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.

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) {

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();
+	}
+
 }