Reuse generated entity instantiators from `ClassGeneratingEntityInstantiator` instances.

We now reuse generated ObjectInstantiator classes instead of attempting to redefine classes with the same name. Redefinition leads to LinkageError and thus to the use of reflection-based instantiators and an increased allocation rate due to the failed attempt of class redeclaration.

Closes: #2446.

src/main/java/org/springframework/data/mapping/model/ClassGeneratingEntityInstantiator.java

@@ -124,6 +124,13 @@ class ClassGeneratingEntityInstantiator implements EntityInstantiator {
 		try {
 			return doCreateEntityInstantiator(entity);
 		} catch (Throwable ex) {
+
+			if (LOGGER.isDebugEnabled()) {
+				LOGGER.debug(
+						String.format("Cannot create entity instantiator for %s. Falling back to ReflectionEntityInstantiator.",
+								entity.getName()),
+						ex);
+			}
 			return ReflectionEntityInstantiator.INSTANCE;
 		}
 	}
@@ -336,12 +343,22 @@ class ClassGeneratingEntityInstantiator implements EntityInstantiator {
 				@Nullable PreferredConstructor<?, ?> constructor) {
 
 			String className = generateClassName(entity);
-			byte[] bytecode = generateBytecode(className, entity, constructor);
-
 			Class<?> type = entity.getType();
+			ClassLoader classLoader = type.getClassLoader();
+
+			if (ClassUtils.isPresent(className, classLoader)) {
+
+				try {
+					return ClassUtils.forName(className, classLoader);
+				} catch (Exception o_O) {
+					throw new IllegalStateException(o_O);
+				}
+			}
+
+			byte[] bytecode = generateBytecode(className, entity, constructor);
 
 			try {
-				return ReflectUtils.defineClass(className, bytecode, type.getClassLoader(), type.getProtectionDomain(), type);
+				return ReflectUtils.defineClass(className, bytecode, classLoader, type.getProtectionDomain(), type);
 			} catch (Exception e) {
 				throw new IllegalStateException(e);
 			}

src/test/java/org/springframework/data/mapping/model/ClassGeneratingEntityInstantiatorUnitTests.java

@@ -23,6 +23,7 @@ import static org.springframework.data.util.ClassTypeInformation.from;
 import java.lang.reflect.Constructor;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Map;
 import java.util.stream.IntStream;
 
 import org.junit.jupiter.api.Test;
@@ -39,6 +40,8 @@ import org.springframework.data.mapping.PreferredConstructor;
 import org.springframework.data.mapping.PreferredConstructor.Parameter;
 import org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.ObjectInstantiator;
 import org.springframework.data.mapping.model.ClassGeneratingEntityInstantiatorUnitTests.Outer.Inner;
+import org.springframework.data.util.TypeInformation;
+import org.springframework.test.util.ReflectionTestUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -210,6 +213,9 @@ class ClassGeneratingEntityInstantiatorUnitTests<P extends PersistentProperty<P>
 			assertThat(instance).isInstanceOf(ObjCtorNoArgs.class);
 			assertThat(((ObjCtorNoArgs) instance).ctorInvoked).isTrue();
 		});
+
+		ClassGeneratingEntityInstantiator classGeneratingEntityInstantiator = new ClassGeneratingEntityInstantiator();
+		classGeneratingEntityInstantiator.createInstance(entity, provider);
 	}
 
 	@Test // DATACMNS-578, DATACMNS-1126
@@ -360,6 +366,26 @@ class ClassGeneratingEntityInstantiatorUnitTests<P extends PersistentProperty<P>
 		assertThat(this.instance.shouldUseReflectionEntityInstantiator(entity)).isTrue();
 	}
 
+	@Test // GH-2446
+	void shouldReuseGeneratedClasses() {
+
+		prepareMocks(ProtectedInnerClass.class);
+
+		this.instance.createInstance(entity, provider);
+
+		ClassGeneratingEntityInstantiator instantiator = new ClassGeneratingEntityInstantiator();
+		instantiator.createInstance(entity, provider);
+
+		Map<TypeInformation<?>, EntityInstantiator> first = (Map<TypeInformation<?>, EntityInstantiator>) ReflectionTestUtils
+				.getField(this.instance, "entityInstantiators");
+
+		Map<TypeInformation<?>, EntityInstantiator> second = (Map<TypeInformation<?>, EntityInstantiator>) ReflectionTestUtils
+				.getField(instantiator, "entityInstantiators");
+
+		assertThat(first.get(null)).isNotNull().isNotInstanceOf(Enum.class);
+		assertThat(second.get(null)).isNotNull().isNotInstanceOf(Enum.class);
+	}
+
 	@Test // DATACMNS-1422
 	void shouldUseReflectionIfFrameworkTypesNotVisible() throws Exception {