Support any HttpEntity implementing ResolvableTypeProvider

Closes gh-22931

Author: jhoeller

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -25,9 +25,11 @@
 
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.ResolvableTypeProvider;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
+import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedMultiValueMap;
@@ -263,13 +265,13 @@ public HttpEntity<?> build() {
 	 * @param <T> the type contained in the publisher
 	 * @param <P> the publisher
 	 */
-	public static final class PublisherEntity<T, P extends Publisher<T>> extends HttpEntity<P> {
+	public static final class PublisherEntity<T, P extends Publisher<T>> extends HttpEntity<P>
+			implements ResolvableTypeProvider  {
 
 		private final ResolvableType resolvableType;
 
-
-		private PublisherEntity(@Nullable MultiValueMap<String, String> headers, P publisher,
-				ResolvableType resolvableType) {
+		PublisherEntity(
+				@Nullable MultiValueMap<String, String> headers, P publisher, ResolvableType resolvableType) {
 
 			super(publisher, headers);
 			Assert.notNull(publisher, "'publisher' must not be null");
@@ -280,6 +282,8 @@ private PublisherEntity(@Nullable MultiValueMap<String, String> headers, P publi
 		/**
 		 * Return the element type for the {@code Publisher} body.
 		 */
+		@Override
+		@NonNull
 		public ResolvableType getResolvableType() {
 			return this.resolvableType;
 		}

spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java

@@ -34,6 +34,7 @@
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
+import org.springframework.core.ResolvableTypeProvider;
 import org.springframework.core.codec.CharSequenceEncoder;
 import org.springframework.core.codec.CodecException;
 import org.springframework.core.codec.Hints;
@@ -46,7 +47,6 @@
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpOutputMessage;
-import org.springframework.http.client.MultipartBodyBuilder;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.FormHttpMessageWriter;
 import org.springframework.http.codec.HttpMessageWriter;
@@ -265,11 +265,8 @@ private <T> Flux<DataBuffer> encodePart(byte[] boundary, String name, T value, D
 			outputHeaders.putAll(httpEntity.getHeaders());
 			body = httpEntity.getBody();
 			Assert.state(body != null, "MultipartHttpMessageWriter only supports HttpEntity with body");
-
-			if (httpEntity instanceof MultipartBodyBuilder.PublisherEntity<?, ?>) {
-				MultipartBodyBuilder.PublisherEntity<?, ?> publisherEntity =
-						(MultipartBodyBuilder.PublisherEntity<?, ?>) httpEntity;
-				resolvableType = publisherEntity.getResolvableType();
+			if (httpEntity instanceof ResolvableTypeProvider) {
+				resolvableType = ((ResolvableTypeProvider) httpEntity).getResolvableType();
 			}
 		}
 		else {