ResponseEntity is like @ResponseBody but with status and headers. For example:

  • Java

  • Kotlin

public ResponseEntity<String> handle() {
	String body = ... ;
	String etag = ... ;
	return ResponseEntity.ok().eTag(etag).body(body);
fun handle(): ResponseEntity<String> {
	val body: String = ...
	val etag: String = ...
	return ResponseEntity.ok().eTag(etag).build(body)

WebFlux supports using a single value reactive type to produce the ResponseEntity asynchronously, and/or single and multi-value reactive types for the body. This allows a variety of async responses with ResponseEntity as follows:

  • ResponseEntity<Mono<T>> or ResponseEntity<Flux<T>> make the response status and headers known immediately while the body is provided asynchronously at a later point. Use Mono if the body consists of 0..1 values or Flux if it can produce multiple values.

  • Mono<ResponseEntity<T>> provides all three — response status, headers, and body, asynchronously at a later point. This allows the response status and headers to vary depending on the outcome of asynchronous request handling.

  • Mono<ResponseEntity<Mono<T>>> or Mono<ResponseEntity<Flux<T>>> are yet another possible, albeit less common alternative. They provide the response status and headers asynchronously first and then the response body, also asynchronously, second.