Keep YAML entries that haven an empty array value

Prior to this commit, a YAML entry that define an empty array value was
lost. This commit makes sure to flag it with an empty String, which
corresponds as an empty comma separated list of entries in the
properties format.

Issue: SPR-16769

spring-beans/src/main/java/org/springframework/beans/factory/config/YamlProcessor.java

@@ -293,10 +293,14 @@ public abstract class YamlProcessor {
 				// Need a compound key
 				@SuppressWarnings("unchecked")
 				Collection<Object> collection = (Collection<Object>) value;
-				int count = 0;
-				for (Object object : collection) {
-					buildFlattenedMap(result,
-							Collections.singletonMap("[" + (count++) + "]", object), key);
+				if (collection.isEmpty()) {
+					result.put(key, "");
+				} else {
+					int count = 0;
+					for (Object object : collection) {
+						buildFlattenedMap(result, Collections.singletonMap(
+								"[" + (count++) + "]", object), key);
+					}
 				}
 			}
 			else {

spring-beans/src/test/java/org/springframework/beans/factory/config/YamlMapFactoryBeanTests.java

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -19,6 +19,7 @@ package org.springframework.beans.factory.config;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.junit.Test;
@@ -116,6 +117,20 @@ public class YamlMapFactoryBeanTests {
 		assertEquals(Integer.valueOf(3), sub.get("key1.key2"));
 	}
 
+	@Test
+	public void mapWithEmptyArrayValue() {
+		this.factory.setResources(new ByteArrayResource("a: alpha\ntest: []".getBytes()));
+		assertTrue(this.factory.getObject().containsKey("test"));
+		assertEquals(((List<?>)this.factory.getObject().get("test")).size(), 0);
+	}
+
+	@Test
+	public void mapWithEmptyValue() {
+		this.factory.setResources(new ByteArrayResource("a: alpha\ntest:".getBytes()));
+		assertTrue(this.factory.getObject().containsKey("test"));
+		assertNull(this.factory.getObject().get("test"));
+	}
+
 	@Test
 	public void testDuplicateKey() throws Exception {
 		this.factory.setResources(new ByteArrayResource("mymap:\n  foo: bar\nmymap:\n  bar: foo".getBytes()));

spring-beans/src/test/java/org/springframework/beans/factory/config/YamlPropertiesFactoryBeanTests.java

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -207,6 +207,15 @@ public class YamlPropertiesFactoryBeanTests {
 		assertThat(properties.getProperty("spam"), equalTo(""));
 	}
 
+	@Test
+	public void testLoadEmptyArrayValue() {
+		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
+		factory.setResources(new ByteArrayResource("a: alpha\ntest: []".getBytes()));
+		Properties properties = factory.getObject();
+		assertThat(properties.getProperty("a"), equalTo("alpha"));
+		assertThat(properties.getProperty("test"), equalTo(""));
+	}
+
 	@Test
 	public void testLoadArrayOfString() throws Exception {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();