Jackson2JsonDecoder should support empty JSON array

Before this commit, the Jackson2JsonDecoder was not able to decode an
empty JSON array (`[]`). After this commit, it is.

Issue: SPR-15685

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonDecoder.java

@@ -111,20 +111,23 @@ public class Jackson2JsonDecoder extends Jackson2CodecSupport implements HttpMes
 				this.objectMapper.readerFor(javaType));
 
 		return objectDecoder.decode(inputStream, elementType, mimeType, hints)
-				.map(dataBuffer -> {
+				.flatMap(dataBuffer -> {
+					if (dataBuffer.readableByteCount() == 0) {
+						return Mono.empty();
+					}
 					try {
 						Object value = reader.readValue(dataBuffer.asInputStream());
 						DataBufferUtils.release(dataBuffer);
-						return value;
+						return Mono.just(value);
 					}
 					catch (InvalidDefinitionException ex) {
-						throw new CodecException("Type definition error: " + ex.getType(), ex);
+						return Mono.error(new CodecException("Type definition error: " + ex.getType(), ex));
 					}
 					catch (JsonProcessingException ex) {
-						throw new DecodingException("JSON decoding error: " + ex.getOriginalMessage(), ex);
+						return Mono.error(new DecodingException("JSON decoding error: " + ex.getOriginalMessage(), ex));
 					}
 					catch (IOException ex) {
-						throw new DecodingException("I/O error while parsing input stream", ex);
+						return Mono.error(new DecodingException("I/O error while parsing input stream", ex));
 					}
 				});
 	}

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonDecoderTests.java

 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,9 +35,7 @@ import org.springframework.http.codec.Pojo;
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonMap;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNull;
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.*;
 import static org.springframework.core.ResolvableType.forClass;
 import static org.springframework.http.MediaType.APPLICATION_JSON;
 import static org.springframework.http.MediaType.APPLICATION_XML;
@@ -116,6 +114,19 @@ public class Jackson2JsonDecoderTests extends AbstractDataBufferAllocatingTestCa
 				.verifyComplete();
 	}
 
+	@Test
+	public void decodeEmptyArrayToFlux() throws Exception {
+		Flux<DataBuffer> source = Flux.just(stringBuffer("[]"));
+
+		ResolvableType elementType = forClass(Pojo.class);
+		Flux<Object> flux = new Jackson2JsonDecoder().decode(source, elementType, null,
+				emptyMap());
+
+		StepVerifier.create(flux)
+				.expectNextCount(0)
+				.verifyComplete();
+	}
+
 	@Test
 	public void fieldLevelJsonView() throws Exception {
 		Flux<DataBuffer> source = Flux.just(
@@ -164,6 +175,18 @@ public class Jackson2JsonDecoderTests extends AbstractDataBufferAllocatingTestCa
 				.verifyComplete();
 	}
 
+	@Test
+	public void decodeEmptyArrayToMono() throws Exception {
+		Flux<DataBuffer> source = Flux.just(stringBuffer("[]"));
+		ResolvableType elementType = forClass(Pojo.class);
+		Mono<Object> mono = new Jackson2JsonDecoder().decodeToMono(source, elementType,
+				null, emptyMap());
+
+		StepVerifier.create(mono)
+				.expectNextCount(0)
+				.verifyComplete();
+	}
+
 	@Test
 	public void invalidData() throws Exception {
 		Flux<DataBuffer> source = Flux.just(stringBuffer( "{\"foofoo\": \"foofoo\", \"barbar\": \"barbar\"}"));