From e8da02db47a3e5b56dbc86507d9fd0eb2db5d5e7 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.

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 29186d0a19a3..d3d007bea96c 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
@@ -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;
 		}
 
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 f3b05812da7f..e82cf02f6fda 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;
@@ -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);