Remove origin support for empty YAML maps

Adding origin support caused an unexpected and unwanted change
in behavior where configuration property binding would fail. The
failure would occur because there was no way to convert from the
entry in the environment that represents the empty map to the
target type.

The commit changes the YAML loader to drop empty maps,
effectively reverting the map portion of
3bcbb0e594 and gh-21704. This aligns
the behavior with the decision we made in gh-19095.

Origin support for an empty list has been retained as it does not
have a negative effect on configuration property binding. Prior to
these changes, an empty YAML list was mapped to an origin tracked
value that contains an empty list. Fully reverting
3bcbb0e594 would have resulted in an
empty YAML list being mapped to an empty string. To avoid adding a
collection type to the environment, we now map an empty YAML list
to an origin tracked value that contains an empty string.

Closes gh-35403

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/OriginTrackedYamlLoader.java

@@ -29,11 +29,11 @@ import org.yaml.snakeyaml.constructor.BaseConstructor;
 import org.yaml.snakeyaml.constructor.Constructor;
 import org.yaml.snakeyaml.constructor.SafeConstructor;
 import org.yaml.snakeyaml.error.Mark;
-import org.yaml.snakeyaml.nodes.CollectionNode;
 import org.yaml.snakeyaml.nodes.MappingNode;
 import org.yaml.snakeyaml.nodes.Node;
 import org.yaml.snakeyaml.nodes.NodeTuple;
 import org.yaml.snakeyaml.nodes.ScalarNode;
+import org.yaml.snakeyaml.nodes.SequenceNode;
 import org.yaml.snakeyaml.nodes.Tag;
 import org.yaml.snakeyaml.representer.Representer;
 import org.yaml.snakeyaml.resolver.Resolver;
@@ -105,8 +105,8 @@ class OriginTrackedYamlLoader extends YamlProcessor {
 
 		@Override
 		protected Object constructObject(Node node) {
-			if (node instanceof CollectionNode && ((CollectionNode<?>) node).getValue().isEmpty()) {
-				return constructTrackedObject(node, super.constructObject(node));
+			if (node instanceof SequenceNode sequenceNode && sequenceNode.getValue().isEmpty()) {
+				return constructTrackedObject(node, "");
 			}
 			if (node instanceof ScalarNode) {
 				if (!(node instanceof KeyScalarNode)) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/env/OriginTrackedYamlLoaderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -17,7 +17,6 @@
 package org.springframework.boot.env;
 
 import java.nio.charset.StandardCharsets;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
@@ -115,18 +114,19 @@ class OriginTrackedYamlLoaderTests {
 	void processEmptyAndNullValues() {
 		OriginTrackedValue empty = getValue("empty");
 		OriginTrackedValue nullValue = getValue("null-value");
+		OriginTrackedValue emptyList = getValue("emptylist");
 		assertThat(empty.getValue()).isEqualTo("");
 		assertThat(getLocation(empty)).isEqualTo("27:8");
 		assertThat(nullValue.getValue()).isEqualTo("");
 		assertThat(getLocation(nullValue)).isEqualTo("28:13");
+		assertThat(emptyList.getValue()).isEqualTo("");
+		assertThat(getLocation(emptyList)).isEqualTo("29:12");
 	}
 
 	@Test
-	void processEmptyListAndMap() {
-		OriginTrackedValue emptymap = getValue("emptymap");
-		OriginTrackedValue emptylist = getValue("emptylist");
-		assertThat(emptymap.getValue()).isEqualTo(Collections.emptyMap());
-		assertThat(emptylist.getValue()).isEqualTo(Collections.emptyList());
+	void emptyMapsAreDropped() {
+		Object emptyMap = getValue("emptymap");
+		assertThat(emptyMap).isNull();
 	}
 
 	@Test
@@ -194,11 +194,12 @@ class OriginTrackedYamlLoaderTests {
 		assertThat(loaded).isNotEmpty();
 	}
 
-	private OriginTrackedValue getValue(String name) {
+	@SuppressWarnings("unchecked")
+	private <T> T getValue(String name) {
 		if (this.result == null) {
 			this.result = this.loader.load();
 		}
-		return (OriginTrackedValue) this.result.get(0).get(name);
+		return (T) this.result.get(0).get(name);
 	}
 
 	private String getLocation(OriginTrackedValue value) {