Allow recursive binding in Maps

Update `Binder` so that Maps containing references to themselves may
be bound. The existing stack-overflow protection (required when binding
a bean to a non enumerable source) now only applies to bean properties.

Fixes gh-9801

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

@@ -183,18 +183,18 @@ public class Binder {
 		Assert.notNull(target, "Target must not be null");
 		handler = (handler != null ? handler : BindHandler.DEFAULT);
 		Context context = new Context();
-		T bound = bind(name, target, handler, context);
+		T bound = bind(name, target, handler, context, false);
 		return BindResult.of(bound);
 	}
 
 	protected final <T> T bind(ConfigurationPropertyName name, Bindable<T> target,
-			BindHandler handler, Context context) {
+			BindHandler handler, Context context, boolean skipIfHasBoundBean) {
 		context.clearConfigurationProperty();
 		try {
 			if (!handler.onStart(name, target, context)) {
 				return null;
 			}
-			Object bound = bindObject(name, target, handler, context);
+			Object bound = bindObject(name, target, handler, context, skipIfHasBoundBean);
 			return handleBindResult(name, target, handler, context, bound);
 		}
 		catch (Exception ex) {
@@ -234,7 +234,8 @@ public class Binder {
 	}
 
 	private <T> Object bindObject(ConfigurationPropertyName name, Bindable<T> target,
-			BindHandler handler, Context context) throws Exception {
+			BindHandler handler, Context context, boolean skipIfHasBoundBean)
+					throws Exception {
 		ConfigurationProperty property = findProperty(name, context);
 		if (property == null && containsNoDescendantOf(context.streamSources(), name)) {
 			return null;
@@ -246,7 +247,7 @@ public class Binder {
 		if (property != null) {
 			return bindProperty(name, target, handler, context, property);
 		}
-		return bindBean(name, target, handler, context);
+		return bindBean(name, target, handler, context, skipIfHasBoundBean);
 	}
 
 	private AggregateBinder<?> getAggregateBinder(Bindable<?> target, Context context) {
@@ -266,7 +267,8 @@ public class Binder {
 	private <T> Object bindAggregate(ConfigurationPropertyName name, Bindable<T> target,
 			BindHandler handler, Context context, AggregateBinder<?> aggregateBinder) {
 		AggregateElementBinder elementBinder = (itemName, itemTarget, source) -> {
-			Supplier<?> supplier = () -> bind(itemName, itemTarget, handler, context);
+			Supplier<?> supplier = () -> bind(itemName, itemTarget, handler, context,
+					false);
 			return context.withSource(source, supplier);
 		};
 		return context.withIncreasedDepth(
@@ -290,15 +292,16 @@ public class Binder {
 	}
 
 	private Object bindBean(ConfigurationPropertyName name, Bindable<?> target,
-			BindHandler handler, Context context) {
+			BindHandler handler, Context context, boolean skipIfHasBoundBean) {
 		if (containsNoDescendantOf(context.streamSources(), name)
 				|| isUnbindableBean(name, target, context)) {
 			return null;
 		}
 		BeanPropertyBinder propertyBinder = (propertyName, propertyTarget) -> bind(
-				name.append(propertyName), propertyTarget, handler, context);
+				name.append(propertyName), propertyTarget, handler, context, true);
 		Class<?> type = target.getType().resolve();
-		if (context.hasBoundBean(type)) {
+		if (skipIfHasBoundBean && context.hasBoundBean(type)) {
+			System.err.println(type + " " + name);
 			return null;
 		}
 		return context.withBean(type, () -> {

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

@@ -251,10 +251,12 @@ public class BinderTests {
 	@Test
 	public void bindToValidatedBeanWithResourceAndNonEnumerablePropertySource() {
 		ConfigurationPropertySources.from(new PropertySource<String>("test") {
+
 			@Override
 			public Object getProperty(String name) {
 				return null;
 			}
+
 		}).forEach(this.sources::add);
 		Validator validator = new SpringValidatorAdapter(Validation.byDefaultProvider()
 				.configure().buildValidatorFactory().getValidator());
@@ -262,6 +264,14 @@ public class BinderTests {
 				new ValidationBindHandler(validator));
 	}
 
+	@Test
+	public void bindToBeanWithCycle() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		this.sources.add(source.nonIterable());
+		Bindable<CycleBean1> target = Bindable.of(CycleBean1.class);
+		this.binder.bind("foo", target);
+	}
+
 	public static class JavaBean {
 
 		private String value;
@@ -303,4 +313,32 @@ public class BinderTests {
 
 	}
 
+	public static class CycleBean1 {
+
+		private CycleBean2 two;
+
+		public CycleBean2 getTwo() {
+			return this.two;
+		}
+
+		public void setTwo(CycleBean2 two) {
+			this.two = two;
+		}
+
+	}
+
+	public static class CycleBean2 {
+
+		private CycleBean1 one;
+
+		public CycleBean1 getOne() {
+			return this.one;
+		}
+
+		public void setOne(CycleBean1 one) {
+			this.one = one;
+		}
+
+	}
+
 }

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

@@ -19,6 +19,7 @@ package org.springframework.boot.context.properties.bind;
 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;
@@ -515,6 +516,19 @@ public class MapBinderTests {
 		assertThat(map).containsEntry("x [B] y", "[ball]");
 	}
 
+	@Test
+	public void nestedMapsShouldNotBindToNull() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.value", "one");
+		source.put("foo.foos.foo1.value", "two");
+		source.put("foo.foos.foo2.value", "three");
+		this.sources.add(source);
+		BindResult<NestableFoo> foo = this.binder.bind("foo", NestableFoo.class);
+		assertThat(foo.get().getValue()).isNotNull();
+		assertThat(foo.get().getFoos().get("foo1").getValue()).isEqualTo("two");
+		assertThat(foo.get().getFoos().get("foo2").getValue()).isEqualTo("three");
+	}
+
 	private <K, V> Bindable<Map<K, V>> getMapBindable(Class<K> keyGeneric,
 			ResolvableType valueType) {
 		ResolvableType keyType = ResolvableType.forClass(keyGeneric);
@@ -547,4 +561,24 @@ public class MapBinderTests {
 
 	}
 
+	static class NestableFoo {
+
+		private Map<String, NestableFoo> foos = new LinkedHashMap<>();
+
+		private String value;
+
+		public Map<String, NestableFoo> getFoos() {
+			return this.foos;
+		}
+
+		public String getValue() {
+			return this.value;
+		}
+
+		public void setValue(String value) {
+			this.value = value;
+		}
+
+	}
+
 }