Shared empty InjectionMetadata/LifecycleMetadata instance

Closes gh-22570

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -443,7 +443,7 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 
 	private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
 		if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
-			return new InjectionMetadata(clazz, Collections.emptyList());
+			return InjectionMetadata.EMPTY;
 		}
 
 		List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
@@ -496,7 +496,7 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 		}
 		while (targetClass != null && targetClass != Object.class);
 
-		return new InjectionMetadata(clazz, elements);
+		return InjectionMetadata.forElements(elements, clazz);
 	}
 
 	@Nullable

spring-beans/src/main/java/org/springframework/beans/factory/annotation/InitDestroyAnnotationBeanPostProcessor.java

@@ -80,6 +80,24 @@ import org.springframework.util.ReflectionUtils;
 public class InitDestroyAnnotationBeanPostProcessor
 		implements DestructionAwareBeanPostProcessor, MergedBeanDefinitionPostProcessor, PriorityOrdered, Serializable {
 
+	private final transient LifecycleMetadata emptyLifecycleMetadata =
+			new LifecycleMetadata(Object.class, Collections.emptyList(), Collections.emptyList()) {
+				@Override
+				public void checkConfigMembers(RootBeanDefinition beanDefinition) {
+				}
+				@Override
+				public void invokeInitMethods(Object target, String beanName) {
+				}
+				@Override
+				public void invokeDestroyMethods(Object target, String beanName) {
+				}
+				@Override
+				public boolean hasDestroyMethods() {
+					return false;
+				}
+			};
+
+
 	protected transient Log logger = LogFactory.getLog(getClass());
 
 	@Nullable
@@ -200,7 +218,7 @@ public class InitDestroyAnnotationBeanPostProcessor
 
 	private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {
 		if (!AnnotationUtils.isCandidateClass(clazz, Arrays.asList(this.initAnnotationType, this.destroyAnnotationType))) {
-			return new LifecycleMetadata(clazz, Collections.emptyList(), Collections.emptyList());
+			return this.emptyLifecycleMetadata;
 		}
 
 		List<LifecycleElement> initMethods = new ArrayList<>();
@@ -233,7 +251,8 @@ public class InitDestroyAnnotationBeanPostProcessor
 		}
 		while (targetClass != null && targetClass != Object.class);
 
-		return new LifecycleMetadata(clazz, initMethods, destroyMethods);
+		return (initMethods.isEmpty() && destroyMethods.isEmpty() ? this.emptyLifecycleMetadata :
+				new LifecycleMetadata(clazz, initMethods, destroyMethods));
 	}
 
 

spring-beans/src/main/java/org/springframework/beans/factory/annotation/InjectionMetadata.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -22,6 +22,7 @@ import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Member;
 import java.lang.reflect.Method;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -47,6 +48,23 @@ import org.springframework.util.ReflectionUtils;
  */
 public class InjectionMetadata {
 
+	/**
+	 * An empty {@code InjectionMetadata} instance with no-op callbacks.
+	 * @since 5.2
+	 */
+	public static final InjectionMetadata EMPTY = new InjectionMetadata(Object.class, Collections.emptyList()) {
+		@Override
+		public void checkConfigMembers(RootBeanDefinition beanDefinition) {
+		}
+		@Override
+		public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) {
+		}
+		@Override
+		public void clear(@Nullable PropertyValues pvs) {
+		}
+	};
+
+
 	private static final Log logger = LogFactory.getLog(InjectionMetadata.class);
 
 	private final Class<?> targetClass;
@@ -57,6 +75,14 @@ public class InjectionMetadata {
 	private volatile Set<InjectedElement> checkedElements;
 
 
+	/**
+	 * Create a new {@code InjectionMetadata instance}.
+	 * <p>Preferably use {@link #forElements} for reusing the {@link #EMPTY}
+	 * instance in case of no elements.
+	 * @param targetClass the target class
+	 * @param elements the associated elements to inject
+	 * @see #forElements
+	 */
 	public InjectionMetadata(Class<?> targetClass, Collection<InjectedElement> elements) {
 		this.targetClass = targetClass;
 		this.injectedElements = elements;
@@ -108,6 +134,24 @@ public class InjectionMetadata {
 	}
 
 
+	/**
+	 * Return an {@code InjectionMetadata} instance, possibly for empty elements.
+	 * @param elements the elements to inject (possibly empty)
+	 * @param clazz the target class
+	 * @return a new {@code InjectionMetadata} instance,
+	 * or {@link #EMPTY} in case of no elements
+	 * @since 5.2
+	 */
+	public static InjectionMetadata forElements(Collection<InjectedElement> elements, Class<?> clazz) {
+		return (elements.isEmpty() ? InjectionMetadata.EMPTY : new InjectionMetadata(clazz, elements));
+	}
+
+	/**
+	 * Check whether the given injection metadata needs to be refreshed.
+	 * @param metadata the existing metadata instance
+	 * @param clazz the current target class
+	 * @return {@code true} indicating a refresh, {@code false} otherwise
+	 */
 	public static boolean needsRefresh(@Nullable InjectionMetadata metadata, Class<?> clazz) {
 		return (metadata == null || metadata.targetClass != clazz);
 	}

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

@@ -369,7 +369,7 @@ public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBean
 
 	private InjectionMetadata buildResourceMetadata(final Class<?> clazz) {
 		if (!AnnotationUtils.isCandidateClass(clazz, resourceAnnotationTypes)) {
-			return new InjectionMetadata(clazz, Collections.emptyList());
+			return InjectionMetadata.EMPTY;
 		}
 
 		List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
@@ -448,7 +448,7 @@ public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBean
 		}
 		while (targetClass != null && targetClass != Object.class);
 
-		return new InjectionMetadata(clazz, elements);
+		return InjectionMetadata.forElements(elements, clazz);
 	}
 
 	/**

spring-orm/src/main/java/org/springframework/orm/jpa/support/PersistenceAnnotationBeanPostProcessor.java

@@ -24,7 +24,6 @@ import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -416,7 +415,7 @@ public class PersistenceAnnotationBeanPostProcessor
 
 	private InjectionMetadata buildPersistenceMetadata(final Class<?> clazz) {
 		if (!AnnotationUtils.isCandidateClass(clazz, Arrays.asList(PersistenceContext.class, PersistenceUnit.class))) {
-			return new InjectionMetadata(clazz, Collections.emptyList());
+			return InjectionMetadata.EMPTY;
 		}
 
 		List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
@@ -459,7 +458,7 @@ public class PersistenceAnnotationBeanPostProcessor
 		}
 		while (targetClass != null && targetClass != Object.class);
 
-		return new InjectionMetadata(clazz, elements);
+		return InjectionMetadata.forElements(elements, clazz);
 	}
 
 	/**