Preserve property ordering in SpringIterableConfigurationPropertySource

Fixes gh-21470

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SpringIterableConfigurationPropertySource.java

@@ -19,9 +19,9 @@ package org.springframework.boot.context.properties.source;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.ConcurrentModificationException;
-import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Objects;
@@ -259,7 +259,7 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 		}
 
 		private <K, V> Map<K, V> cloneOrCreate(Map<K, V> source, int size) {
-			return (source != null) ? new HashMap<>(source) : new HashMap<>(size);
+			return (source != null) ? new LinkedHashMap<>(source) : new LinkedHashMap<>(size);
 		}
 
 		private void addParents(Map<ConfigurationPropertyName, Set<ConfigurationPropertyName>> descendants,

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/SpringIterableConfigurationPropertySourceTests.java

@@ -224,6 +224,20 @@ class SpringIterableConfigurationPropertySourceTests {
 		assertThat(adapter.stream()).hasSize(2);
 	}
 
+	@Test
+	void orderOfUnderlyingSourceIsPreserved() {
+		Map<String, Object> map = new LinkedHashMap<>();
+		map.put("test.map.alpha", "value1");
+		map.put("test.map.bravo", "value2");
+		map.put("test.map.charlie", "value3");
+		map.put("test.map.delta", "value4");
+		EnumerablePropertySource<?> source = new OriginTrackedMapPropertySource("test", map, true);
+		SpringIterableConfigurationPropertySource propertySource = new SpringIterableConfigurationPropertySource(source,
+				DefaultPropertyMapper.INSTANCE);
+		assertThat(propertySource.stream().map(ConfigurationPropertyName::toString)).containsExactly("test.map.alpha",
+				"test.map.bravo", "test.map.charlie", "test.map.delta");
+	}
+
 	/**
 	 * Test {@link PropertySource} that's also an {@link OriginLookup}.
 	 *