Add support for merging two BeanRegistrationAotContribution instances

Closes gh-31446

spring-beans/src/main/java/org/springframework/beans/factory/aot/BeanRegistrationAotContribution.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -19,6 +19,7 @@ package org.springframework.beans.factory.aot;
 import java.util.function.UnaryOperator;
 
 import org.springframework.aot.generate.GenerationContext;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -82,4 +83,31 @@ public interface BeanRegistrationAotContribution {
 		};
 	}
 
+	/**
+	 * Create a contribution that applies the contribution of the first contribution
+	 * followed by the second contribution. Any contribution can be {@code null} to be
+	 * ignored and the concatenated contribution is {@code null} if both inputs are
+	 * {@code null}.
+	 * @param a the first contribution
+	 * @param b the second contribution
+	 * @return the concatenation of the two contributions, or {@code null} if
+	 * they are both {@code null}.
+	 * @since 6.1
+	 */
+	@Nullable
+	static BeanRegistrationAotContribution concat(@Nullable BeanRegistrationAotContribution a,
+			@Nullable BeanRegistrationAotContribution b) {
+
+		if (a == null) {
+			return b;
+		}
+		if (b == null) {
+			return a;
+		}
+		return (generationContext, beanRegistrationCode) -> {
+			a.applyTo(generationContext, beanRegistrationCode);
+			b.applyTo(generationContext, beanRegistrationCode);
+		};
+	}
+
 }

spring-beans/src/test/java/org/springframework/beans/factory/aot/BeanRegistrationAotContributionTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2002-2023 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.aot;
+
+import org.junit.jupiter.api.Test;
+import org.mockito.InOrder;
+
+import org.springframework.aot.test.generate.TestGenerationContext;
+import org.springframework.beans.testfixture.beans.factory.aot.MockBeanRegistrationCode;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+/**
+ * Tests for {@link BeanRegistrationAotContribution}.
+ *
+ * @author Stephane Nicoll
+ */
+class BeanRegistrationAotContributionTests {
+
+	@Test
+	void concatWithBothNullReturnsNull() {
+		assertThat(BeanRegistrationAotContribution.concat(null, null)).isNull();
+	}
+
+	@Test
+	void concatWithFirstNullReturnsSecondAsIs() {
+		BeanRegistrationAotContribution contribution = mock(BeanRegistrationAotContribution.class);
+		assertThat(BeanRegistrationAotContribution.concat(null, contribution)).isSameAs(contribution);
+		verifyNoInteractions(contribution);
+	}
+
+	@Test
+	void concatWithSecondNullReturnsFirstAsIs() {
+		BeanRegistrationAotContribution contribution = mock(BeanRegistrationAotContribution.class);
+		assertThat(BeanRegistrationAotContribution.concat(contribution, null)).isSameAs(contribution);
+		verifyNoInteractions(contribution);
+	}
+
+	@Test
+	void concatApplyContributionsInOrder() {
+		BeanRegistrationAotContribution first = mock(BeanRegistrationAotContribution.class);
+		BeanRegistrationAotContribution second = mock(BeanRegistrationAotContribution.class);
+		BeanRegistrationAotContribution combined = BeanRegistrationAotContribution.concat(first, second);
+		assertThat(combined).isNotNull();
+		TestGenerationContext generationContext = new TestGenerationContext();
+		BeanRegistrationCode beanRegistrationCode = new MockBeanRegistrationCode(generationContext);
+		combined.applyTo(generationContext, beanRegistrationCode);
+		InOrder ordered = inOrder(first, second);
+		ordered.verify(first).applyTo(generationContext, beanRegistrationCode);
+		ordered.verify(second).applyTo(generationContext, beanRegistrationCode);
+	}
+
+}