Support multipart/mixed and related in DefaultServerWebExchange

See gh-29671

framework-docs/src/docs/asciidoc/web/webflux.adoc

@@ -613,8 +613,8 @@ The `DefaultServerWebExchange` uses the configured `HttpMessageReader` to parse
 ----
 
 The `DefaultServerWebExchange` uses the configured
-`HttpMessageReader<MultiValueMap<String, Part>>` to parse `multipart/form-data` content
-into a `MultiValueMap`.
+`HttpMessageReader<MultiValueMap<String, Part>>` to parse `multipart/form-data`,  
+`multipart/mixed` and `multipart/related` content into a `MultiValueMap`.
 By default, this is the `DefaultPartHttpMessageReader`, which does not have any third-party
 dependencies.
 Alternatively, the `SynchronossPartHttpMessageReader` can be used, which is based on the
@@ -805,9 +805,9 @@ consistently for access to the cached form data versus reading from the raw requ
 ==== Multipart
 
 `MultipartHttpMessageReader` and `MultipartHttpMessageWriter` support decoding and
-encoding "multipart/form-data" content. In turn `MultipartHttpMessageReader` delegates to
-another `HttpMessageReader` for the actual parsing to a `Flux<Part>` and then simply
-collects the parts into a `MultiValueMap`.
+encoding "multipart/form-data", "multipart/mixed" and "multipart/related" content. 
+In turn `MultipartHttpMessageReader` delegates to another `HttpMessageReader` 
+for the actual parsing to a `Flux<Part>` and then simply collects the parts into a `MultiValueMap`.
 By default, the `DefaultPartHttpMessageReader` is used, but this can be changed through the
 `ServerCodecConfigurer`.
 For more information about the `DefaultPartHttpMessageReader`, refer to the

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -163,9 +163,11 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 
 		try {
 			MediaType contentType = request.getHeaders().getContentType();
-			if (MediaType.MULTIPART_FORM_DATA.isCompatibleWith(contentType)) {
+			if (MediaType.MULTIPART_FORM_DATA.isCompatibleWith(contentType) ||
+				MediaType.MULTIPART_MIXED.isCompatibleWith(contentType) ||
+				MediaType.MULTIPART_RELATED.isCompatibleWith(contentType)) {
 				return ((HttpMessageReader<MultiValueMap<String, Part>>) configurer.getReaders().stream()
-						.filter(reader -> reader.canRead(MULTIPART_DATA_TYPE, MediaType.MULTIPART_FORM_DATA))
+						.filter(reader -> reader.canRead(MULTIPART_DATA_TYPE, contentType))
 						.findFirst()
 						.orElseThrow(() -> new IllegalStateException("No multipart HttpMessageReader.")))
 						.readMono(MULTIPART_DATA_TYPE, request, Hints.from(Hints.LOG_PREFIX_HINT, logPrefix))

spring-web/src/test/java/org/springframework/http/server/reactive/MultipartHttpHandlerIntegrationTests.java

@@ -32,6 +32,7 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.multipart.FilePart;
 import org.springframework.http.codec.multipart.FormFieldPart;
 import org.springframework.http.codec.multipart.Part;
+import org.springframework.http.converter.FormHttpMessageConverter;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.client.RestTemplate;
@@ -55,14 +56,38 @@ class MultipartHttpHandlerIntegrationTests extends AbstractHttpHandlerIntegratio
 	}
 
 	@ParameterizedHttpServerTest
-	void getFormParts(HttpServer httpServer) throws Exception {
+	void getFormPartsFormdata(HttpServer httpServer) throws Exception {
+		performTest(httpServer, MediaType.MULTIPART_FORM_DATA);
+	}
+
+	@ParameterizedHttpServerTest
+	void getFormPartsMixed(HttpServer httpServer) throws Exception {
+		performTest(httpServer, MediaType.MULTIPART_MIXED);
+	}
+
+	@ParameterizedHttpServerTest
+	void getFormPartsRelated(HttpServer httpServer) throws Exception {
+		RestTemplate restTemplate = new RestTemplate();
+		restTemplate.getMessageConverters().stream()
+				.filter(FormHttpMessageConverter.class::isInstance)
+				.map(FormHttpMessageConverter.class::cast)
+				.findFirst()
+				.orElseThrow()
+				.addSupportedMediaTypes(MediaType.MULTIPART_RELATED);
+		performTest(httpServer, MediaType.MULTIPART_RELATED, restTemplate);
+	}
+
+	private void performTest(HttpServer httpServer, MediaType mediaType) throws Exception {
+		performTest(httpServer, mediaType, new RestTemplate());
+	}
+
+	private void performTest(HttpServer httpServer, MediaType mediaType, RestTemplate restTemplate) throws Exception {
 		startServer(httpServer);
 
 		@SuppressWarnings("resource")
-		RestTemplate restTemplate = new RestTemplate();
 		RequestEntity<MultiValueMap<String, Object>> request = RequestEntity
 				.post(URI.create("http://localhost:" + port + "/form-parts"))
-				.contentType(MediaType.MULTIPART_FORM_DATA)
+				.contentType(mediaType)
 				.body(generateBody());
 		ResponseEntity<Void> response = restTemplate.exchange(request, Void.class);
 		assertThat(response.getStatusCode()).isEqualTo(HttpStatus.OK);