Improve handling of default values when printing the banner

Previously, default values for the following properties did not work:

- application.title
- application.formatted-version
- application.version
- spring-boot.formatted-version
- spring-boot.version

Instead of the default value, an empty string was used instead. For
example, ${application.title:Title} would be replaced with "" rather
than "Title" when the application title was unavailable.

This commit improves the ResourceBanner so that a placeholder's
default value is used. An empty string will still be used when no
default value is provided. For example, ${application.title} will
be replaced with "". As before, custom properties that are not
well-known will not be replaced at all. For example
${custom.property} will remain as-is in the printed banner when
the custom.property has not been set.

Fixes gh-44137

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/ResourceBanner.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -85,22 +85,34 @@ public void printBanner(Environment environment, Class<?> sourceClass, PrintStre
 	 * @return a mutable list of property resolvers
 	 */
 	protected List<PropertyResolver> getPropertyResolvers(Environment environment, Class<?> sourceClass) {
-		MutablePropertySources sources = new MutablePropertySources();
-		if (environment instanceof ConfigurableEnvironment configurableEnvironment) {
-			configurableEnvironment.getPropertySources().forEach(sources::addLast);
-		}
-		sources.addLast(getTitleSource(sourceClass));
-		sources.addLast(getAnsiSource());
-		sources.addLast(getVersionSource(sourceClass));
 		List<PropertyResolver> resolvers = new ArrayList<>();
-		resolvers.add(new PropertySourcesPropertyResolver(sources));
+		resolvers.add(new PropertySourcesPropertyResolver(createNullDefaultSources(environment, sourceClass)));
+		resolvers.add(new PropertySourcesPropertyResolver(createEmptyDefaultSources(sourceClass)));
 		return resolvers;
 	}
 
-	private MapPropertySource getTitleSource(Class<?> sourceClass) {
+	private MutablePropertySources createNullDefaultSources(Environment environment, Class<?> sourceClass) {
+		MutablePropertySources nullDefaultSources = new MutablePropertySources();
+		if (environment instanceof ConfigurableEnvironment configurableEnvironment) {
+			configurableEnvironment.getPropertySources().forEach(nullDefaultSources::addLast);
+		}
+		nullDefaultSources.addLast(getTitleSource(sourceClass, null));
+		nullDefaultSources.addLast(getAnsiSource());
+		nullDefaultSources.addLast(getVersionSource(sourceClass, null));
+		return nullDefaultSources;
+	}
+
+	private MutablePropertySources createEmptyDefaultSources(Class<?> sourceClass) {
+		MutablePropertySources emptyDefaultSources = new MutablePropertySources();
+		emptyDefaultSources.addLast(getTitleSource(sourceClass, ""));
+		emptyDefaultSources.addLast(getVersionSource(sourceClass, ""));
+		return emptyDefaultSources;
+	}
+
+	private MapPropertySource getTitleSource(Class<?> sourceClass, String defaultValue) {
 		String applicationTitle = getApplicationTitle(sourceClass);
 		Map<String, Object> titleMap = Collections.singletonMap("application.title",
-				(applicationTitle != null) ? applicationTitle : "");
+				(applicationTitle != null) ? applicationTitle : defaultValue);
 		return new MapPropertySource("title", titleMap);
 	}
 
@@ -119,18 +131,18 @@ private AnsiPropertySource getAnsiSource() {
 		return new AnsiPropertySource("ansi", true);
 	}
 
-	private MapPropertySource getVersionSource(Class<?> sourceClass) {
-		return new MapPropertySource("version", getVersionsMap(sourceClass));
+	private MapPropertySource getVersionSource(Class<?> sourceClass, String defaultValue) {
+		return new MapPropertySource("version", getVersionsMap(sourceClass, defaultValue));
 	}
 
-	private Map<String, Object> getVersionsMap(Class<?> sourceClass) {
+	private Map<String, Object> getVersionsMap(Class<?> sourceClass, String defaultValue) {
 		String appVersion = getApplicationVersion(sourceClass);
 		String bootVersion = getBootVersion();
 		Map<String, Object> versions = new HashMap<>();
-		versions.put("application.version", getVersionString(appVersion, false));
-		versions.put("spring-boot.version", getVersionString(bootVersion, false));
-		versions.put("application.formatted-version", getVersionString(appVersion, true));
-		versions.put("spring-boot.formatted-version", getVersionString(bootVersion, true));
+		versions.put("application.version", getVersionString(appVersion, false, defaultValue));
+		versions.put("spring-boot.version", getVersionString(bootVersion, false, defaultValue));
+		versions.put("application.formatted-version", getVersionString(appVersion, true, defaultValue));
+		versions.put("spring-boot.formatted-version", getVersionString(bootVersion, true, defaultValue));
 		return versions;
 	}
 
@@ -143,9 +155,9 @@ protected String getBootVersion() {
 		return SpringBootVersion.getVersion();
 	}
 
-	private String getVersionString(String version, boolean format) {
+	private String getVersionString(String version, boolean format, String fallback) {
 		if (version == null) {
-			return "";
+			return fallback;
 		}
 		return format ? " (v" + version + ")" : version;
 	}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/ResourceBannerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -69,6 +69,14 @@ void renderWithoutVersions() {
 		assertThat(banner).startsWith("banner 1  ");
 	}
 
+	@Test
+	void renderWithoutVersionsWithDefaultValues() {
+		Resource resource = new ByteArrayResource(
+				"banner ${a} ${spring-boot.version:X.Y.Z} ${application.version:A.B.C}".getBytes());
+		String banner = printBanner(resource, null, null, null);
+		assertThat(banner).startsWith("banner 1 X.Y.Z A.B.C");
+	}
+
 	@Test
 	void renderFormattedVersions() {
 		Resource resource = new ByteArrayResource(
@@ -80,9 +88,18 @@ void renderFormattedVersions() {
 	@Test
 	void renderWithoutFormattedVersions() {
 		Resource resource = new ByteArrayResource(
-				"banner ${a}${spring-boot.formatted-version}${application.formatted-version}".getBytes());
+				"banner ${a} ${spring-boot.formatted-version} ${application.formatted-version}".getBytes());
 		String banner = printBanner(resource, null, null, null);
-		assertThat(banner).startsWith("banner 1");
+		assertThat(banner).startsWith("banner 1  ");
+	}
+
+	@Test
+	void renderWithoutFormattedVersionsWithDefaultValues() {
+		Resource resource = new ByteArrayResource(
+				"banner ${a} ${spring-boot.formatted-version:(vX.Y.Z)} ${application.formatted-version:(vA.B.C)}"
+					.getBytes());
+		String banner = printBanner(resource, null, null, null);
+		assertThat(banner).startsWith("banner 1 (vX.Y.Z) (vA.B.C)");
 	}
 
 	@Test
@@ -131,6 +148,13 @@ void renderWithoutTitle() {
 		assertThat(banner).startsWith("banner  1");
 	}
 
+	@Test
+	void renderWithoutTitleWithDefaultValue() {
+		Resource resource = new ByteArrayResource("banner ${application.title:Default Title} ${a}".getBytes());
+		String banner = printBanner(resource, null, null, null);
+		assertThat(banner).startsWith("banner Default Title 1");
+	}
+
 	@Test
 	void renderWithDefaultValues() {
 		Resource resource = new ByteArrayResource(