What's the difference between `@RequestBody` and `@RequestParam`?

`@RequestBody` deserializes the entire HTTP request body (typically JSON) into a Java object via Jackson, used for POST/PUT payloads. `@RequestParam` reads a single named value from the URL query string or form data, used for simple scalar inputs like `?page=2`. Mixing them up — expecting a `@RequestParam` when the client actually sent a JSON body — is a common source of 400 errors.