This content originally appeared on DEV Community and was authored by realNameHidden
🟢 Imagine this scenario:
Normally, in Spring Boot (MVC), when you write:
@GetMapping("/hello")
public String sayHello() {
return "Hello World!";
}
👉 Here, the method returns a String directly.
The request → executes the method → response is returned immediately.
🟡 Now, what if the response will come later?
Sometimes data doesn’t come instantly (e.g., fetching from DB, calling another API, adding a delay).
Instead of blocking the thread, we want to return a “promise” that will complete later.
That’s where Mono comes in.
Think of Mono as a box that will contain ONE value in the future.
🔹 Example 1: Basic Mono
@GetMapping("/hello")
public Mono<String> sayHello() {
return Mono.just("Hello Reactive World!");
}
👉 Here:
Mono.just("Hello Reactive World!") means:
"I promise I will give you this string — just not necessarily right now."
When the browser calls /hello, Spring WebFlux unwraps the Mono and sends the string.
🔹 Example 2: Mono with Delay
@GetMapping("/delayed")
public Mono<String> delayedHello() {
return Mono.just("Hello after delay!")
.delayElement(Duration.ofSeconds(2)); // respond after 2 seconds
}
👉 Here:
The response comes after 2 seconds, but the server doesn’t block the thread during the wait.
That means the server can serve other requests meanwhile.
🔹 Example 3: Mono with an Object
record User(String id, String name) {}
@GetMapping("/user")
public Mono<User> getUser() {
User user = new User("101", "Alice");
return Mono.just(user);
}
}
👉 Browser Response:
{
"id": "101",
"name": "Alice"
}
}
🟢 Very Simple Analogy
Think of Mono as:
A parcel delivery service 📦
You place an order → you get a tracking ID immediately (Mono).
The actual parcel (data) may come later.
When it arrives, you open it and see the item.
So, Mono = “a container for ONE thing that comes now or later.”
✅ Use Mono when:
Your API returns a single item asynchronously.
You want non-blocking performance in Spring WebFlux.
This content originally appeared on DEV Community and was authored by realNameHidden
realNameHidden | Sciencx (2025-08-31T06:01:31+00:00) Mono in Spring Boot Explained with Simple Examples. Retrieved from https://www.scien.cx/2025/08/31/mono-in-spring-boot-explained-with-simple-examples/
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