Fallback Locale other than the system Locale through setDefaultLocale

Closes gh-23977

spring-context/src/main/java/org/springframework/context/support/AbstractResourceBasedMessageSource.java

 /*
- * 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.
@@ -17,6 +17,7 @@
 package org.springframework.context.support;
 
 import java.util.LinkedHashSet;
+import java.util.Locale;
 import java.util.Set;
 
 import org.springframework.lang.Nullable;
@@ -43,6 +44,9 @@ public abstract class AbstractResourceBasedMessageSource extends AbstractMessage
 
 	private boolean fallbackToSystemLocale = true;
 
+	@Nullable
+	private Locale defaultLocale;
+
 	private long cacheMillis = -1;
 
 
@@ -143,6 +147,7 @@ public abstract class AbstractResourceBasedMessageSource extends AbstractMessage
 	 * {@code java.util.ResourceBundle}. However, this is often not desirable
 	 * in an application server environment, where the system Locale is not relevant
 	 * to the application at all: set this flag to "false" in such a scenario.
+	 * @see #setDefaultLocale
 	 */
 	public void setFallbackToSystemLocale(boolean fallbackToSystemLocale) {
 		this.fallbackToSystemLocale = fallbackToSystemLocale;
@@ -152,11 +157,46 @@ public abstract class AbstractResourceBasedMessageSource extends AbstractMessage
 	 * Return whether to fall back to the system Locale if no files for a specific
 	 * Locale have been found.
 	 * @since 4.3
+	 * @deprecated as of 5.2.2, in favor of {@link #getDefaultLocale()}
 	 */
+	@Deprecated
 	protected boolean isFallbackToSystemLocale() {
 		return this.fallbackToSystemLocale;
 	}
 
+	/**
+	 * Specify a default Locale to fall back to, as an alternative to falling back
+	 * to the system Locale.
+	 * <p>Default is to fall back to the system Locale. You may override this with
+	 * a locally specified default Locale here, or enforce no fallback locale at all
+	 * through disabling {@link #setFallbackToSystemLocale "fallbackToSystemLocale"}.
+	 * @since 5.2.2
+	 * @see #setFallbackToSystemLocale
+	 * @see #getDefaultLocale()
+	 */
+	public void setDefaultLocale(@Nullable Locale defaultLocale) {
+		this.defaultLocale = defaultLocale;
+	}
+
+	/**
+	 * Determine a default Locale to fall back to: either a locally specified default
+	 * Locale or the system Locale, or {@code null} for no fallback locale at all.
+	 * @since 5.2.2
+	 * @see #setDefaultLocale
+	 * @see #setFallbackToSystemLocale
+	 * @see Locale#getDefault()
+	 */
+	@Nullable
+	protected Locale getDefaultLocale() {
+		if (this.defaultLocale != null) {
+			return this.defaultLocale;
+		}
+		if (this.fallbackToSystemLocale) {
+			return Locale.getDefault();
+		}
+		return null;
+	}
+
 	/**
 	 * Set the number of seconds to cache loaded properties files.
 	 * <ul>

spring-context/src/main/java/org/springframework/context/support/ReloadableResourceBundleMessageSource.java

 /*
- * 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.
@@ -237,6 +237,7 @@ public class ReloadableResourceBundleMessageSource extends AbstractResourceBased
 		if (mergedHolder != null) {
 			return mergedHolder;
 		}
+
 		Properties mergedProps = newProperties();
 		long latestTimestamp = -1;
 		String[] basenames = StringUtils.toStringArray(getBasenameSet());
@@ -253,6 +254,7 @@ public class ReloadableResourceBundleMessageSource extends AbstractResourceBased
 				}
 			}
 		}
+
 		mergedHolder = new PropertiesHolder(mergedProps, latestTimestamp);
 		PropertiesHolder existing = this.cachedMergedProperties.putIfAbsent(locale, mergedHolder);
 		if (existing != null) {
@@ -279,10 +281,15 @@ public class ReloadableResourceBundleMessageSource extends AbstractResourceBased
 				return filenames;
 			}
 		}
+
+		// Filenames for given Locale
 		List<String> filenames = new ArrayList<>(7);
 		filenames.addAll(calculateFilenamesForLocale(basename, locale));
-		if (isFallbackToSystemLocale() && !locale.equals(Locale.getDefault())) {
-			List<String> fallbackFilenames = calculateFilenamesForLocale(basename, Locale.getDefault());
+
+		// Filenames for default Locale, if any
+		Locale defaultLocale = getDefaultLocale();
+		if (defaultLocale != null && !defaultLocale.equals(locale)) {
+			List<String> fallbackFilenames = calculateFilenamesForLocale(basename, defaultLocale);
 			for (String fallbackFilename : fallbackFilenames) {
 				if (!filenames.contains(fallbackFilename)) {
 					// Entry for fallback locale that isn't already in filenames list.
@@ -290,7 +297,10 @@ public class ReloadableResourceBundleMessageSource extends AbstractResourceBased
 				}
 			}
 		}
+
+		// Filename for default bundle file
 		filenames.add(basename);
+
 		if (localeMap == null) {
 			localeMap = new ConcurrentHashMap<>();
 			Map<Locale, List<String>> existing = this.cachedFilenames.putIfAbsent(basename, localeMap);

spring-context/src/main/java/org/springframework/context/support/ResourceBundleMessageSource.java

@@ -461,7 +461,8 @@ public class ResourceBundleMessageSource extends AbstractResourceBasedMessageSou
 		@Override
 		@Nullable
 		public Locale getFallbackLocale(String baseName, Locale locale) {
-			return (isFallbackToSystemLocale() ? super.getFallbackLocale(baseName, locale) : null);
+			Locale defaultLocale = getDefaultLocale();
+			return (defaultLocale != null && !defaultLocale.equals(locale) ? defaultLocale : null);
 		}
 
 		@Override