[[mvc-ann-return-types]]
= Return Values

[.small]#xref:web/webflux/controller/ann-methods/return-types.adoc[See equivalent in the Reactive stack]#

The next table describes the supported controller method return values. Reactive types are
supported for all return values.

[cols="1,2", options="header"]
|===
| Controller method return value | Description

| `@ResponseBody`
| The return value is converted through `HttpMessageConverter` implementations and written to the
  response. See xref:web/webmvc/mvc-controller/ann-methods/responsebody.adoc[`@ResponseBody`].

| `HttpEntity<B>`, `ResponseEntity<B>`
| The return value that specifies the full response (including HTTP headers and body) is to be converted
  through `HttpMessageConverter` implementations and written to the response.
  See xref:web/webmvc/mvc-controller/ann-methods/responseentity.adoc[ResponseEntity].

| `HttpHeaders`
| For returning a response with headers and no body.

| `ErrorResponse`, `ProblemDetail`
| To render an RFC 9457 error response with details in the body,
  see xref:web/webmvc/mvc-ann-rest-exceptions.adoc[Error Responses]

| `String`
| A view name to be resolved with `ViewResolver` implementations and used together with the implicit
  model -- determined through command objects and `@ModelAttribute` methods. The handler
  method can also programmatically enrich the model by declaring a `Model` argument
  (see xref:web/webmvc/mvc-controller/ann-requestmapping.adoc#mvc-ann-requestmapping-registration[Explicit Registrations]).

| `View`
| A `View` instance to use for rendering together with the implicit model -- determined
  through command objects and `@ModelAttribute` methods. The handler method can also
  programmatically enrich the model by declaring a `Model` argument
  (see xref:web/webmvc/mvc-controller/ann-requestmapping.adoc#mvc-ann-requestmapping-registration[Explicit Registrations]).

| `java.util.Map`, `org.springframework.ui.Model`
| Attributes to be added to the implicit model, with the view name implicitly determined
  through a `RequestToViewNameTranslator`.

| `@ModelAttribute`
| An attribute to be added to the model, with the view name implicitly determined through
  a `RequestToViewNameTranslator`.

  Note that `@ModelAttribute` is optional. See "Any other return value" at the end of
  this table.

| `ModelAndView` object
| The view and model attributes to use and, optionally, a response status.

| `FragmentsRendering`, `Collection<ModelAndView>`
| For rendering one or more fragments each with its own view and model.
  See xref:web/webmvc-view/mvc-fragments.adoc[HTML Fragments] for more details.

| `void`
| A method with a `void` return type (or `null` return value) is considered to have fully
  handled the response if it also has a `ServletResponse`, an `OutputStream` argument, or
  an `@ResponseStatus` annotation. The same is also true if the controller has made a positive
  `ETag` or `lastModified` timestamp check (see xref:web/webmvc/mvc-caching.adoc#mvc-caching-etag-lastmodified[Controllers] for details).

  If none of the above is true, a `void` return type can also indicate "`no response body`" for
  REST controllers or a default view name selection for HTML controllers.

| `DeferredResult<V>`
| Produce any of the preceding return values asynchronously from any thread -- for example, as a
  result of some event or callback. See xref:web/webmvc/mvc-ann-async.adoc[Asynchronous Requests] and xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-deferredresult[`DeferredResult`].

| `Callable<V>`
| Produce any of the above return values asynchronously in a Spring MVC-managed thread.
  See xref:web/webmvc/mvc-ann-async.adoc[Asynchronous Requests] and xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-callable[`Callable`].

| `ListenableFuture<V>`,
  `java.util.concurrent.CompletionStage<V>`,
  `java.util.concurrent.CompletableFuture<V>`
| Alternative to `DeferredResult`, as a convenience (for example, when an underlying service
  returns one of those).

| `ResponseBodyEmitter`, `SseEmitter`
| Emit a stream of objects asynchronously to be written to the response with
  `HttpMessageConverter` implementations. Also supported as the body of a `ResponseEntity`.
  See xref:web/webmvc/mvc-ann-async.adoc[Asynchronous Requests] and xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-http-streaming[HTTP Streaming].

| `StreamingResponseBody`
| Write to the response `OutputStream` asynchronously. Also supported as the body of a
  `ResponseEntity`. See xref:web/webmvc/mvc-ann-async.adoc[Asynchronous Requests] and xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-http-streaming[HTTP Streaming].

| Reactor and other reactive types registered via `ReactiveAdapterRegistry`
| A single value type, for example, `Mono`, is comparable to returning `DeferredResult`.
  A multi-value type, for example, `Flux`, may be treated as a stream depending on the requested
  media type, for example, "text/event-stream", "application/json+stream", or otherwise is
  collected to a List and rendered as a single value. See xref:web/webmvc/mvc-ann-async.adoc[Asynchronous Requests] and
  xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-reactive-types[Reactive Types].

| Other return values
| If a return value remains unresolved in any other way, it is treated as a model
  attribute, unless it is a simple type as determined by
  {spring-framework-api}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty],
  in which case it remains unresolved.
|===
