From a584311178fc3873cd085b16c571873da148b7c8 Mon Sep 17 00:00:00 2001
From: Rossen Stoyanchev <rstoyanchev@pivotal.io>
Date: Thu, 7 Jul 2016 05:36:19 -0400
Subject: [PATCH] Fix StringDecoder#decodeToMono issue with empty input

Before this change decodeToMono always created a StringBuilder to
aggregate resulting in an "" (empty string) rather than an empty
Mono for an empty input stream.

Now we aggregate in the DataBuffer instead and then decode to String.
---
 .../core/codec/StringDecoder.java             | 18 +++++++-------
 .../core/codec/StringDecoderTests.java        | 24 ++++++++++++++++++-
 2 files changed, 31 insertions(+), 11 deletions(-)

diff --git a/spring-web-reactive/src/main/java/org/springframework/core/codec/StringDecoder.java b/spring-web-reactive/src/main/java/org/springframework/core/codec/StringDecoder.java
index 51dc6a5eb5..a96e14f536 100644
--- a/spring-web-reactive/src/main/java/org/springframework/core/codec/StringDecoder.java
+++ b/spring-web-reactive/src/main/java/org/springframework/core/codec/StringDecoder.java
@@ -91,16 +91,16 @@ public class StringDecoder extends AbstractDecoder<String> {
 		if (this.splitOnNewline) {
 			inputFlux = Flux.from(inputStream).flatMap(StringDecoder::splitOnNewline);
 		}
-		return decodeInternal(inputFlux, mimeType);
+		return inputFlux.map(buffer ->  decodeDataBuffer(buffer, mimeType));
 	}
 
 	@Override
 	public Mono<String> decodeToMono(Publisher<DataBuffer> inputStream, ResolvableType elementType,
 			MimeType mimeType, Object... hints) {
 
-		return decodeInternal(Flux.from(inputStream), mimeType).
-				collect(StringBuilder::new, StringBuilder::append).
-				map(StringBuilder::toString);
+		return Flux.from(inputStream)
+				.reduce(DataBuffer::write)
+				.map(buffer -> decodeDataBuffer(buffer, mimeType));
 	}
 
 	private static Flux<DataBuffer> splitOnNewline(DataBuffer dataBuffer) {
@@ -120,13 +120,11 @@ public class StringDecoder extends AbstractDecoder<String> {
 		return Flux.fromIterable(results);
 	}
 
-	private Flux<String> decodeInternal(Flux<DataBuffer> inputFlux, MimeType mimeType) {
+	private String decodeDataBuffer(DataBuffer dataBuffer, MimeType mimeType) {
 		Charset charset = getCharset(mimeType);
-		return inputFlux.map(dataBuffer -> {
-			CharBuffer charBuffer = charset.decode(dataBuffer.asByteBuffer());
-			DataBufferUtils.release(dataBuffer);
-			return charBuffer.toString();
-		});
+		CharBuffer charBuffer = charset.decode(dataBuffer.asByteBuffer());
+		DataBufferUtils.release(dataBuffer);
+		return charBuffer.toString();
 	}
 
 	private Charset getCharset(MimeType mimeType) {
diff --git a/spring-web-reactive/src/test/java/org/springframework/core/codec/StringDecoderTests.java b/spring-web-reactive/src/test/java/org/springframework/core/codec/StringDecoderTests.java
index f3d4bb43b0..43492ae6a9 100644
--- a/spring-web-reactive/src/test/java/org/springframework/core/codec/StringDecoderTests.java
+++ b/spring-web-reactive/src/test/java/org/springframework/core/codec/StringDecoderTests.java
@@ -73,7 +73,18 @@ public class StringDecoderTests extends AbstractDataBufferAllocatingTestCase {
 	}
 
 	@Test
-	public void decodeEmpty() throws InterruptedException {
+	public void decodeEmptyFlux() throws InterruptedException {
+		Flux<DataBuffer> source = Flux.empty();
+		Flux<String> output = this.decoder.decode(source, ResolvableType.forClass(String.class), null);
+
+		TestSubscriber.subscribe(output)
+				.assertNoError()
+				.assertComplete()
+				.assertNoValues();
+	}
+
+	@Test
+	public void decodeEmptyString() throws InterruptedException {
 		Flux<DataBuffer> source = Flux.just(stringBuffer(""));
 		Flux<String> output = this.decoder.decode(source, ResolvableType.forClass(String.class), null);
 
@@ -92,4 +103,15 @@ public class StringDecoderTests extends AbstractDataBufferAllocatingTestCase {
 				.assertValues("foobarbaz");
 	}
 
+	@Test
+	public void decodeToMonoWithEmptyFlux() throws InterruptedException {
+		Flux<DataBuffer> source = Flux.empty();
+		Mono<String> output = this.decoder.decodeToMono(source, ResolvableType.forClass(String.class), null);
+
+		TestSubscriber.subscribe(output)
+				.assertNoError()
+				.assertComplete()
+				.assertNoValues();
+	}
+
 }