Support StreamingHttpOutputMessage in RestClient

spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java

@@ -477,7 +477,14 @@ public <T> RequestBodySpec body(T body, ParameterizedTypeReference<T> bodyType)
 
 		@Override
 		public RequestBodySpec body(StreamingHttpOutputMessage.Body body) {
-			this.body = request -> body.writeTo(request.getBody());
+			this.body = request -> {
+				if (request instanceof StreamingHttpOutputMessage streamingMessage) {
+					streamingMessage.setBody(body);
+				}
+				else {
+					body.writeTo(request.getBody());
+				}
+			};
 			return this;
 		}
 

spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.client;
 
+import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
@@ -559,6 +560,27 @@ void postUserAsJsonWithJsonView(ClientHttpRequestFactory requestFactory) {
 		});
 	}
 
+	@ParameterizedRestClientTest
+	void postStreamingMessageBody(ClientHttpRequestFactory requestFactory) {
+		startServer(requestFactory);
+
+		prepareResponse(response -> response.setResponseCode(200));
+
+		ResponseEntity<Void> result = this.restClient.post()
+				.uri("/streaming/body")
+				.body(new ByteArrayInputStream("test-data".getBytes(UTF_8))::transferTo)
+				.retrieve()
+				.toBodilessEntity();
+
+		assertThat(result.getStatusCode()).isEqualTo(HttpStatus.OK);
+
+		expectRequestCount(1);
+		expectRequest(request -> {
+			assertThat(request.getPath()).isEqualTo("/streaming/body");
+			assertThat(request.getBody().readUtf8()).isEqualTo("test-data");
+		});
+	}
+
 	@ParameterizedRestClientTest // gh-31361
 	public void postForm(ClientHttpRequestFactory requestFactory) {
 		startServer(requestFactory);