Support binding to immutable maps

Closes gh-13323

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/MapBinder.java

@@ -90,22 +90,32 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 		if (existingMap == null) {
 			return additional;
 		}
-		existingMap.putAll(additional);
-		return copyIfPossible(existingMap);
+		try {
+			existingMap.putAll(additional);
+			return copyIfPossible(existingMap);
+		}
+		catch (UnsupportedOperationException ex) {
+			Map<Object, Object> result = createNewMap(additional.getClass(), existingMap);
+			result.putAll(additional);
+			return result;
+		}
 	}
 
 	private Map<Object, Object> copyIfPossible(Map<Object, Object> map) {
 		try {
-			Map<Object, Object> result = CollectionFactory.createMap(map.getClass(),
-					map.size());
-			result.putAll(map);
-			return result;
+			return createNewMap(map.getClass(), map);
 		}
 		catch (Exception ex) {
 			return map;
 		}
 	}
 
+	private Map<Object, Object> createNewMap(Class<?> mapClass, Map<Object, Object> map) {
+		Map<Object, Object> result = CollectionFactory.createMap(mapClass, map.size());
+		result.putAll(map);
+		return result;
+	}
+
 	private class EntryBinder {
 
 		private final ConfigurationPropertyName root;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/MapBinderTests.java

@@ -615,6 +615,22 @@ public class MapBinderTests {
 		assertThat(result.getItems()).containsExactly(entry("a", "b"));
 	}
 
+	@Test
+	public void bindToImmutableMapShouldReturnPopulatedCollection() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.values.c", "d");
+		source.put("foo.values.e", "f");
+		this.sources.add(source);
+		Map<String, String> result = this.binder
+				.bind("foo.values",
+						STRING_STRING_MAP
+								.withExistingValue(Collections.singletonMap("a", "b")))
+				.get();
+		assertThat(result).hasSize(3);
+		assertThat(result.entrySet()).containsExactly(entry("a", "b"), entry("c", "d"),
+				entry("e", "f"));
+	}
+
 	private <K, V> Bindable<Map<K, V>> getMapBindable(Class<K> keyGeneric,
 			ResolvableType valueType) {
 		ResolvableType keyType = ResolvableType.forClass(keyGeneric);