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

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

@@ -41,6 +41,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
  * @author Juergen Hoeller
  * @author Rick Evans
  * @author Sam Brannen
+ * @author Vedran Pavic
  */
 class CookieLocaleResolverTests {
 
@@ -410,4 +411,24 @@ class CookieLocaleResolverTests {
 		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 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Vedran Pavic
  */
 class SessionLocaleResolverTests {
 
@@ -94,4 +97,25 @@ class SessionLocaleResolverTests {
 		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"));
+	}
+
 }