Introduce fallback flag and annotation (as companion to primary)

Closes gh-26241

Author: jhoeller

spring-beans/src/main/java/org/springframework/beans/factory/config/BeanDefinition.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -178,6 +178,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	 * Set whether this bean is a primary autowire candidate.
 	 * <p>If this value is {@code true} for exactly one bean among multiple
 	 * matching candidates, it will serve as a tie-breaker.
+	 * @see #setFallback
 	 */
 	void setPrimary(boolean primary);
 
@@ -186,6 +187,21 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	 */
 	boolean isPrimary();
 
+	/**
+	 * Set whether this bean is a fallback autowire candidate.
+	 * <p>If this value is {@code true} for all beans but one among multiple
+	 * matching candidates, the remaining bean will be selected.
+	 * @since 6.2
+	 * @see #setPrimary
+	 */
+	void setFallback(boolean fallback);
+
+	/**
+	 * Return whether this bean is a fallback autowire candidate.
+	 * @since 6.2
+	 */
+	boolean isFallback();
+
 	/**
 	 * Specify the factory bean to use, if any.
 	 * This the name of the bean to call the specified factory method on.

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

@@ -189,6 +189,8 @@ public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccess
 
 	private boolean primary = false;
 
+	private boolean fallback = false;
+
 	private final Map<String, AutowireCandidateQualifier> qualifiers = new LinkedHashMap<>();
 
 	@Nullable
@@ -288,6 +290,7 @@ protected AbstractBeanDefinition(BeanDefinition original) {
 			setAutowireCandidate(originalAbd.isAutowireCandidate());
 			setDefaultCandidate(originalAbd.isDefaultCandidate());
 			setPrimary(originalAbd.isPrimary());
+			setFallback(originalAbd.isFallback());
 			copyQualifiersFrom(originalAbd);
 			setInstanceSupplier(originalAbd.getInstanceSupplier());
 			setNonPublicAccessAllowed(originalAbd.isNonPublicAccessAllowed());
@@ -365,6 +368,7 @@ public void overrideFrom(BeanDefinition other) {
 			setAutowireCandidate(otherAbd.isAutowireCandidate());
 			setDefaultCandidate(otherAbd.isDefaultCandidate());
 			setPrimary(otherAbd.isPrimary());
+			setFallback(otherAbd.isFallback());
 			copyQualifiersFrom(otherAbd);
 			setInstanceSupplier(otherAbd.getInstanceSupplier());
 			setNonPublicAccessAllowed(otherAbd.isNonPublicAccessAllowed());
@@ -742,6 +746,7 @@ public boolean isDefaultCandidate() {
 	 * Set whether this bean is a primary autowire candidate.
 	 * <p>Default is {@code false}. If this value is {@code true} for exactly one
 	 * bean among multiple matching candidates, it will serve as a tie-breaker.
+	 * @see #setFallback
 	 */
 	@Override
 	public void setPrimary(boolean primary) {
@@ -756,6 +761,25 @@ public boolean isPrimary() {
 		return this.primary;
 	}
 
+	/**
+	 * Set whether this bean is a fallback autowire candidate.
+	 * <p>Default is {@code false}. If this value is {@code true} for all beans but
+	 * one among multiple matching candidates, the remaining bean will be selected.
+	 * @since 6.2
+	 * @see #setPrimary
+	 */
+	public void setFallback(boolean fallback) {
+		this.fallback = fallback;
+	}
+
+	/**
+	 * Return whether this bean is a fallback autowire candidate.
+	 * @since 6.2
+	 */
+	public boolean isFallback() {
+		return this.fallback;
+	}
+
 	/**
 	 * Register a qualifier to be used for autowire candidate resolution,
 	 * keyed by the qualifier's type name.

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

@@ -1796,6 +1796,7 @@ protected String determineAutowireCandidate(Map<String, Object> candidates, Depe
 	@Nullable
 	protected String determinePrimaryCandidate(Map<String, Object> candidates, Class<?> requiredType) {
 		String primaryBeanName = null;
+		// First pass: identify unique primary candidate
 		for (Map.Entry<String, Object> entry : candidates.entrySet()) {
 			String candidateBeanName = entry.getKey();
 			Object beanInstance = entry.getValue();
@@ -1816,6 +1817,19 @@ else if (candidateLocal) {
 				}
 			}
 		}
+		// Second pass: identify unique non-fallback candidate
+		if (primaryBeanName == null) {
+			for (Map.Entry<String, Object> entry : candidates.entrySet()) {
+				String candidateBeanName = entry.getKey();
+				Object beanInstance = entry.getValue();
+				if (!isFallback(candidateBeanName, beanInstance)) {
+					if (primaryBeanName != null) {
+						return null;
+					}
+					primaryBeanName = candidateBeanName;
+				}
+			}
+		}
 		return primaryBeanName;
 	}
 
@@ -1878,6 +1892,23 @@ protected boolean isPrimary(String beanName, Object beanInstance) {
 				parent.isPrimary(transformedBeanName, beanInstance));
 	}
 
