Adds option for setting custom content type for modify request body.

fixes gh-492

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java

@@ -63,7 +63,18 @@ public class ModifyRequestBodyGatewayFilterFactory
 					.flatMap(o -> config.rewriteFunction.apply(exchange, o));
 
 			BodyInserter bodyInserter = BodyInserters.fromPublisher(modifiedBody, config.getOutClass());
-			CachedBodyOutputMessage outputMessage = new CachedBodyOutputMessage(exchange, exchange.getRequest().getHeaders());
+			HttpHeaders headers = new HttpHeaders();
+			headers.putAll(exchange.getRequest().getHeaders());
+
+			// the new content type will be computed by bodyInserter
+			// and then set in the request decorator
+			headers.remove(HttpHeaders.CONTENT_LENGTH);
+
+			// if the body is changing content types, set it here, to the bodyInserter will know about it
+			if (config.getContentType() != null) {
+				headers.set(HttpHeaders.CONTENT_TYPE, config.getContentType());
+			}
+			CachedBodyOutputMessage outputMessage = new CachedBodyOutputMessage(exchange, headers);
 			return bodyInserter.insert(outputMessage,  new BodyInserterContext())
 					// .log("modify_request", Level.INFO)
 					.then(Mono.defer(() -> {
@@ -71,10 +82,15 @@ public class ModifyRequestBodyGatewayFilterFactory
 								exchange.getRequest()) {
 							@Override
 							public HttpHeaders getHeaders() {
+								long contentLength = headers.getContentLength();
 								HttpHeaders httpHeaders = new HttpHeaders();
 								httpHeaders.putAll(super.getHeaders());
-								// TODO: this causes a 'HTTP/1.1 411 Length Required' on httpbin.org
-								httpHeaders.set(HttpHeaders.TRANSFER_ENCODING, "chunked");
+								if (contentLength > 0) {
+									httpHeaders.setContentLength(contentLength);
+								} else {
+									// TODO: this causes a 'HTTP/1.1 411 Length Required' on httpbin.org
+									httpHeaders.set(HttpHeaders.TRANSFER_ENCODING, "chunked");
+								}
 								return httpHeaders;
 							}
 
@@ -92,7 +108,12 @@ public class ModifyRequestBodyGatewayFilterFactory
 	public static class Config {
 		private Class inClass;
 		private Class outClass;
+
+		private String contentType;
+
+		@Deprecated
 		private Map<String, Object> inHints;
+		@Deprecated
 		private Map<String, Object> outHints;
 
 		private RewriteFunction rewriteFunction;
@@ -115,19 +136,23 @@ public class ModifyRequestBodyGatewayFilterFactory
 			return this;
 		}
 
+		@Deprecated
 		public Map<String, Object> getInHints() {
 			return inHints;
 		}
 
+		@Deprecated
 		public Config setInHints(Map<String, Object> inHints) {
 			this.inHints = inHints;
 			return this;
 		}
 
+		@Deprecated
 		public Map<String, Object> getOutHints() {
 			return outHints;
 		}
 
+		@Deprecated
 		public Config setOutHints(Map<String, Object> outHints) {
 			this.outHints = outHints;
 			return this;
@@ -149,5 +174,14 @@ public class ModifyRequestBodyGatewayFilterFactory
 			this.rewriteFunction = rewriteFunction;
 			return this;
 		}
+
+		public String getContentType() {
+			return contentType;
+		}
+
+		public Config setContentType(String contentType) {
+			this.contentType = contentType;
+			return this;
+		}
 	}
 }

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/route/builder/GatewayFilterSpec.java

@@ -206,6 +206,21 @@ public class GatewayFilterSpec extends UriSpec {
 				.apply(c -> c.setRewriteFunction(inClass, outClass, rewriteFunction)));
 	}
 
+	/**
+	 * A filter that can be used to modify the request body.
+	 * This filter is BETA and may be subject to change in a future release.
+	 * @param inClass the class to convert the incoming request body to
+	 * @param outClass the class the Gateway will add to the request before it is routed
+	 * @param newContentType the new Content-Type header to be sent
+	 * @param rewriteFunction the {@link RewriteFunction} that transforms the request body
+	 * @param <T> the original request body class
+	 * @param <R> the new request body class
+	 * @return a {@link GatewayFilterSpec} that can be used to apply additional filters
+	 */
+	public <T, R> GatewayFilterSpec modifyRequestBody(Class<T> inClass, Class<R> outClass, String newContentType, RewriteFunction<T, R> rewriteFunction) {
+		return filter(getBean(ModifyRequestBodyGatewayFilterFactory.class)
+				.apply(c -> c.setRewriteFunction(inClass, outClass, rewriteFunction).setContentType(newContentType)));
+	}
 	/**
 	 * A filter that can be used to modify the response body
 	 * This filter is BETA and may be subject to change in a future release.

spring-cloud-gateway-sample/src/main/java/org/springframework/cloud/gateway/sample/GatewaySampleApplication.java

@@ -70,10 +70,8 @@ public class GatewaySampleApplication {
 				)
 				.route("rewrite_request_obj", r -> r.host("*.rewriterequestobj.org")
 					.filters(f -> f.prefixPath("/httpbin")
-							//TODO: add as configuration to modifyRequestBody
-							.setRequestHeader("Content-Type", MediaType.APPLICATION_JSON_VALUE)
 							.addResponseHeader("X-TestHeader", "rewrite_request")
-							.modifyRequestBody(String.class, Hello.class,
+							.modifyRequestBody(String.class, Hello.class, MediaType.APPLICATION_JSON_VALUE,
 									(exchange, s) -> {
 										return Mono.just(new Hello(s.toUpperCase()));
 									})