Support repeatable multipart write

Closes gh-34859

spring-web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

@@ -28,6 +28,7 @@ import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 
 import org.jspecify.annotations.Nullable;
 
@@ -485,9 +486,18 @@ public class FormHttpMessageConverter implements HttpMessageConverter<MultiValue
 		outputMessage.getHeaders().setContentType(contentType);
 
 		if (outputMessage instanceof StreamingHttpOutputMessage streamingOutputMessage) {
-			streamingOutputMessage.setBody(outputStream -> {
-				writeParts(outputStream, parts, boundary);
-				writeEnd(outputStream, boundary);
+			boolean repeatable = checkPartsRepeatable(parts);
+			streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
+				@Override
+				public void writeTo(OutputStream outputStream) throws IOException {
+					FormHttpMessageConverter.this.writeParts(outputStream, parts, boundary);
+					writeEnd(outputStream, boundary);
+				}
+
+				@Override
+				public boolean repeatable() {
+					return repeatable;
+				}
 			});
 		}
 		else {
@@ -496,6 +506,35 @@ public class FormHttpMessageConverter implements HttpMessageConverter<MultiValue
 		}
 	}
 
+	@SuppressWarnings({"unchecked", "ConstantValue"})
+	private <T> boolean checkPartsRepeatable(MultiValueMap<String, Object> map) {
+		return map.entrySet().stream().allMatch(e -> e.getValue().stream().filter(Objects::nonNull).allMatch(part -> {
+			HttpHeaders headers = null;
+			Object body = part;
+			if (part instanceof HttpEntity<?> entity) {
+				headers = entity.getHeaders();
+				body = entity.getBody();
+				Assert.state(body != null, "Empty body for part '" + e.getKey() + "': " + part);
+			}
+			HttpMessageConverter<?> converter = findConverterFor(e.getKey(), headers, body);
+			return (converter instanceof AbstractHttpMessageConverter<?> ahmc &&
+					((AbstractHttpMessageConverter<T>) ahmc).supportsRepeatableWrites((T) body));
+		}));
+	}
+
+	private @Nullable HttpMessageConverter<?> findConverterFor(
+			String name, @Nullable HttpHeaders headers, Object body) {
+
+		Class<?> partType = body.getClass();
+		MediaType contentType = (headers != null ? headers.getContentType() : null);
+		for (HttpMessageConverter<?> converter : this.partConverters) {
+			if (converter.canWrite(partType, contentType)) {
+				return converter;
+			}
+		}
+		return null;
+	}
+
 	/**
 	 * When {@link #setMultipartCharset(Charset)} is configured (i.e. RFC 2047,
 	 * {@code encoded-word} syntax) we need to use ASCII for part headers, or
@@ -521,32 +560,27 @@ public class FormHttpMessageConverter implements HttpMessageConverter<MultiValue
 	@SuppressWarnings("unchecked")
 	private void writePart(String name, HttpEntity<?> partEntity, OutputStream os) throws IOException {
 		Object partBody = partEntity.getBody();
-		if (partBody == null) {
-			throw new IllegalStateException("Empty body for part '" + name + "': " + partEntity);
-		}
-		Class<?> partType = partBody.getClass();
+		Assert.state(partBody != null, "Empty body for part '" + name + "': " + partEntity);
 		HttpHeaders partHeaders = partEntity.getHeaders();
 		MediaType partContentType = partHeaders.getContentType();
-		for (HttpMessageConverter<?> messageConverter : this.partConverters) {
-			if (messageConverter.canWrite(partType, partContentType)) {
-				Charset charset = isFilenameCharsetSet() ? StandardCharsets.US_ASCII : this.charset;
-				HttpOutputMessage multipartMessage = new MultipartHttpOutputMessage(os, charset);
-				String filename = getFilename(partBody);
-				ContentDisposition.Builder cd = ContentDisposition.formData()
-						.name(name);
-				if (filename != null) {
-					cd.filename(filename, this.multipartCharset);
-				}
-				multipartMessage.getHeaders().setContentDisposition(cd.build());
-				if (!partHeaders.isEmpty()) {
-					multipartMessage.getHeaders().putAll(partHeaders);
-				}
-				((HttpMessageConverter<Object>) messageConverter).write(partBody, partContentType, multipartMessage);
-				return;
+		HttpMessageConverter<?> converter = findConverterFor(name, partHeaders, partBody);
+		if (converter != null) {
+			Charset charset = isFilenameCharsetSet() ? StandardCharsets.US_ASCII : this.charset;
+			HttpOutputMessage multipartMessage = new MultipartHttpOutputMessage(os, charset);
+			String filename = getFilename(partBody);
+			ContentDisposition.Builder cd = ContentDisposition.formData().name(name);
+			if (filename != null) {
+				cd.filename(filename, this.multipartCharset);
+			}
+			multipartMessage.getHeaders().setContentDisposition(cd.build());
+			if (!partHeaders.isEmpty()) {
+				multipartMessage.getHeaders().putAll(partHeaders);
 			}
+			((HttpMessageConverter<Object>) converter).write(partBody, partContentType, multipartMessage);
+			return;
 		}
-		throw new HttpMessageNotWritableException("Could not write request: no suitable HttpMessageConverter " +
-				"found for request type [" + partType.getName() + "]");
+		throw new HttpMessageNotWritableException("Could not write request: " +
+				"no suitable HttpMessageConverter found for request type [" + partBody.getClass().getName() + "]");
 	}
 
