Optional @RequestPart Mono arg resolves to Mono.empty

Closes gh-23060

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestPartMethodArgumentResolver.java

@@ -99,13 +99,9 @@ public class RequestPartMethodArgumentResolver extends AbstractMessageReaderArgu
 			// Mono<Part> or Flux<Part>
 			MethodParameter elementType = parameter.nested();
 			if (Part.class.isAssignableFrom(elementType.getNestedParameterType())) {
-				parts = (adapter.isMultiValue() ? parts : parts.take(1));
 				return Mono.just(adapter.fromPublisher(parts));
 			}
-			// We have to decode the content for each part, one at a time
-			if (adapter.isMultiValue()) {
-				return Mono.just(decodePartValues(parts, elementType, bindingContext, exchange, isRequired));
-			}
+			return Mono.just(adapter.fromPublisher(decodePartValues(parts, elementType, bindingContext, exchange, isRequired)));
 		}
 
 		// <T> or Mono<T>

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestPartMethodArgumentResolverTests.java

@@ -60,6 +60,7 @@ import static org.springframework.web.method.MvcAnnotationPredicates.*;
 /**
  * Unit tests for {@link RequestPartMethodArgumentResolver}.
  * @author Rossen Stoyanchev
+ * @author Ilya Lukyanovich
  */
 public class RequestPartMethodArgumentResolverTests {
 
@@ -131,6 +132,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("James", actual.get(1).getName());
 	}
 
+	@Test
+	public void listPersonNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(List.class, Person.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		List<Person> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual).isEmpty();
+	}
+
 	@Test
 	public void monoPerson() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Mono.class, Person.class);
@@ -141,6 +151,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("Jones", actual.block().getName());
 	}
 
+	@Test
+	public void monoPersonNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(Mono.class, Person.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		Mono<Person> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual.block()).isNull();
+	}
+
 	@Test
 	public void fluxPerson() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Flux.class, Person.class);
@@ -154,6 +173,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("James", persons.get(1).getName());
 	}
 
+	@Test
+	public void fluxPersonNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(Flux.class, Person.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		Flux<Person> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual.collectList().block()).isEmpty();
+	}
+
 	@Test
 	public void part() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Part.class);
@@ -177,6 +205,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("{\"name\":\"James\"}", partToUtf8String(actual.get(1)));
 	}
 
+	@Test
+	public void listPartNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(List.class, Part.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		List<Part> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual).isEmpty();
+	}
+
 	@Test
 	public void monoPart() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Mono.class, Part.class);
@@ -188,6 +225,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("{\"name\":\"Jones\"}", partToUtf8String(part));
 	}
 
+	@Test
+	public void monoPartNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(Mono.class, Part.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		Mono<Part> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual.block()).isNull();
+	}
+
 	@Test
 	public void fluxPart() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Flux.class, Part.class);
@@ -201,6 +247,15 @@ public class RequestPartMethodArgumentResolverTests {
 		assertEquals("{\"name\":\"James\"}", partToUtf8String(parts.get(1)));
 	}
 
+	@Test
+	public void fluxPartNotRequired() {
+		MethodParameter param = this.testMethod.annot(requestPart().notRequired()).arg(Flux.class, Part.class);
+		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
+		Flux<Part> actual = resolveArgument(param, bodyBuilder);
+
+		assertThat(actual.collectList().block()).isEmpty();
+	}
+
 	@Test
 	public void personRequired() {
 		MethodParameter param = this.testMethod.annot(requestPart()).arg(Person.class);
@@ -278,7 +333,13 @@ public class RequestPartMethodArgumentResolverTests {
 			@RequestPart("name") Flux<Part> partFlux,
 			@RequestPart("name") List<Part> partList,
 			@RequestPart(name = "anotherPart", required = false) Person anotherPerson,
+			@RequestPart(name = "name", required = false) Mono<Person> anotherPersonMono,
+			@RequestPart(name = "name", required = false) Flux<Person> anotherPersonFlux,
+			@RequestPart(name = "name", required = false) List<Person> anotherPersonList,
 			@RequestPart(name = "anotherPart", required = false) Part anotherPart,
+			@RequestPart(name = "name", required = false) Mono<Part> anotherPartMono,
+			@RequestPart(name = "name", required = false) Flux<Part> anotherPartFlux,
+			@RequestPart(name = "name", required = false) List<Part> anotherPartList,
 			Person notAnnotated) {}