Simplify default locale/timezone resolution in cookie/session locale resolvers

At present, the customization of the default locale and timezone
resolution in CookieLocaleResolver and SessionLocaleResolver requires
subclassing them and overriding determineDefaultLocale and/or
determineDefaultTimeZone methods.

This commit simplifies resolution of the default locale and timezone
resolution by introducing dedicated functions for these purposes, thus
allowing customization without needing to resort to subclassing the
locale resolvers.

Closes gh-27609

Author: vpavic

spring-webmvc/src/main/java/org/springframework/web/servlet/i18n/CookieLocaleResolver.java

@@ -18,6 +18,7 @@
 
 import java.util.Locale;
 import java.util.TimeZone;
+import java.util.function.Function;
 
 import jakarta.servlet.http.Cookie;
 import jakarta.servlet.http.HttpServletRequest;
@@ -50,6 +51,7 @@
  *
  * @author Juergen Hoeller
  * @author Jean-Pierre Pawlak
+ * @author Vedran Pavic
  * @since 27.02.2003
  * @see #setDefaultLocale
  * @see #setDefaultTimeZone
@@ -94,6 +96,15 @@ public class CookieLocaleResolver extends CookieGenerator implements LocaleConte
 	@Nullable
 	private TimeZone defaultTimeZone;
 
+	private Function<HttpServletRequest, Locale> defaultLocaleFunction = request -> {
+		Locale defaultLocale = getDefaultLocale();
+		if (defaultLocale == null) {
+			defaultLocale = request.getLocale();
+		}
+		return defaultLocale;
+	};
+
+	private Function<HttpServletRequest, TimeZone> defaultTimeZoneFunction = request -> getDefaultTimeZone();
 
 	/**
 	 * Create a new instance of {@link CookieLocaleResolver} using the
@@ -137,8 +148,8 @@ public boolean isLanguageTagCompliant() {
 	 * @since 5.1.7
 	 * @see #setDefaultLocale
 	 * @see #setDefaultTimeZone
-	 * @see #determineDefaultLocale
-	 * @see #determineDefaultTimeZone
+	 * @see #setDefaultLocaleFunction(Function)
+	 * @see #setDefaultTimeZoneFunction(Function)
 	 */
 	public void setRejectInvalidCookies(boolean rejectInvalidCookies) {
 		this.rejectInvalidCookies = rejectInvalidCookies;
@@ -186,6 +197,35 @@ protected TimeZone getDefaultTimeZone() {
 		return this.defaultTimeZone;
 	}
 
+	/**
+	 * Set the function used to determine the default locale for the given request,
+	 * called if no {@link Locale} session attribute has been found.
+	 * <p>The default implementation returns the specified default locale,
+	 * if any, else falls back to the request's accept-header locale.
+	 * @param defaultLocaleFunction the function used to determine the default locale
+	 * @since 6.0
+	 * @see #setDefaultLocale
+	 * @see jakarta.servlet.http.HttpServletRequest#getLocale()
+	 */
+	public void setDefaultLocaleFunction(Function<HttpServletRequest, Locale> defaultLocaleFunction) {
+		Assert.notNull(defaultLocaleFunction, "defaultLocaleFunction must not be null");
+		this.defaultLocaleFunction = defaultLocaleFunction;
+	}
+
+	/**
+	 * Set the function used to determine the default time zone for the given request,
+	 * called if no {@link TimeZone} session attribute has been found.
+	 * <p>The default implementation returns the specified default time zone,
+	 * if any, or {@code null} otherwise.
+	 * @param defaultTimeZoneFunction the function used to determine the default time zone
+	 * @since 6.0
+	 * @see #setDefaultTimeZone
+	 */
+	public void setDefaultTimeZoneFunction(Function<HttpServletRequest, TimeZone> defaultTimeZoneFunction) {
+		Assert.notNull(defaultTimeZoneFunction, "defaultTimeZoneFunction must not be null");
+		this.defaultTimeZoneFunction = defaultTimeZoneFunction;
+	}
+
 
