted to all subsequent subscribers.
Mono.defer() creates a new Mono instance for each subscriber, with the value being generated at the moment of subscription.
| Feature | Mono.just() | Mono.defer() |
|---|---|---|
| Execution Time | Eager: Value is computed when Mono.just() is called (during pipeline assembly). |
Lazy: Value is computed only when a subscriber subscribes (at subscription time). |
| Value to Subscribers | All subscribers receive the exact same pre-computed value. | Each subscriber can receive a different, freshly computed value. |
| Input Type | Takes a direct value T (must be non-null). |
Takes a Supplier<Mono<T>> (a function that returns a Mono). |
| Use Case | Use for static, constant, or already-computed non-null values. | Use for dynamic values, values that need to be up-to-date, or when the value fetching has side effects or can fail. |
In the Mono.just() case, if a delay occurs between subscriptions, all subscribers still receive the original timestamp. With Mono.defer(), each subscription triggers the System.currentTimeMillis() call again, ensuring the most current timestamp is used.
// Using Mono.just() Mono<Long> justMono = Mono.just(System.currentTimeMillis()); // Time captured NOW, when 'justMono' is created. // Subscribe multiple times: all print the same time justMono.subscribe(time -> System.out.println("Just Time: " + time)); justMono.subscribe(time -> System.out.println("Just Time: " + time)); // Using Mono.defer() Mono<Long> deferMono = Mono.defer(() -> Mono.just(System.currentTimeMillis())); // The supplier (lambda) is executed only upon subscription. // Subscribe multiple times: each can print a different time deferMono.subscribe(time -> System.out.println("Defer Time: " + time)); // Small delay here deferMono.subscribe(time -> System.out.println("Defer Time: " + time));
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