Add public variant getDeclaredMethods method

Add a public variant of `getDeclaredMethods` that defensively copies the
cached methods array. This is often more faster and more convenient
for users than calling `doWithLocalMethods`. We still retain most of the
benefits of the cache, namely fewer security manager calls and not as
many `Method` instances being created.

Closes gh-22580

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 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.
@@ -360,6 +360,13 @@ public class ReflectionUtilsTests {
 		assertThat(totalMs, Matchers.lessThan(10L));
 	}
 
+	@Test
+	public void getDecalredMethodsReturnsCopy() {
+		Method[] m1 = ReflectionUtils.getDeclaredMethods(A.class);
+		Method[] m2 = ReflectionUtils.getDeclaredMethods(A.class);
+		assertThat(m1, not(sameInstance(m2)));
+	}
+
 	private static class ListSavingMethodCallback implements ReflectionUtils.MethodCallback {
 
 		private List<String> methodNames = new LinkedList<>();