 	@Override
 	public Locale resolveLocale(HttpServletRequest request) {
@@ -260,9 +300,9 @@ private void parseLocaleCookieIfNecessary(HttpServletRequest request) {
 			}
 
 			request.setAttribute(LOCALE_REQUEST_ATTRIBUTE_NAME,
-					(locale != null ? locale : determineDefaultLocale(request)));
+					(locale != null ? locale : this.defaultLocaleFunction.apply(request)));
 			request.setAttribute(TIME_ZONE_REQUEST_ATTRIBUTE_NAME,
-					(timeZone != null ? timeZone : determineDefaultTimeZone(request)));
+					(timeZone != null ? timeZone : this.defaultTimeZoneFunction.apply(request)));
 		}
 	}
 
@@ -291,9 +331,9 @@ public void setLocaleContext(HttpServletRequest request, @Nullable HttpServletRe
 			removeCookie(response);
 		}
 		request.setAttribute(LOCALE_REQUEST_ATTRIBUTE_NAME,
-				(locale != null ? locale : determineDefaultLocale(request)));
+				(locale != null ? locale : this.defaultLocaleFunction.apply(request)));
 		request.setAttribute(TIME_ZONE_REQUEST_ATTRIBUTE_NAME,
-				(timeZone != null ? timeZone : determineDefaultTimeZone(request)));
+				(timeZone != null ? timeZone : this.defaultTimeZoneFunction.apply(request)));
 	}
 
 
@@ -335,13 +375,11 @@ protected String toLocaleValue(Locale locale) {
 	 * @return the default locale (never {@code null})
 	 * @see #setDefaultLocale
 	 * @see jakarta.servlet.http.HttpServletRequest#getLocale()
+	 * @deprecated as of 6.0, in favor of {@link #setDefaultLocaleFunction(Function)}
 	 */
+	@Deprecated
 	protected Locale determineDefaultLocale(HttpServletRequest request) {
-		Locale defaultLocale = getDefaultLocale();
-		if (defaultLocale == null) {
-			defaultLocale = request.getLocale();
-		}
-		return defaultLocale;
+		return this.defaultLocaleFunction.apply(request);
 	}
 
 	/**
@@ -352,10 +390,12 @@ protected Locale determineDefaultLocale(HttpServletRequest request) {
 	 * @param request the request to resolve the time zone for
 	 * @return the default time zone (or {@code null} if none defined)
 	 * @see #setDefaultTimeZone
+	 * @deprecated as of 6.0, in favor of {@link #setDefaultTimeZoneFunction(Function)}
 	 */
+	@Deprecated
 	@Nullable
 	protected TimeZone determineDefaultTimeZone(HttpServletRequest request) {
-		return getDefaultTimeZone();
+		return this.defaultTimeZoneFunction.apply(request);
 	}
 
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/i18n/SessionLocaleResolver.java

@@ -18,13 +18,15 @@
 
 import java.util.Locale;
 import java.util.TimeZone;
+import java.util.function.Function;
 
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
 
 import org.springframework.context.i18n.LocaleContext;
 import org.springframework.context.i18n.TimeZoneAwareLocaleContext;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 import org.springframework.web.util.WebUtils;
 
 /**
@@ -54,6 +56,7 @@
  * against the current {@code HttpServletRequest}.
  *
  * @author Juergen Hoeller
+ * @author Vedran Pavic
  * @since 27.02.2003
  * @see #setDefaultLocale
  * @see #setDefaultTimeZone
@@ -85,6 +88,15 @@ public class SessionLocaleResolver extends AbstractLocaleContextResolver {
 
 	private String timeZoneAttributeName = TIME_ZONE_SESSION_ATTRIBUTE_NAME;
 
