[FLINK-7583] [REST] Use static constant for CONTENT_TYPE header

This closes #4646.

flink-runtime/src/main/java/org/apache/flink/runtime/rest/RestClient.java

@@ -27,6 +27,7 @@ import org.apache.flink.runtime.rest.messages.MessageParameters;
 import org.apache.flink.runtime.rest.messages.RequestBody;
 import org.apache.flink.runtime.rest.messages.ResponseBody;
 import org.apache.flink.runtime.rest.util.RestClientException;
+import org.apache.flink.runtime.rest.util.RestConstants;
 import org.apache.flink.runtime.rest.util.RestMapperUtils;
 import org.apache.flink.util.FlinkRuntimeException;
 import org.apache.flink.util.Preconditions;
@@ -152,7 +153,7 @@ public class RestClient {
 		FullHttpRequest httpRequest = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, messageHeaders.getHttpMethod().getNettyHttpMethod(), targetUrl, payload);
 		httpRequest.headers()
 			.add(HttpHeaders.Names.CONTENT_LENGTH, payload.capacity())
-			.add(HttpHeaders.Names.CONTENT_TYPE, "application/json; charset=" + ConfigConstants.DEFAULT_CHARSET.name())
+			.add(HttpHeaders.Names.CONTENT_TYPE, RestConstants.REST_CONTENT_TYPE)
 			.set(HttpHeaders.Names.HOST, targetAddress + ':' + targetPort)
 			.set(HttpHeaders.Names.CONNECTION, HttpHeaders.Values.CLOSE);
 

flink-runtime/src/main/java/org/apache/flink/runtime/rest/handler/AbstractRestHandler.java

@@ -25,6 +25,7 @@ import org.apache.flink.runtime.rest.messages.MessageHeaders;
 import org.apache.flink.runtime.rest.messages.MessageParameters;
 import org.apache.flink.runtime.rest.messages.RequestBody;
 import org.apache.flink.runtime.rest.messages.ResponseBody;
+import org.apache.flink.runtime.rest.util.RestConstants;
 import org.apache.flink.runtime.rest.util.RestMapperUtils;
 
 import org.apache.flink.shaded.netty4.io.netty.buffer.ByteBuf;
@@ -192,7 +193,7 @@ public abstract class AbstractRestHandler<R extends RequestBody, P extends Respo
 	private static void sendResponse(@Nonnull ChannelHandlerContext ctx, @Nonnull HttpRequest httpRequest, @Nonnull HttpResponseStatus statusCode, @Nonnull String message) {
 		HttpResponse response = new DefaultHttpResponse(HTTP_1_1, statusCode);
 
-		response.headers().set(CONTENT_TYPE, "application/json");
+		response.headers().set(CONTENT_TYPE, RestConstants.REST_CONTENT_TYPE);
 
 		if (HttpHeaders.isKeepAlive(httpRequest)) {
 			response.headers().set(CONNECTION, HttpHeaders.Values.KEEP_ALIVE);

flink-runtime/src/main/java/org/apache/flink/runtime/rest/util/RestConstants.java

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.runtime.rest.util;
+
+import org.apache.flink.configuration.ConfigConstants;
+
+/**
+ * This class contains constants to be used by rest components.
+ */
+public class RestConstants {
+	public static final String REST_CONTENT_TYPE = "application/json; charset=" + ConfigConstants.DEFAULT_CHARSET.name();
+}