Exclude Part from nested constructor binding in WebFlux

See gh-31778

spring-web/src/main/java/org/springframework/web/bind/support/WebExchangeDataBinder.java

@@ -20,6 +20,8 @@ import java.util.List;
 import java.util.Map;
 import java.util.TreeMap;
 
+import org.springframework.core.MethodParameter;
+import org.springframework.web.multipart.MultipartFile;
 import reactor.core.publisher.Mono;
 
 import org.springframework.beans.MutablePropertyValues;
@@ -87,6 +89,13 @@ public class WebExchangeDataBinder extends WebDataBinder {
 				.then();
 	}
 
+	@Override
+	protected boolean shouldConstructArgument(MethodParameter param) {
+		Class<?> type = param.nestedIfOptional().getNestedParameterType();
+		return (super.shouldConstructArgument(param) &&
+				!MultipartFile.class.isAssignableFrom(type) && !Part.class.isAssignableFrom(type));
+	}
+
 	/**
 	 * Bind query parameters, form data, or multipart form data to the binder target.
 	 * @param exchange the current exchange

spring-web/src/test/java/org/springframework/web/bind/support/WebExchangeDataBinderTests.java

@@ -28,6 +28,7 @@ import reactor.core.publisher.Mono;
 
 import org.springframework.beans.testfixture.beans.ITestBean;
 import org.springframework.beans.testfixture.beans.TestBean;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
@@ -219,6 +220,19 @@ public class WebExchangeDataBinderTests {
 		assertThat(bean.getSomePartList().get(1).filename()).isEqualTo("spring.png");
 	}
 
+	@Test
+	public void testConstructorMultipart() throws Exception {
+		WebExchangeDataBinder binder = new WebExchangeDataBinder(null);
+		binder.setTargetType(ResolvableType.forClass(ConstructorMultipartBean.class));
+
+		MultiValueMap<String, Object> data = new LinkedMultiValueMap<>();
+		data.add("part", new ClassPathResource("org/springframework/http/codec/multipart/foo.txt"));
+		binder.construct(exchangeMultipart(data)).block(Duration.ofMillis(5000));
+		ConstructorMultipartBean bean = (ConstructorMultipartBean) binder.getTarget();
+
+		assertThat(bean.getPart().filename()).isEqualTo("foo.txt");
+		assertThat(bean.getNullableFilePart()).isNull();
+	}
 
 
 	private ServerWebExchange exchange(MultiValueMap<String, String> formData) {
@@ -313,4 +327,24 @@ public class WebExchangeDataBinderTests {
 		}
 	}
 
+	private static class ConstructorMultipartBean {
+		private final FilePart part;
+		private final FilePart nullableFilePart;
+
+		public ConstructorMultipartBean(
+				FilePart part,
+				FilePart nullableFilePart
+		) {
+			this.part = part;
+			this.nullableFilePart = nullableFilePart;
+		}
+
+		public FilePart getPart() {
+			return part;
+		}
+
+		public FilePart getNullableFilePart() {
+			return nullableFilePart;
+		}
+	}
 }