Defer creation of maps in MapBinder

Closes gh-39375

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

@@ -53,18 +53,22 @@ protected boolean isAllowRecursiveBinding(ConfigurationPropertySource source) {
 	@Override
 	protected Object bindAggregate(ConfigurationPropertyName name, Bindable<?> target,
 			AggregateElementBinder elementBinder) {
-		Map<Object, Object> map = CollectionFactory
-			.createMap((target.getValue() != null) ? Map.class : target.getType().resolve(Object.class), 0);
 		Bindable<?> resolvedTarget = resolveTarget(target);
 		boolean hasDescendants = hasDescendants(name);
-		for (ConfigurationPropertySource source : getContext().getSources()) {
-			if (!ConfigurationPropertyName.EMPTY.equals(name)) {
+		if (!hasDescendants && !ConfigurationPropertyName.EMPTY.equals(name)) {
+			for (ConfigurationPropertySource source : getContext().getSources()) {
 				ConfigurationProperty property = source.getConfigurationProperty(name);
-				if (property != null && !hasDescendants) {
+				if (property != null) {
 					getContext().setConfigurationProperty(property);
 					Object result = getContext().getPlaceholdersResolver().resolvePlaceholders(property.getValue());
 					return getContext().getConverter().convert(result, target);
 				}
+			}
+		}
+		Map<Object, Object> map = CollectionFactory
+			.createMap((target.getValue() != null) ? Map.class : target.getType().resolve(Object.class), 0);
+		for (ConfigurationPropertySource source : getContext().getSources()) {
+			if (!ConfigurationPropertyName.EMPTY.equals(name)) {
 				source = source.filter(name::isAncestorOf);
 			}
 			new EntryBinder(name, resolvedTarget, elementBinder).bindEntries(source, map);

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

@@ -17,13 +17,15 @@
 package org.springframework.boot.context.properties.bind;
 
 import java.net.InetAddress;
+import java.util.AbstractMap;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
+import java.util.Set;
 import java.util.stream.Collectors;
 
 import org.junit.jupiter.api.Test;
@@ -33,6 +35,7 @@
 
 import org.springframework.boot.context.properties.bind.BinderTests.ExampleEnum;
 import org.springframework.boot.context.properties.bind.BinderTests.JavaBean;
+import org.springframework.boot.context.properties.bind.MapBinderTests.CustomMapWithoutDefaultCtor.CustomMap;
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
 import org.springframework.boot.context.properties.source.ConfigurationPropertySource;
 import org.springframework.boot.context.properties.source.MapConfigurationPropertySource;
@@ -78,7 +81,7 @@ class MapBinderTests {
 
 	private final List<ConfigurationPropertySource> sources = new ArrayList<>();
 
-	private Binder binder = new Binder(this.sources);
+	private final Binder binder = new Binder(this.sources);
 
 	@Test
 	void bindToMapShouldReturnPopulatedMap() {
@@ -315,15 +318,13 @@ void bindToMapWithPlaceholdersShouldBeGreedyForScalars() {
 		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(environment, "foo=boo");
 		MockConfigurationPropertySource source = new MockConfigurationPropertySource("foo.aaa.bbb.ccc", "baz-${foo}");
 		this.sources.add(source);
-		this.binder = new Binder(this.sources, new PropertySourcesPlaceholdersResolver(environment));
-		Map<String, ExampleEnum> result = this.binder.bind("foo", Bindable.mapOf(String.class, ExampleEnum.class))
-			.get();
+		Binder binder = new Binder(this.sources, new PropertySourcesPlaceholdersResolver(environment));
+		Map<String, ExampleEnum> result = binder.bind("foo", Bindable.mapOf(String.class, ExampleEnum.class)).get();
 		assertThat(result).containsEntry("aaa.bbb.ccc", ExampleEnum.BAZ_BOO);
 	}
 
 	@Test
 	void bindToMapWithNoPropertiesShouldReturnUnbound() {
-		this.binder = new Binder(this.sources);
 		BindResult<Map<String, ExampleEnum>> result = this.binder.bind("foo",
 				Bindable.mapOf(String.class, ExampleEnum.class));
 		assertThat(result.isBound()).isFalse();
@@ -624,6 +625,18 @@ void bindToMapWithPlaceholdersShouldResolve() {
 		assertThat(map).containsKey("bcd");
 	}
 
+	@Test
+	void bindToCustomMapWithoutCtorAndConverterShouldResolve() {
+		DefaultConversionService conversionService = new DefaultConversionService();
+		conversionService.addConverter(new CustomMapConverter());
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.custom-map", "value");
+		this.sources.add(source);
+		Binder binder = new Binder(this.sources, null, conversionService, null);
+		CustomMapWithoutDefaultCtor result = binder.bind("foo", Bindable.of(CustomMapWithoutDefaultCtor.class)).get();
+		assertThat(result.getCustomMap().getSource()).isEqualTo("value");
+	}
+
 	private <K, V> Bindable<Map<K, V>> getMapBindable(Class<K> keyGeneric, ResolvableType valueType) {
 		ResolvableType keyType = ResolvableType.forClass(keyGeneric);
 		return Bindable.of(ResolvableType.forClassWithGenerics(Map.class, keyType, valueType));
@@ -761,6 +774,48 @@ void setAddresses(Map<String, ? extends List<? extends InetAddress>> addresses)
 
 	}
 
+	static class CustomMapWithoutDefaultCtor {
+
+		private final CustomMap customMap;
+
+		CustomMapWithoutDefaultCtor(CustomMap customMap) {
+			this.customMap = customMap;
+		}
+
+		CustomMap getCustomMap() {
+			return this.customMap;
+		}
+
+		static final class CustomMap extends AbstractMap<String, Object> {
+
+			private final String source;
+
+			CustomMap(String source) {
+				this.source = source;
+			}
+
+			@Override
+			public Set<Entry<String, Object>> entrySet() {
+				return Collections.emptySet();
+			}
+
+			String getSource() {
+				return this.source;
+			}
+
+		}
+
+	}
+
+	private static final class CustomMapConverter implements Converter<String, CustomMap> {
+
+		@Override
+		public CustomMap convert(String source) {
+			return new CustomMap(source);
+		}
+
+	}
+
 	private static final class InvocationArgument<T> implements Answer<T> {
 
 		private final int index;