Duplicate values should bind properly to List

Fixes gh-10106

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

@@ -19,7 +19,6 @@ package org.springframework.boot.context.properties.bind;
 import java.util.function.Supplier;
 
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
-import org.springframework.core.ResolvableType;
 
 /**
  * Internal strategy used by {@link Binder} to bind aggregates (Maps, Lists, Arrays).
@@ -47,9 +46,7 @@ abstract class AggregateBinder<T> {
 	public final Object bind(ConfigurationPropertyName name, Bindable<?> target,
 			AggregateElementBinder itemBinder) {
 		Supplier<?> value = target.getValue();
-		Class<?> type = (value == null ? target.getType().resolve()
-				: ResolvableType.forClass(AggregateBinder.class, getClass())
-						.resolveGeneric());
+		Class<?> type = (value == null ? target.getType().resolve() : null);
 		Object result = bind(name, target, itemBinder, type);
 		if (result == null || value == null || value.get() == null) {
 			return result;

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

@@ -17,6 +17,7 @@
 package org.springframework.boot.context.properties.bind;
 
 import java.util.Collection;
+import java.util.List;
 
 import org.springframework.boot.context.properties.source.ConfigurationPropertyName;
 import org.springframework.core.CollectionFactory;
@@ -37,10 +38,12 @@ class CollectionBinder extends IndexedElementsBinder<Collection<Object>> {
 	@Override
 	protected Object bind(ConfigurationPropertyName name, Bindable<?> target,
 			AggregateElementBinder elementBinder, Class<?> type) {
+		Class<?> collectionType = (type != null ? type
+				: ResolvableType.forClassWithGenerics(List.class, Object.class).resolve());
 		IndexedCollectionSupplier collection = new IndexedCollectionSupplier(
-				() -> CollectionFactory.createCollection(type, 0));
+				() -> CollectionFactory.createCollection(collectionType, 0));
 		ResolvableType elementType = target.getType().asCollection().getGeneric();
-		bindIndexed(name, target, elementBinder, collection, target.getType(),
+		bindIndexed(name, target, elementBinder, collection, ResolvableType.forClass(collectionType),
 				elementType);
 		if (collection.wasSupplied()) {
 			return collection.get();

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

@@ -48,7 +48,9 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 	@Override
 	protected Object bind(ConfigurationPropertyName name, Bindable<?> target,
 			AggregateElementBinder elementBinder, Class<?> type) {
-		Map<Object, Object> map = CollectionFactory.createMap(type, 0);
+		Class<?> mapType = (type != null ? type
+				: ResolvableType.forClassWithGenerics(Map.class, Object.class, Object.class).resolve());
+		Map<Object, Object> map = CollectionFactory.createMap(mapType, 0);
 		Bindable<?> resolvedTarget = resolveTarget(target);
 		for (ConfigurationPropertySource source : getContext().getSources()) {
 			if (!ConfigurationPropertyName.EMPTY.equals(name)) {

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

@@ -18,6 +18,7 @@ package org.springframework.boot.context.properties.bind;
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
@@ -316,10 +317,45 @@ public class CollectionBinderTests {
 		assertThat(result.getItems()).containsExactly("a", "b", "c", "d");
 	}
 
+	@Test
+	public void bindToCollectionWithNoDefaultConstructor() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.items", "a,b,c,c");
+		this.sources.add(source);
+		ExampleCustomBean result = this.binder
+				.bind("foo", ExampleCustomBean.class).get();
+		assertThat(result.getItems()).hasSize(4);
+		assertThat(result.getItems()).containsExactly("a", "b", "c", "c");
+	}
+
+	@Test
+	public void bindToListShouldAllowDuplicateValues() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.items", "a,b,c,c");
+		this.sources.add(source);
+		ExampleCollectionBean result = this.binder
+				.bind("foo", ExampleCollectionBean.class).get();
+		assertThat(result.getItems()).hasSize(5);
+		assertThat(result.getItems()).containsExactly("a", "b", "c", "c", "d");
+	}
+
+	@Test
+	public void bindToSetShouldNotAllowDuplicateValues() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.items-set", "a,b,c,c");
+		this.sources.add(source);
+		ExampleCollectionBean result = this.binder
+				.bind("foo", ExampleCollectionBean.class).get();
+		assertThat(result.getItemsSet()).hasSize(3);
+		assertThat(result.getItemsSet()).containsExactly("a", "b", "c");
+	}
+
 	public static class ExampleCollectionBean {
 
 		private List<String> items = new ArrayList<>();
 
+		private Set<String> itemsSet = new HashSet<>();
+
 		public List<String> getItems() {
 			return this.items;
 		}
@@ -328,6 +364,36 @@ public class CollectionBinderTests {
 			this.items.add("d");
 		}
 
+		public Set<String> getItemsSet() {
+			return this.itemsSet;
+		}
+
+		public void setItemsSet(Set<String> itemsSet) {
+			this.itemsSet = itemsSet;
+		}
+	}
+
+	public static class ExampleCustomBean {
+
+		private MyCustomList items = new MyCustomList(Collections.singletonList("foo"));
+
+		public MyCustomList getItems() {
+			return this.items;
+		}
+
+		public void setItems(MyCustomList items) {
+			this.items = items;
+		}
+	}
+
+	public static class MyCustomList extends ArrayList {
+
+		private List<String> items = new ArrayList<>(Collections.singletonList("foo"));
+
+		public MyCustomList(List<String> items) {
+			this.items = items;
+		}
+
 	}
 
 }