Ensure that JSON is written to response body

Issue: SPR-15988

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

@@ -356,7 +356,9 @@ public class ProtobufHttpMessageConverter extends AbstractHttpMessageConverter<M
 				throws IOException {
 
 			if (contentType.isCompatibleWith(APPLICATION_JSON)) {
-				this.printer.appendTo(message, new OutputStreamWriter(output, charset));
+				OutputStreamWriter writer = new OutputStreamWriter(output, charset);
+				this.printer.appendTo(message, writer);
+				writer.flush();
 			}
 			else {
 				throw new IOException("protobuf-java-util does not support " + contentType + " format");

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

@@ -17,8 +17,10 @@
 package org.springframework.http.converter.protobuf;
 
 import java.io.IOException;
+import java.nio.charset.Charset;
 
 import com.google.protobuf.Message;
+import com.google.protobuf.util.JsonFormat;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -36,6 +38,7 @@ import static org.mockito.Mockito.*;
  *
  * @author Alex Antonov
  * @author Juergen Hoeller
+ * @author Andreas Ahlenstorf
  */
 public class ProtobufHttpMessageConverterTests {
 
@@ -104,7 +107,7 @@ public class ProtobufHttpMessageConverterTests {
 	}
 
 	@Test
-	public void write() throws IOException {
+	public void writeProtobuf() throws IOException {
 		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
 		MediaType contentType = ProtobufHttpMessageConverter.PROTOBUF;
 		this.converter.write(this.testMsg, contentType, outputMessage);
@@ -121,6 +124,54 @@ public class ProtobufHttpMessageConverterTests {
 		assertEquals("sample.proto", schemaHeader);
 	}
 
+	@Test
+	public void writeJsonWithGoogleProtobuf() throws IOException {
+		this.converter = new ProtobufHttpMessageConverter(
+				new ProtobufHttpMessageConverter.ProtobufJavaUtilSupport(null, null),
+				this.registryInitializer);
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		MediaType contentType = MediaType.APPLICATION_JSON_UTF8;
+		this.converter.write(this.testMsg, contentType, outputMessage);
+
+		assertEquals(contentType, outputMessage.getHeaders().getContentType());
+
+		final String body = outputMessage.getBodyAsString(Charset.forName("UTF-8"));
+		assertFalse("body is empty", body.isEmpty());
+
+		Msg.Builder builder = Msg.newBuilder();
+		JsonFormat.parser().merge(body, builder);
+		assertEquals(this.testMsg, builder.build());
+
+		assertNull(outputMessage.getHeaders().getFirst(
+				ProtobufHttpMessageConverter.X_PROTOBUF_MESSAGE_HEADER));
+		assertNull(outputMessage.getHeaders().getFirst(
+				ProtobufHttpMessageConverter.X_PROTOBUF_SCHEMA_HEADER));
+	}
+
+	@Test
+	public void writeJsonWithJavaFormat() throws IOException {
+		this.converter = new ProtobufHttpMessageConverter(
+				new ProtobufHttpMessageConverter.ProtobufJavaFormatSupport(),
+				this.registryInitializer);
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		MediaType contentType = MediaType.APPLICATION_JSON_UTF8;
+		this.converter.write(this.testMsg, contentType, outputMessage);
+
+		assertEquals(contentType, outputMessage.getHeaders().getContentType());
+
+		final String body = outputMessage.getBodyAsString(Charset.forName("UTF-8"));
+		assertFalse("body is empty", body.isEmpty());
+
+		Msg.Builder builder = Msg.newBuilder();
+		JsonFormat.parser().merge(body, builder);
+		assertEquals(this.testMsg, builder.build());
+
+		assertNull(outputMessage.getHeaders().getFirst(
+				ProtobufHttpMessageConverter.X_PROTOBUF_MESSAGE_HEADER));
+		assertNull(outputMessage.getHeaders().getFirst(
+				ProtobufHttpMessageConverter.X_PROTOBUF_SCHEMA_HEADER));
+	}
+
 	@Test
 	public void defaultContentType() throws Exception {
 		assertEquals(ProtobufHttpMessageConverter.PROTOBUF, this.converter.getDefaultContentType(this.testMsg));