 	/**

spring-web/src/test/java/org/springframework/http/converter/FormHttpMessageConverterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -40,6 +40,7 @@ import org.springframework.core.io.Resource;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
+import org.springframework.http.StreamingHttpOutputMessage;
 import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
 import org.springframework.http.converter.xml.SourceHttpMessageConverter;
 import org.springframework.util.LinkedMultiValueMap;
@@ -204,7 +205,7 @@ class FormHttpMessageConverterTests {
 		parameters.put("charset", UTF_8.name());
 		parameters.put("foo", "bar");
 
-		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		StreamingMockHttpOutputMessage outputMessage = new StreamingMockHttpOutputMessage();
 		this.converter.write(parts, new MediaType("multipart", "form-data", parameters), outputMessage);
 
 		final MediaType contentType = outputMessage.getHeaders().getContentType();
@@ -248,6 +249,8 @@ class FormHttpMessageConverterTests {
 		item = items.get(5);
 		assertThat(item.getFieldName()).isEqualTo("json");
 		assertThat(item.getContentType()).isEqualTo("application/json");
+
+		assertThat(outputMessage.wasRepeatable()).isTrue();
 	}
 
 	@Test
@@ -286,7 +289,7 @@ class FormHttpMessageConverterTests {
 		parameters.put("charset", UTF_8.name());
 		parameters.put("foo", "bar");
 
-		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		StreamingMockHttpOutputMessage outputMessage = new StreamingMockHttpOutputMessage();
 		this.converter.write(parts, new MediaType("multipart", "form-data", parameters), outputMessage);
 
 		final MediaType contentType = outputMessage.getHeaders().getContentType();
@@ -330,6 +333,8 @@ class FormHttpMessageConverterTests {
 		item = items.get(5);
 		assertThat(item.getFieldName()).isEqualTo("xml");
 		assertThat(item.getContentType()).isEqualTo("text/xml");
+
+		assertThat(outputMessage.wasRepeatable()).isFalse();
 	}
 
 	@Test  // SPR-13309
@@ -444,6 +449,27 @@ class FormHttpMessageConverterTests {
 	}
 
 
+	private static class StreamingMockHttpOutputMessage extends MockHttpOutputMessage implements StreamingHttpOutputMessage {
+
+		private boolean repeatable;
+
+		public boolean wasRepeatable() {
+			return this.repeatable;
+		}
+
+		@Override
+		public void setBody(Body body) {
+			try {
+				this.repeatable = body.repeatable();
+				body.writeTo(getBody());
+			}
+			catch (IOException ex) {
+				throw new RuntimeException(ex);
+			}
+		}
+	}
+
+
 	private static class MockHttpOutputMessageRequestContext implements UploadContext {
 
 		private final MockHttpOutputMessage outputMessage;