Improve docs on streaming via StreamingHttpOutputMessage

Closes gh-35700

spring-web/src/main/java/org/springframework/http/StreamingHttpOutputMessage.java

@@ -20,10 +20,16 @@ import java.io.IOException;
 import java.io.OutputStream;
 
 /**
- * Represents an HTTP output message that allows for setting a streaming body.
+ * Contract for {@code HttpOutputMessage} implementations to expose the ability
- * Note that such messages typically do not support {@link #getBody()} access.
+ * to stream request body content by writing to an {@link OutputStream} from
+ * a callback.
+ *
+ * <p>The {@link #setBody(Body)} method provides the option to stream, and is
+ * mutually exclusive use of {@link #getBody()}, which instead returns an
+ * {@code OutputStream} that aggregates the request body before sending it.
  *
  * @author Arjen Poutsma
+ * @author Rossen Stoyanchev
  * @since 4.0
  * @see #setBody
  */
@@ -31,15 +37,17 @@ public interface StreamingHttpOutputMessage extends HttpOutputMessage {
 
 	/**
 	 * Set the streaming body callback for this message.
+	 * <p>Note that this is mutually exclusive with {@link #getBody()}, which
+	 * may instead aggregate the request body before sending it.
 	 * @param body the streaming body callback
 	 */
 	void setBody(Body body);
 
 
 	/**
-	 * Defines the contract for bodies that can be written directly to an
+	 * Contract to stream request body content to an {@link OutputStream}.
-	 * {@link OutputStream}. Useful with HTTP client libraries that provide
+	 * In some HTTP client libraries this is only possible indirectly through a
-	 * indirect access to an {@link OutputStream} via a callback mechanism.
+	 * callback mechanism.
 	 */
 	@FunctionalInterface
 	interface Body {

spring-web/src/main/java/org/springframework/http/client/AbstractStreamingClientHttpRequest.java

@@ -20,17 +20,24 @@ import java.io.IOException;
 import java.io.OutputStream;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.StreamingHttpOutputMessage;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.FastByteArrayOutputStream;
 
 /**
- * Abstract base for {@link ClientHttpRequest} that also implement
+ * Extension of {@link AbstractClientHttpRequest} that adds the ability to stream
- * {@link StreamingHttpOutputMessage}. Ensures that headers and
+ * request body content directly to the underlying HTTP client library through
- * body are not written multiple times.
+ * the {@link StreamingHttpOutputMessage} contract.
+ *
+ * <p>It is necessary to call {@link #setBody} and stream the request body through
+ * a callback for access to the {@code OutputStream}. The alternative to call
+ * {@link #getBody()} is also supported as a fallback, but that does not stream,
+ * and returns an aggregating {@code OutputStream} instead.
  *
  * @author Arjen Poutsma
+ * @author Rossen Stoyanchev
  * @since 6.1
  */
 abstract class AbstractStreamingClientHttpRequest extends AbstractClientHttpRequest
@@ -43,6 +50,12 @@ abstract class AbstractStreamingClientHttpRequest extends AbstractClientHttpRequ
 	private FastByteArrayOutputStream bodyStream;
 
 
+	/**
+	 * Implements the {@link HttpOutputMessage} contract for request body content.
+	 * <p>Note that this method does not result in streaming, and the returned
+	 * {@code OutputStream} aggregates the full content in a byte[] before
+	 * sending. To use streaming, call {@link #setBody} instead.
+	 */
 	@Override
 	protected final OutputStream getBodyInternal(HttpHeaders headers) {
 		Assert.state(this.body == null, "Invoke either getBody or setBody; not both");
@@ -53,6 +66,10 @@ abstract class AbstractStreamingClientHttpRequest extends AbstractClientHttpRequ
 		return this.bodyStream;
 	}
 
+	/**
+	 * Implements the {@link StreamingHttpOutputMessage} contract for writing
+	 * request body by streaming directly to the underlying HTTP client.
+	 */
 	@Override
 	public final void setBody(Body body) {
 		Assert.notNull(body, "Body must not be null");
@@ -73,12 +90,14 @@ abstract class AbstractStreamingClientHttpRequest extends AbstractClientHttpRequ
 
 
 	/**
-	 * Abstract template method that writes the given headers and content to the HTTP request.
+	 * Abstract method for concrete implementations to write the headers and
-	 * @param headers the HTTP headers
+	 * {@link StreamingHttpOutputMessage.Body} to the HTTP request.
+	 * @param headers the HTTP headers for the request
 	 * @param body the HTTP body, may be {@code null} if no body was {@linkplain #setBody(Body) set}
 	 * @return the response object for the executed request
 	 * @since 6.1
 	 */
-	protected abstract ClientHttpResponse executeInternal(HttpHeaders headers, @Nullable Body body) throws IOException;
+	protected abstract ClientHttpResponse executeInternal(
+			HttpHeaders headers, @Nullable Body body) throws IOException;
 
 }

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

@@ -19,6 +19,7 @@ package org.springframework.web.client;
 import java.io.IOException;
 import java.lang.reflect.Type;
 
+import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.client.ClientHttpRequest;
 
 /**
@@ -44,6 +45,12 @@ public interface RequestCallback {
 	 * Gets called by {@link RestTemplate#execute} with an opened {@code ClientHttpRequest}.
 	 * Does not need to care about closing the request or about handling errors:
 	 * this will all be handled by the {@code RestTemplate}.
+	 * <p><strong>Note:</strong> In order to stream request body content directly
+	 * to the underlying HTTP library, implementations must check if the request
+	 * is an implementation of {@link org.springframework.http.StreamingHttpOutputMessage},
+	 * and set the request body through it. Use of the {@link HttpOutputMessage#getBody()}
+	 * is also supported, but results in full content aggregation prior to execution.
+	 * All built-in request implementations support {@code StreamingHttpOutputMessage}.
 	 * @param request the active HTTP request
 	 * @throws IOException in case of I/O errors
 	 */