From 8d6117e419058addcaee5a45370f0276b28602e5 Mon Sep 17 00:00:00 2001
From: Daniil Razorenov <daniltmb@gmail.com>
Date: Mon, 23 Jun 2025 22:54:28 +0500
Subject: [PATCH] Support StreamingHttpOutputMessage in RestClient

This commit allows RestClient to handle StreamingHttpOutputMessage
properly by checking the type of the request and invoking setBody()
when appropriate. This improves interoperability with components that
expect streamed output.

A new integration test has been added to verify the functionality.

See gh-35102

Signed-off-by: Daniil Razorenov <daniltmb@gmail.com>
---
 .../web/client/DefaultRestClient.java         |  9 +++++++-
 .../client/RestClientIntegrationTests.java    | 22 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java b/spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java
index 95e485d5b10..36ce64db342 100644
--- a/spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java
+++ b/spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java
@@ -466,7 +466,14 @@ final class DefaultRestClient implements RestClient {
 
 		@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;
 		}
 
diff --git a/spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java b/spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java
index c1a726181d6..4fa39300121 100644
--- a/spring-web/src/test/java/org/springframework/web/client/RestClientIntegrationTests.java
+++ b/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;
@@ -529,6 +530,27 @@ class RestClientIntegrationTests {
 		});
 	}
 
+	@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);