Document default value of server.tomcat.resource.cache-ttl

Closes gh-47252

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -1016,7 +1016,7 @@ public class ServerProperties {
 			/**
 			 * Time-to-live of the static resource cache.
 			 */
-			private Duration cacheTtl;
+			private Duration cacheTtl = Duration.ofSeconds(5);
 
 			public boolean isAllowCaching() {
 				return this.allowCaching;

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizerTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.web.embedded;
 
+import java.time.Duration;
 import java.util.Locale;
 import java.util.function.Consumer;
 
@@ -314,6 +315,19 @@ class TomcatWebServerFactoryCustomizerTests {
 		assertThat(remoteIpValve.getTrustedProxies()).isEqualTo("proxy1|proxy2");
 	}
 
+	@Test
+	void resourceCacheMatchesDefault() {
+		ServerProperties properties = new ServerProperties();
+		customizeAndRunServer((server) -> {
+			Tomcat tomcat = server.getTomcat();
+			Context context = (Context) tomcat.getHost().findChildren()[0];
+			assertThat(properties.getTomcat().getResource().isAllowCaching())
+				.isEqualTo(context.getResources().isCachingAllowed());
+			assertThat(properties.getTomcat().getResource().getCacheTtl())
+				.isEqualTo(Duration.ofMillis(context.getResources().getCacheTtl()));
+		});
+	}
+
 	@Test
 	void customStaticResourceAllowCaching() {
 		bind("server.tomcat.resource.allow-caching=false");
@@ -326,7 +340,7 @@ class TomcatWebServerFactoryCustomizerTests {
 
 	@Test
 	void customStaticResourceCacheTtl() {
-		bind("server.tomcat.resource.cache-ttl=10000");
+		bind("server.tomcat.resource.cache-ttl=10s");
 		customizeAndRunServer((server) -> {
 			Tomcat tomcat = server.getTomcat();
 			Context context = (Context) tomcat.getHost().findChildren()[0];