Properties treated as Map<String, String> for bind

Fixes gh-9152

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

@@ -18,6 +18,7 @@ package org.springframework.boot.context.properties.bind;
 
 import java.util.Collection;
 import java.util.Map;
+import java.util.Properties;
 
 import org.springframework.boot.context.properties.bind.convert.BinderConversionService;
 import org.springframework.boot.context.properties.source.ConfigurationProperty;
@@ -44,15 +45,24 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 	protected Object bind(ConfigurationPropertyName name, Bindable<?> target,
 			AggregateElementBinder elementBinder, Class<?> type) {
 		Map<Object, Object> map = CollectionFactory.createMap(type, 0);
+		Bindable<?> resolvedTarget = resolveTarget(target);
 		for (ConfigurationPropertySource source : getContext().getSources()) {
 			if (!ConfigurationPropertyName.EMPTY.equals(name)) {
 				source = source.filter(name::isAncestorOf);
 			}
-			new EntryBinder(name, target, elementBinder).bindEntries(source, map);
+			new EntryBinder(name, resolvedTarget, elementBinder).bindEntries(source, map);
 		}
 		return (map.isEmpty() ? null : map);
 	}
 
+	private Bindable<?> resolveTarget(Bindable<?> target) {
+		Class<?> type = target.getType().resolve();
+		if (Properties.class.isAssignableFrom(type)) {
+			return Bindable.mapOf(String.class, String.class);
+		}
+		return target;
+	}
+
 	@Override
 	protected Map<Object, Object> merge(Map<Object, Object> existing,
 			Map<Object, Object> additional) {

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

@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Properties;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -372,4 +373,19 @@ public class MapBinderTests {
 				eq(target), any(), isA(Map.class));
 	}
 
+	@Test
+	public void bindToPropertiesShouldBeEquivalentToMapOfStringString() throws Exception {
+		this.sources
+				.add(new MockConfigurationPropertySource("foo.bar.baz", "1", "line1"));
+		BindHandler handler = mock(BindHandler.class,
+				withSettings().defaultAnswer(Answers.CALLS_REAL_METHODS));
+		Bindable<Properties> target = Bindable.of(Properties.class);
+		this.binder.bind("foo", target, handler);
+		InOrder inOrder = inOrder(handler);
+		inOrder.verify(handler).onSuccess(eq(ConfigurationPropertyName.of("foo.bar.baz")),
+				eq(Bindable.of(String.class)), any(), eq("1"));
+		inOrder.verify(handler).onSuccess(eq(ConfigurationPropertyName.of("foo")),
+				eq(target), any(), isA(Properties.class));
+	}
+
 }