+	/**
+	 * Return whether the bean definition for the given bean name has been
+	 * marked as a fallback bean.
+	 * @param beanName the name of the bean
+	 * @param beanInstance the corresponding bean instance (can be {@code null})
+	 * @return whether the given bean qualifies as fallback
+	 * @since 6.2
+	 */
+	protected boolean isFallback(String beanName, Object beanInstance) {
+		String transformedBeanName = transformedBeanName(beanName);
+		if (containsBeanDefinition(transformedBeanName)) {
+			return getMergedLocalBeanDefinition(transformedBeanName).isFallback();
+		}
+		return (getParentBeanFactory() instanceof DefaultListableBeanFactory parent &&
+				parent.isFallback(transformedBeanName, beanInstance));
+	}
+
 	/**
 	 * Return the priority assigned for the given bean instance by
 	 * the {@code jakarta.annotation.Priority} annotation.

spring-context/src/main/java/org/springframework/context/annotation/AnnotationConfigUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -246,6 +246,9 @@ else if (abd.getMetadata() != metadata) {
 		if (metadata.isAnnotated(Primary.class.getName())) {
 			abd.setPrimary(true);
 		}
+		if (metadata.isAnnotated(Fallback.class.getName())) {
+			abd.setFallback(true);
+		}
 		AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
 		if (dependsOn != null) {
 			abd.setDependsOn(dependsOn.getStringArray("value"));

spring-context/src/main/java/org/springframework/context/annotation/Fallback.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2002-2024 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.context.annotation;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Indicates that a bean qualifies as a fallback autowire candidate.
+ * This is a companion and alternative to the {@link Primary} annotation.
+ *
+ * <p>If all beans but one among multiple matching candidates are marked
+ * as a fallback, the remaining bean will be selected.
+ *
+ * @author Juergen Hoeller
+ * @since 6.2
+ * @see Primary
+ * @see Lazy
+ * @see Bean
+ * @see org.springframework.beans.factory.config.BeanDefinition#setFallback
+ */
+@Target({ElementType.TYPE, ElementType.METHOD})
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+public @interface Fallback {
+
+}

spring-context/src/main/java/org/springframework/context/annotation/Primary.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -77,10 +77,12 @@
  * @author Chris Beams
  * @author Juergen Hoeller
  * @since 3.0
+ * @see Fallback
  * @see Lazy
  * @see Bean
  * @see ComponentScan
  * @see org.springframework.stereotype.Component
+ * @see org.springframework.beans.factory.config.BeanDefinition#setPrimary
  */
 @Target({ElementType.TYPE, ElementType.METHOD})
 @Retention(RetentionPolicy.RUNTIME)

spring-context/src/test/java/org/springframework/context/annotation/configuration/BeanMethodQualificationTests.java

@@ -30,7 +30,9 @@
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Fallback;
 import org.springframework.context.annotation.Lazy;
+import org.springframework.context.annotation.Primary;
 import org.springframework.context.annotation.Scope;
 import org.springframework.context.annotation.ScopedProxyMode;
 import org.springframework.core.annotation.AliasFor;
@@ -80,6 +82,26 @@ void scopedProxy() {
 		ctx.close();
 	}
 
+	@Test
+	void primary() {
+		AnnotationConfigApplicationContext ctx =
+				new AnnotationConfigApplicationContext(PrimaryConfig.class, StandardPojo.class);
+		StandardPojo pojo = ctx.getBean(StandardPojo.class);
+		assertThat(pojo.testBean.getName()).isEqualTo("interesting");
+		assertThat(pojo.testBean2.getName()).isEqualTo("boring");
+		ctx.close();
+	}
+
+	@Test
+	void fallback() {
+		AnnotationConfigApplicationContext ctx =
+				new AnnotationConfigApplicationContext(FallbackConfig.class, StandardPojo.class);
+		StandardPojo pojo = ctx.getBean(StandardPojo.class);
+		assertThat(pojo.testBean.getName()).isEqualTo("interesting");
+		assertThat(pojo.testBean2.getName()).isEqualTo("boring");
+		ctx.close();
+	}
+
 	@Test
 	void customWithLazyResolution() {
 		AnnotationConfigApplicationContext ctx =
@@ -201,6 +223,58 @@ public TestBean testBean2(TestBean testBean1) {
 		}
 	}
 
+	@Configuration
+	static class PrimaryConfig {
+
+		@Bean @Qualifier("interesting") @Primary
+		public static TestBean testBean1() {
+			return new TestBean("interesting");
+		}
+
+		@Bean @Qualifier("interesting")
+		public static TestBean testBean1x() {
+			return new TestBean("interesting");
+		}
+
+		@Bean @Boring @Primary
+		public TestBean testBean2(TestBean testBean1) {
+			TestBean tb = new TestBean("boring");
+			tb.setSpouse(testBean1);
+			return tb;
+		}
+
+		@Bean @Boring
+		public TestBean testBean2x() {
+			return new TestBean("boring");
+		}
+	}
+
+	@Configuration
+	static class FallbackConfig {
+
+		@Bean @Qualifier("interesting")
+		public static TestBean testBean1() {
+			return new TestBean("interesting");
+		}
+
+		@Bean @Qualifier("interesting") @Fallback
+		public static TestBean testBean1x() {
+			return new TestBean("interesting");
+		}
+
+		@Bean @Boring
+		public TestBean testBean2(TestBean testBean1) {
+			TestBean tb = new TestBean("boring");
+			tb.setSpouse(testBean1);
+			return tb;
+		}
+
+		@Bean @Boring @Fallback
+		public TestBean testBean2x() {
+			return new TestBean("boring");
+		}
+	}
+
 	@Component @Lazy
 	static class StandardPojo {