Introduce isLambdaClass() as a public utility in ClassUtils

This commit extracts isLambda() from AopProxyUtils and makes it
publicly available as ClassUtils.isLambdaClass().

This is a prerequisite for gh-28209.

spring-core/src/test/java/org/springframework/util/ClassUtilsTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 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.
@@ -31,6 +31,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Set;
+import java.util.function.Supplier;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Nested;
@@ -408,6 +409,29 @@ class ClassUtilsTests {
 		assertThat(ClassUtils.isPrimitiveOrWrapper(type)).isTrue();
 	}
 
+	@Test
+	void isLambda() {
+		assertIsLambda(ClassUtilsTests.staticLambdaExpression);
+		assertIsLambda(ClassUtilsTests::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());
+	}
+
 
 	@Nested
 	class GetStaticMethodTests {
@@ -500,4 +524,38 @@ class ClassUtilsTests {
 		}
 	}
 
+	private static void assertIsLambda(Supplier<String> supplier) {
+		assertThat(ClassUtils.isLambdaClass(supplier.getClass())).isTrue();
+	}
+
+	private static void assertIsNotLambda(Supplier<String> supplier) {
+		assertThat(ClassUtils.isLambdaClass(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";
+		}
+	}
+
 }