Ensure Spring AOP generates JDK dynamic proxies for lambdas

Prior to this commit, if AOP proxy generation was configured with
proxyTargetClass=true (which is the default behavior in recent versions
of Spring Boot), beans implemented as lambda expressions or method
references could not be proxied with CGLIB on Java 16 or higher without
specifying `--add-opens java.base/java.lang=ALL-UNNAMED`.

This commit addresses this shortcoming by ensuring that beans
implemented as lambda expressions or method references are always
proxied using a JDK dynamic proxy even if proxyTargetClass=true.

Closes gh-27971

spring-aop/src/test/java/org/springframework/aop/framework/AopProxyUtilsTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * 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.
@@ -19,6 +19,7 @@ package org.springframework.aop.framework;
 import java.lang.reflect.Proxy;
 import java.util.Arrays;
 import java.util.List;
+import java.util.function.Supplier;
 
 import org.junit.jupiter.api.Test;
 
@@ -32,6 +33,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
 /**
  * @author Rod Johnson
  * @author Chris Beams
+ * @author Sam Brannen
  */
 public class AopProxyUtilsTests {
 
@@ -132,4 +134,61 @@ public class AopProxyUtilsTests {
 				AopProxyUtils.proxiedUserInterfaces(proxy));
 	}
 
+	@Test
+	void isLambda() {
+		assertIsLambda(AopProxyUtilsTests.staticLambdaExpression);
+		assertIsLambda(AopProxyUtilsTests::staticStringFactory);
+
+		assertIsLambda(this.instanceLambdaExpression);
+		assertIsLambda(this::instanceStringFactory);
+	}
+
+	@Test
+	void isNotLambda() {
+		assertIsNotLambda(new EnigmaSupplier());
+
+		assertIsNotLambda(new Supplier<String>() {
+			@Override
+			public String get() {
+				return "anonymous inner class";
+			}
+		});
+
+		assertIsNotLambda(new Fake$$LambdaSupplier());
+	}
+
+	private static void assertIsLambda(Supplier<String> supplier) {
+		assertThat(AopProxyUtils.isLambda(supplier.getClass())).isTrue();
+	}
+
+	private static void assertIsNotLambda(Supplier<String> supplier) {
+		assertThat(AopProxyUtils.isLambda(supplier.getClass())).isFalse();
+	}
+
+	private static final Supplier<String> staticLambdaExpression = () -> "static lambda expression";
+
+	private final Supplier<String> instanceLambdaExpression = () -> "instance lambda expressions";
+
+	private static String staticStringFactory() {
+		return "static string factory";
+	}
+
+	private String instanceStringFactory() {
+		return "instance string factory";
+	}
+
+	private static class EnigmaSupplier implements Supplier<String> {
+		@Override
+		public String get() {
+			return "enigma";
+		}
+	}
+
+	private static class Fake$$LambdaSupplier implements Supplier<String> {
+		@Override
+		public String get() {
+			return "fake lambda";
+		}
+	}
+
 }