+	private Function<HttpServletRequest, Locale> defaultLocaleFunction = request -> {
+		Locale defaultLocale = getDefaultLocale();
+		if (defaultLocale == null) {
+			defaultLocale = request.getLocale();
+		}
+		return defaultLocale;
+	};
+
+	private Function<HttpServletRequest, TimeZone> defaultTimeZoneFunction = request -> getDefaultTimeZone();
 
 	/**
 	 * Specify the name of the corresponding attribute in the {@code HttpSession},
@@ -106,12 +118,40 @@ public void setTimeZoneAttributeName(String timeZoneAttributeName) {
 		this.timeZoneAttributeName = timeZoneAttributeName;
 	}
 
+	/**
+	 * Set the function used to determine the default locale for the given request,
+	 * called if no {@link Locale} session attribute has been found.
+	 * <p>The default implementation returns the specified default locale,
+	 * if any, else falls back to the request's accept-header locale.
+	 * @param defaultLocaleFunction the function used to determine the default locale
+	 * @since 6.0
+	 * @see #setDefaultLocale
+	 * @see jakarta.servlet.http.HttpServletRequest#getLocale()
+	 */
+	public void setDefaultLocaleFunction(Function<HttpServletRequest, Locale> defaultLocaleFunction) {
+		Assert.notNull(defaultLocaleFunction, "defaultLocaleFunction must not be null");
+		this.defaultLocaleFunction = defaultLocaleFunction;
+	}
+
+	/**
+	 * Set the function used to determine the default time zone for the given request,
+	 * called if no {@link TimeZone} session attribute has been found.
+	 * <p>The default implementation returns the specified default time zone,
+	 * if any, or {@code null} otherwise.
+	 * @param defaultTimeZoneFunction the function used to determine the default time zone
+	 * @since 6.0
+	 * @see #setDefaultTimeZone
+	 */
+	public void setDefaultTimeZoneFunction(Function<HttpServletRequest, TimeZone> defaultTimeZoneFunction) {
+		Assert.notNull(defaultTimeZoneFunction, "defaultTimeZoneFunction must not be null");
+		this.defaultTimeZoneFunction = defaultTimeZoneFunction;
+	}
 
 	@Override
 	public Locale resolveLocale(HttpServletRequest request) {
 		Locale locale = (Locale) WebUtils.getSessionAttribute(request, this.localeAttributeName);
 		if (locale == null) {
-			locale = determineDefaultLocale(request);
+			locale = this.defaultLocaleFunction.apply(request);
 		}
 		return locale;
 	}
@@ -123,7 +163,7 @@ public LocaleContext resolveLocaleContext(final HttpServletRequest request) {
 			public Locale getLocale() {
 				Locale locale = (Locale) WebUtils.getSessionAttribute(request, localeAttributeName);
 				if (locale == null) {
-					locale = determineDefaultLocale(request);
+					locale = SessionLocaleResolver.this.defaultLocaleFunction.apply(request);
 				}
 				return locale;
 			}
@@ -132,7 +172,7 @@ public Locale getLocale() {
 			public TimeZone getTimeZone() {
 				TimeZone timeZone = (TimeZone) WebUtils.getSessionAttribute(request, timeZoneAttributeName);
 				if (timeZone == null) {
-					timeZone = determineDefaultTimeZone(request);
+					timeZone = SessionLocaleResolver.this.defaultTimeZoneFunction.apply(request);
 				}
 				return timeZone;
 			}
@@ -167,13 +207,11 @@ public void setLocaleContext(HttpServletRequest request, @Nullable HttpServletRe
 	 * @return the default locale (never {@code null})
 	 * @see #setDefaultLocale
 	 * @see jakarta.servlet.http.HttpServletRequest#getLocale()
+	 * @deprecated as of 6.0, in favor of {@link #setDefaultLocaleFunction(Function)}
 	 */
+	@Deprecated
 	protected Locale determineDefaultLocale(HttpServletRequest request) {
-		Locale defaultLocale = getDefaultLocale();
-		if (defaultLocale == null) {
-			defaultLocale = request.getLocale();
-		}
-		return defaultLocale;
+		return this.defaultLocaleFunction.apply(request);
 	}
 
