Use shared zero length array constants

Update code that's often called so that zero length array results use
a single shared static constant, rather than a new instance for each
call.

Closes gh-23340

Author: philwebb

spring-core/src/main/java/org/springframework/core/annotation/AnnotationTypeMapping.java

@@ -48,6 +48,10 @@
  */
 final class AnnotationTypeMapping {
 
+
+	private static final MirrorSet[] EMPTY_MIRROR_SETS = new MirrorSet[0];
+
+
 	@Nullable
 	private final AnnotationTypeMapping source;
 
@@ -550,7 +554,7 @@ class MirrorSets {
 
 		MirrorSets() {
 			this.assigned = new MirrorSet[attributes.size()];
-			this.mirrorSets = new MirrorSet[0];
+			this.mirrorSets = EMPTY_MIRROR_SETS;
 		}
 
 		void updateFrom(Collection<Method> aliases) {
@@ -575,7 +579,7 @@ void updateFrom(Collection<Method> aliases) {
 				mirrorSet.update();
 				Set<MirrorSet> unique = new LinkedHashSet<>(Arrays.asList(this.assigned));
 				unique.remove(null);
-				this.mirrorSets = unique.toArray(new MirrorSet[0]);
+				this.mirrorSets = unique.toArray(EMPTY_MIRROR_SETS);
 			}
 		}
 

spring-core/src/main/java/org/springframework/util/MethodInvoker.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.
@@ -38,6 +38,9 @@
  */
 public class MethodInvoker {
 
+	private static final Object[] EMPTY_ARGUMENTS = new Object[0];
+
+
 	@Nullable
 	protected Class<?> targetClass;
 
@@ -141,7 +144,7 @@ public void setArguments(Object... arguments) {
 	 * Return the arguments for the method invocation.
 	 */
 	public Object[] getArguments() {
-		return (this.arguments != null ? this.arguments : new Object[0]);
+		return (this.arguments != null ? this.arguments : EMPTY_ARGUMENTS);
 	}
 
 

spring-core/src/main/java/org/springframework/util/ObjectUtils.java

@@ -54,6 +54,7 @@ public abstract class ObjectUtils {
 	private static final String ARRAY_END = "}";
 	private static final String EMPTY_ARRAY = ARRAY_START + ARRAY_END;
 	private static final String ARRAY_ELEMENT_SEPARATOR = ", ";
+	private static final Object[] EMPTY_OBJECT_ARRAY = new Object[0];
 
 
 	/**
@@ -282,14 +283,14 @@ public static Object[] toObjectArray(@Nullable Object source) {
 			return (Object[]) source;
 		}
 		if (source == null) {
-			return new Object[0];
+			return EMPTY_OBJECT_ARRAY;
 		}
 		if (!source.getClass().isArray()) {
 			throw new IllegalArgumentException("Source is not an array: " + source);
 		}
 		int length = Array.getLength(source);
 		if (length == 0) {
-			return new Object[0];
+			return EMPTY_OBJECT_ARRAY;
 		}
 		Class<?> wrapperType = Array.get(source, 0).getClass();
 		Object[] newArray = (Object[]) Array.newInstance(wrapperType, length);

spring-core/src/main/java/org/springframework/util/StringUtils.java

@@ -60,6 +60,8 @@
  */
 public abstract class StringUtils {
 
+	private static final String[] EMPTY_STRING_ARRAY = {};
+
 	private static final String FOLDER_SEPARATOR = "/";
 
 	private static final String WINDOWS_FOLDER_SEPARATOR = "\\";
@@ -898,7 +900,7 @@ public static TimeZone parseTimeZoneString(String timeZoneString) {
 	 * @return the resulting {@code String} array
 	 */
 	public static String[] toStringArray(@Nullable Collection<String> collection) {
-		return (collection != null ? collection.toArray(new String[0]) : new String[0]);
+		return (collection != null || collection.isEmpty() ? collection.toArray(EMPTY_STRING_ARRAY) : EMPTY_STRING_ARRAY);
 	}
 
 	/**
@@ -909,7 +911,7 @@ public static String[] toStringArray(@Nullable Collection<String> collection) {
 	 * @return the resulting {@code String} array
 	 */
 	public static String[] toStringArray(@Nullable Enumeration<String> enumeration) {
-		return (enumeration != null ? toStringArray(Collections.list(enumeration)) : new String[0]);
+		return (enumeration != null ? toStringArray(Collections.list(enumeration)) : EMPTY_STRING_ARRAY);
 	}
 
 	/**
@@ -1151,7 +1153,7 @@ public static String[] tokenizeToStringArray(
 			@Nullable String str, String delimiters, boolean trimTokens, boolean ignoreEmptyTokens) {
 
 		if (str == null) {
-			return new String[0];
+			return EMPTY_STRING_ARRAY;
 		}
 
 		StringTokenizer st = new StringTokenizer(str, delimiters);
@@ -1204,7 +1206,7 @@ public static String[] delimitedListToStringArray(
 			@Nullable String str, @Nullable String delimiter, @Nullable String charsToDelete) {
 
 		if (str == null) {
-			return new String[0];
+			return EMPTY_STRING_ARRAY;
 		}
 		if (delimiter == null) {
 			return new String[] {str};