Make DefaultValueStyler extensible

The `style(Object)` method in DefaultValueStyler delegates to several
internal methods. Prior to this commit those methods were private,
which prevented users from extending DefaultValueStyler without
rewriting most of the functionality.

This commit addresses this by making those methods protected so that
DefaultValueStyler can be extended. In addition, this commit introduces
protected methods for styling null, strings, classes, methods, and
objects that are not covered by any of the other style*() methods.

Closes gh-29380

Author: sbrannen

spring-core/src/main/java/org/springframework/core/style/DefaultValueStyler.java

@@ -36,6 +36,7 @@
  *
  * @author Keith Donald
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 1.2.2
  */
 public class DefaultValueStyler implements ValueStyler {
@@ -53,86 +54,164 @@ public class DefaultValueStyler implements ValueStyler {
 	@Override
 	public String style(@Nullable Object value) {
 		if (value == null) {
-			return NULL;
+			return styleNull();
 		}
-		else if (value instanceof String) {
-			return "\'" + value + "\'";
+		else if (value instanceof String str) {
+			return styleString(str);
 		}
 		else if (value instanceof Class<?> clazz) {
-			return ClassUtils.getShortName(clazz);
+			return styleClass(clazz);
 		}
 		else if (value instanceof Method method) {
-			return method.getName() + "@" + ClassUtils.getShortName(method.getDeclaringClass());
+			return styleMethod(method);
 		}
 		else if (value instanceof Map<?, ?> map) {
-			return style(map);
+			return styleMap(map);
 		}
 		else if (value instanceof Map.Entry<?, ?> entry) {
-			return style(entry);
+			return styleMapEntry(entry);
 		}
 		else if (value instanceof Collection<?> collection) {
-			return style(collection);
+			return styleCollection(collection);
 		}
 		else if (value.getClass().isArray()) {
 			return styleArray(ObjectUtils.toObjectArray(value));
 		}
 		else {
-			return String.valueOf(value);
+			return styleObject(value);
 		}
 	}
 
-	private <K, V> String style(Map<K, V> value) {
-		if (value.isEmpty()) {
+	/**
+	 * Generate a styled version of {@code null}.
+	 * <p>The default implementation returns {@code "[null]"}.
+	 * @return a styled version of {@code null}
+	 * @since 6.0
+	 */
+	protected String styleNull() {
+		return NULL;
+	}
+
+	/**
+	 * Generate a styled version of the supplied {@link String}.
+	 * <p>The default implementation returns the supplied string wrapped in
+	 * single quotes.
+	 * @return a styled version of the supplied string
+	 * @since 6.0
+	 */
+	protected String styleString(String str) {
+		return "\'" + str + "\'";
+	}
+
+	/**
+	 * Generate a styled version of the supplied {@link Class}.
+	 * <p>The default implementation delegates to {@link ClassUtils#getShortName(Class)}.
+	 * @return a styled version of the supplied class
+	 * @since 6.0
+	 */
+	protected String styleClass(Class<?> clazz) {
+		return ClassUtils.getShortName(clazz);
+	}
+
+	/**
+	 * Generate a styled version of the supplied {@link Method}.
+	 * <p>The default implementation returns the method's {@linkplain Method#getName()
+	 * name} and the {@linkplain ClassUtils#getShortName(Class) short name} of the
+	 * method's {@linkplain Method#getDeclaringClass() declaring class}, separated by
+	 * the {@code "@"} symbol.
+	 * @return a styled version of the supplied method
+	 * @since 6.0
+	 */
+	protected String styleMethod(Method method) {
+		return method.getName() + "@" + ClassUtils.getShortName(method.getDeclaringClass());
+	}
+
+	/**
+	 * Generate a styled version of the supplied {@link Map}.
+	 * @return a styled version of the supplied map
+	 * @since 6.0
+	 */
+	protected <K, V> String styleMap(Map<K, V> map) {
+		if (map.isEmpty()) {
 			return EMPTY_MAP;
 		}
 
 		StringJoiner result = new StringJoiner(", ", "[", "]");
-		for (Map.Entry<K, V> entry : value.entrySet()) {
-			result.add(style(entry));
+		for (Map.Entry<K, V> entry : map.entrySet()) {
+			result.add(styleMapEntry(entry));
 		}
 		return MAP + result;
 	}
 
-	private String style(Map.Entry<?, ?> value) {
-		return style(value.getKey()) + " -> " + style(value.getValue());
+	/**
+	 * Generate a styled version of the supplied {@link Map.Entry}.
+	 * @return a styled version of the supplied map entry
+	 * @since 6.0
+	 */
+	protected String styleMapEntry(Map.Entry<?, ?> entry) {
+		return style(entry.getKey()) + " -> " + style(entry.getValue());
 	}
 
-	private String style(Collection<?> value) {
-		String collectionType = getCollectionTypeString(value);
+	/**
+	 * Generate a styled version of the supplied {@link Collection}.
+	 * @return a styled version of the supplied collection
+	 * @since 6.0
+	 */
+	protected String styleCollection(Collection<?> collection) {
+		String collectionType = getCollectionTypeString(collection);
 
-		if (value.isEmpty()) {
+		if (collection.isEmpty()) {
 			return collectionType + EMPTY;
 		}
 
 		StringJoiner result = new StringJoiner(", ", "[", "]");
-		for (Object o : value) {
-			result.add(style(o));
+		for (Object element : collection) {
+			result.add(style(element));
 		}
 		return collectionType + result;
 	}
 
-	private String getCollectionTypeString(Collection<?> value) {
-		if (value instanceof List) {
-			return LIST;
-		}
-		else if (value instanceof Set) {
-			return SET;
-		}
-		else {
-			return COLLECTION;
-		}
-	}
-
-	private String styleArray(Object[] array) {
+	/**
+	 * Generate a styled version of the supplied array.
+	 * @return a styled version of the supplied array
+	 * @since 6.0
+	 */
+	protected String styleArray(Object[] array) {
 		if (array.length == 0) {
 			return ARRAY + '<' + ClassUtils.getShortName(array.getClass().getComponentType()) + '>' + EMPTY;
 		}
 
 		StringJoiner result = new StringJoiner(", ", "[", "]");
-		for (Object o : array) {
-			result.add(style(o));
+		for (Object element : array) {
+			result.add(style(element));
 		}
 		return ARRAY + '<' + ClassUtils.getShortName(array.getClass().getComponentType()) + '>' + result;
 	}
 
+	/**
+	 * Generate a styled version of the supplied {@link Object}.
+	 * <p>This method is only invoked by {@link #style(Object)} as a fallback,
+	 * if none of the other {@code style*()} methods is suitable for the object's
+	 * type.
+	 * <p>The default implementation delegates to {@link String#valueOf(Object)}.
+	 * @return a styled version of the supplied object
+	 * @since 6.0
+	 */
+	protected String styleObject(Object obj) {
+		return String.valueOf(obj);
+	}
+
+
+	private static String getCollectionTypeString(Collection<?> collection) {
+		if (collection instanceof List) {
+			return LIST;
+		}
+		else if (collection instanceof Set) {
+			return SET;
+		}
+		else {
+			return COLLECTION;
+		}
+	}
+
 }