 	/**
@@ -184,10 +222,12 @@ protected Locale determineDefaultLocale(HttpServletRequest request) {
 	 * @param request the request to resolve the time zone for
 	 * @return the default time zone (or {@code null} if none defined)
 	 * @see #setDefaultTimeZone
+	 * @deprecated as of 6.0, in favor of {@link #setDefaultTimeZoneFunction(Function)}
 	 */
+	@Deprecated
 	@Nullable
 	protected TimeZone determineDefaultTimeZone(HttpServletRequest request) {
-		return getDefaultTimeZone();
+		return this.defaultTimeZoneFunction.apply(request);
 	}
 
 }

spring-webmvc/src/test/java/org/springframework/web/servlet/i18n/CookieLocaleResolverTests.java

@@ -41,6 +41,7 @@
  * @author Juergen Hoeller
  * @author Rick Evans
  * @author Sam Brannen
+ * @author Vedran Pavic
  */
 class CookieLocaleResolverTests {
 
@@ -410,4 +411,24 @@ void setLocaleContextToNullWithDefault() {
 		assertThat(localeCookie.getValue()).isEqualTo("");
 	}
 
+	@Test
+	void testCustomDefaultLocaleFunction() {
+		request.addPreferredLocale(Locale.TAIWAN);
+
+		resolver.setDefaultLocaleFunction(request -> Locale.GERMAN);
+
+		assertThat(resolver.resolveLocale(request)).isEqualTo(Locale.GERMAN);
+	}
+
+	@Test
+	void testCustomDefaultTimeZoneFunction() {
+		request.addPreferredLocale(Locale.TAIWAN);
+
+		resolver.setDefaultTimeZoneFunction(request -> TimeZone.getTimeZone("GMT+1"));
+
+		TimeZoneAwareLocaleContext context = (TimeZoneAwareLocaleContext) resolver.resolveLocaleContext(request);
+		assertThat(context.getLocale()).isEqualTo(Locale.TAIWAN);
+		assertThat(context.getTimeZone()).isEqualTo(TimeZone.getTimeZone("GMT+1"));
+	}
+
 }

spring-webmvc/src/test/java/org/springframework/web/servlet/i18n/SessionLocaleResolverTests.java

@@ -17,10 +17,12 @@
 package org.springframework.web.servlet.i18n;
 
 import java.util.Locale;
+import java.util.TimeZone;
 
 import jakarta.servlet.http.HttpSession;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.context.i18n.TimeZoneAwareLocaleContext;
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
 import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
 
@@ -31,6 +33,7 @@
  *
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Vedran Pavic
  */
 class SessionLocaleResolverTests {
 
@@ -94,4 +97,25 @@ void setLocaleToNullLocale() throws Exception {
 		assertThat(resolver.resolveLocale(request)).isEqualTo(Locale.TAIWAN);
 	}
 
+	@Test
+	void testCustomDefaultLocaleFunction() {
+		request.addPreferredLocale(Locale.TAIWAN);
+
+		SessionLocaleResolver resolver = new SessionLocaleResolver();
+		resolver.setDefaultLocaleFunction(request -> Locale.GERMAN);
+
+		assertThat(resolver.resolveLocale(request)).isEqualTo(Locale.GERMAN);
+	}
+
+	@Test
+	void testCustomDefaultTimeZoneFunction() {
+		request.addPreferredLocale(Locale.TAIWAN);
+
+		resolver.setDefaultTimeZoneFunction(request -> TimeZone.getTimeZone("GMT+1"));
+
+		TimeZoneAwareLocaleContext context = (TimeZoneAwareLocaleContext) resolver.resolveLocaleContext(request);
+		assertThat(context.getLocale()).isEqualTo(Locale.TAIWAN);
+		assertThat(context.getTimeZone()).isEqualTo(TimeZone.getTimeZone("GMT+1"));
+	}
+
 }