Expose a way to resolved a merged inner bean definition

This commit extracts the logic of resolving a merged bean definition for
an inner bean to a public method so that other components can reuse it.

Closes gh-28093

Author: snicoll

spring-beans/src/main/java/org/springframework/beans/factory/support/BeanDefinitionValueResolver.java

@@ -23,6 +23,7 @@
 import java.util.Map;
 import java.util.Properties;
 import java.util.Set;
+import java.util.function.BiFunction;
 
 import org.springframework.beans.BeanWrapper;
 import org.springframework.beans.BeanWrapperImpl;
@@ -57,6 +58,7 @@
  *
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Stephane Nicoll
  * @since 1.2
  * @see AbstractAutowireCapableBeanFactory
  */
@@ -143,13 +145,12 @@ else if (value instanceof RuntimeBeanNameReference ref) {
 		}
 		else if (value instanceof BeanDefinitionHolder bdHolder) {
 			// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
-			return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
+			return resolveInnerBean(bdHolder.getBeanName(), bdHolder.getBeanDefinition(),
+					(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));
 		}
 		else if (value instanceof BeanDefinition bd) {
-			// Resolve plain BeanDefinition, without contained name: use dummy name.
-			String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR +
-					ObjectUtils.getIdentityHexString(bd);
-			return resolveInnerBean(argName, innerBeanName, bd);
+			return resolveInnerBean(null, bd,
+					(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));
 		}
 		else if (value instanceof DependencyDescriptor dependencyDescriptor) {
 			Set<String> autowiredBeanNames = new LinkedHashSet<>(4);
@@ -243,6 +244,23 @@ else if (value instanceof NullBean) {
 		}
 	}
 
+	/**
+	 * Resolve an inner bean definition and invoke the specified {@code resolver}
+	 * on its merged bean definition.
+	 * @param innerBeanName the inner bean name (or {@code null} to assign one)
+	 * @param innerBd the inner raw bean definition
+	 * @param resolver the function to invoke to resolve
+	 * @param <T> the type of the resolution
+	 * @return a resolved inner bean, as a result of applying the {@code resolver}
+	 */
+	public <T> T resolveInnerBean(@Nullable String innerBeanName, BeanDefinition innerBd,
+			BiFunction<String, RootBeanDefinition, T> resolver) {
+		String nameToUse = (innerBeanName != null ? innerBeanName : "(inner bean)"
+				+ BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR + ObjectUtils.getIdentityHexString(innerBd));
+		return resolver.apply(nameToUse, this.beanFactory.getMergedBeanDefinition(
+				nameToUse, innerBd, this.beanDefinition));
+	}
+
 	/**
 	 * Evaluate the given value as an expression, if necessary.
 	 * @param value the candidate value (may be an expression)
@@ -362,14 +380,12 @@ private Object resolveReference(Object argName, RuntimeBeanReference ref) {
 	 * Resolve an inner bean definition.
 	 * @param argName the name of the argument that the inner bean is defined for
 	 * @param innerBeanName the name of the inner bean
-	 * @param innerBd the bean definition for the inner bean
+	 * @param mbd the merged bean definition for the inner bean
 	 * @return the resolved inner bean instance
 	 */
 	@Nullable
-	private Object resolveInnerBean(Object argName, String innerBeanName, BeanDefinition innerBd) {
-		RootBeanDefinition mbd = null;
+	private Object resolveInnerBeanValue(Object argName, String innerBeanName, RootBeanDefinition mbd) {
 		try {
-			mbd = this.beanFactory.getMergedBeanDefinition(innerBeanName, innerBd, this.beanDefinition);
 			// Check given bean name whether it is unique. If not already unique,
 			// add counter - increasing the counter until the name is unique.
 			String actualInnerBeanName = innerBeanName;
@@ -400,7 +416,7 @@ private Object resolveInnerBean(Object argName, String innerBeanName, BeanDefini
 			throw new BeanCreationException(
 					this.beanDefinition.getResourceDescription(), this.beanName,
 					"Cannot create inner bean '" + innerBeanName + "' " +
-					(mbd != null && mbd.getBeanClassName() != null ? "of type [" + mbd.getBeanClassName() + "] " : "") +
+					(mbd.getBeanClassName() != null ? "of type [" + mbd.getBeanClassName() + "] " : "") +
 					"while setting " + argName, ex);
 		}
 	}

spring-beans/src/test/java/org/springframework/beans/factory/support/BeanDefinitionValueResolverTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2002-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.beans.factory.support;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link BeanDefinitionValueResolver}.
+ *
+ * @author Stephane Nicoll
+ */
+class BeanDefinitionValueResolverTests {
+
+	@Test
+	void resolveInnerBean() {
+		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
+		RootBeanDefinition parentBd = new RootBeanDefinition();
+		GenericBeanDefinition innerBd = new GenericBeanDefinition();
+		innerBd.setAttribute("test", 42);
+		BeanDefinitionValueResolver bdvr = new BeanDefinitionValueResolver(beanFactory, "test", parentBd);
+		RootBeanDefinition resolvedInnerBd = bdvr.resolveInnerBean(null,innerBd, (name, mbd) -> {
+			assertThat(name).isNotNull().startsWith("(inner bean");
+			return mbd;
+		});
+		assertThat(resolvedInnerBd.getAttribute("test")).isEqualTo(42);
+	}
+
+}