ResponseEntity

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

Java
Kotlin

@GetMapping("/something")
public ResponseEntity<String> handle() {
	String body = ... ;
	String etag = ... ;
	return ResponseEntity.ok().eTag(etag).body(body);
}

@GetMapping("/something")
fun handle(): ResponseEntity<String> {
	val body = ...
	val etag = ...
	return ResponseEntity.ok().eTag(etag).build(body)
}

The body will usually be provided as a value object to be rendered to a corresponding response representation (for example, JSON) by one of the registered HttpMessageConverters.

A ResponseEntity<Resource> can be returned for file content, copying the InputStream content of the provided resource to the response OutputStream. Note that the InputStream should be lazily retrieved by the Resource handle in order to reliably close it after it has been copied to the response. If you are using InputStreamResource for such a purpose, make sure to construct it with an on-demand InputStreamSource (for example, through a lambda expression that retrieves the actual InputStream). Also, custom subclasses of InputStreamResource are only supported in combination with a custom contentLength() implementation which avoids consuming the stream for that purpose.

Spring MVC 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 the following types of async responses:

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.