Lazily initialize DataSize.PATTERN

To avoid unnecessary eager initialization of DataSize.PATTERN, this
commit initializes it lazily in the first invocation of DataSize.parse
by moving PATTERN to a private static nested class.

Closes gh-28560

spring-core/src/main/java/org/springframework/util/unit/DataSize.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 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.
@@ -52,11 +52,6 @@
 @SuppressWarnings("serial")
 public final class DataSize implements Comparable<DataSize>, Serializable {
 
-	/**
-	 * The pattern for parsing.
-	 */
-	private static final Pattern PATTERN = Pattern.compile("^([+\\-]?\\d+)([a-zA-Z]{0,2})$");
-
 	/**
 	 * Bytes per Kilobyte.
 	 */
@@ -179,9 +174,9 @@ public static DataSize parse(CharSequence text) {
 	public static DataSize parse(CharSequence text, @Nullable DataUnit defaultUnit) {
 		Assert.notNull(text, "Text must not be null");
 		try {
-			Matcher matcher = PATTERN.matcher(text);
+			Matcher matcher = DataSizeUtils.PATTERN.matcher(text);
 			Assert.state(matcher.matches(), "Does not match data size pattern");
-			DataUnit unit = determineDataUnit(matcher.group(2), defaultUnit);
+			DataUnit unit = DataSizeUtils.determineDataUnit(matcher.group(2), defaultUnit);
 			long amount = Long.parseLong(matcher.group(1));
 			return DataSize.of(amount, unit);
 		}
@@ -190,11 +185,6 @@ public static DataSize parse(CharSequence text, @Nullable DataUnit defaultUnit)
 		}
 	}
 
-	private static DataUnit determineDataUnit(String suffix, @Nullable DataUnit defaultUnit) {
-		DataUnit defaultUnitToUse = (defaultUnit != null ? defaultUnit : DataUnit.BYTES);
-		return (StringUtils.hasLength(suffix) ? DataUnit.fromSuffix(suffix) : defaultUnitToUse);
-	}
-
 	/**
 	 * Checks if this size is negative, excluding zero.
 	 * @return true if this size has a size less than zero bytes
@@ -271,4 +261,23 @@ public int hashCode() {
 		return Long.hashCode(this.bytes);
 	}
 
+
+	/**
+	 * Static nested class to support lazy loading of the {@link #PATTERN}.
+	 * @since 5.3.21
+	 */
+	private static class DataSizeUtils {
+
+		/**
+		 * The pattern for parsing.
+		 */
+		private static final Pattern PATTERN = Pattern.compile("^([+\\-]?\\d+)([a-zA-Z]{0,2})$");
+
+		private static DataUnit determineDataUnit(String suffix, @Nullable DataUnit defaultUnit) {
+			DataUnit defaultUnitToUse = (defaultUnit != null ? defaultUnit : DataUnit.BYTES);
+			return (StringUtils.hasLength(suffix) ? DataUnit.fromSuffix(suffix) : defaultUnitToUse);
+		}
+
+	}
+
 }