Add ReloadableResourceBundleMessageSource support

Add a `spring.messages.reloadable` configuration property which can be
used to auto-configure a `ReloadableResourceBundleMessageSource` rather
than a `ResourceBundleMessageSource`.

Closes gh-13377

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/context/MessageSourceAutoConfiguration.java

@@ -33,6 +33,8 @@
 import org.springframework.context.annotation.ConditionContext;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.support.AbstractResourceBasedMessageSource;
+import org.springframework.context.support.ReloadableResourceBundleMessageSource;
 import org.springframework.context.support.ResourceBundleMessageSource;
 import org.springframework.core.Ordered;
 import org.springframework.core.io.Resource;
@@ -65,7 +67,9 @@ public MessageSourceProperties messageSourceProperties() {
 
 	@Bean
 	public MessageSource messageSource(MessageSourceProperties properties) {
-		ResourceBundleMessageSource messageSource = new ResourceBundleMessageSource();
+		AbstractResourceBasedMessageSource messageSource = (properties.isReloadable()
+				? new ReloadableResourceBundleMessageSource()
+				: new ResourceBundleMessageSource());
 		if (StringUtils.hasText(properties.getBasename())) {
 			messageSource.setBasenames(StringUtils.commaDelimitedListToStringArray(
 					StringUtils.trimAllWhitespace(properties.getBasename())));

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/context/MessageSourceProperties.java

@@ -71,6 +71,12 @@ public class MessageSourceProperties {
 	 */
 	private boolean useCodeAsDefaultMessage = false;
 
+	/**
+	 * Whether to use a "ReloadableResourceBundleMessageSource" rather than the default
+	 * "ResourceBundleMessageSource". Recommended during development only.
+	 */
+	private boolean reloadable = false;
+
 	public String getBasename() {
 		return this.basename;
 	}
@@ -119,4 +125,12 @@ public void setUseCodeAsDefaultMessage(boolean useCodeAsDefaultMessage) {
 		this.useCodeAsDefaultMessage = useCodeAsDefaultMessage;
 	}
 
+	public boolean isReloadable() {
+		return this.reloadable;
+	}
+
+	public void setReloadable(boolean reloadable) {
+		this.reloadable = reloadable;
+	}
+
 }

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/context/MessageSourceAutoConfigurationTests.java

@@ -21,6 +21,7 @@
 import org.junit.Ignore;
 import org.junit.Test;
 
+import org.springframework.beans.BeansException;
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
@@ -32,6 +33,9 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.PropertySource;
+import org.springframework.context.support.DelegatingMessageSource;
+import org.springframework.context.support.ReloadableResourceBundleMessageSource;
+import org.springframework.context.support.ResourceBundleMessageSource;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -220,6 +224,43 @@ public void existingMessageSourceInParentIsIgnored() {
 								.isEqualTo("bar")));
 	}
 
+	@Test
+	public void testDefaultReloadableValueMessageSource() {
+		this.contextRunner.withPropertyValues("spring.messages.basename:test/messages")
+				.run((context) -> assertThat(getDeclaredMessageSource(context))
+						.isInstanceOf(ResourceBundleMessageSource.class));
+	}
+
+	@Test
+	public void testNotReloadableMessageSource() {
+		this.contextRunner
+				.withPropertyValues("spring.messages.basename:test/messages",
+						"spring.messages.reloadable:false")
+				.run((context) -> assertThat(getDeclaredMessageSource(context))
+						.isInstanceOf(ResourceBundleMessageSource.class));
+	}
+
+	@Test
+	public void testReloadableMessageSource() {
+		this.contextRunner.withPropertyValues("spring.messages.basename:test/messages",
+				"spring.messages.reloadable:true").run((context) -> {
+					assertThat(getDeclaredMessageSource(context))
+							.isInstanceOf(ReloadableResourceBundleMessageSource.class);
+					assertThat(context.getMessage("foo", null, "Foo message", Locale.UK))
+							.isEqualTo("bar");
+				});
+	}
+
+	private MessageSource getDeclaredMessageSource(AssertableApplicationContext context)
+			throws BeansException {
+		MessageSource messageSource = context.getBean(MessageSource.class);
+		if (messageSource instanceof DelegatingMessageSource) {
+			messageSource = ((DelegatingMessageSource) messageSource)
+					.getParentMessageSource();
+		}
+		return messageSource;
+	}
+
 	@Configuration
 	@PropertySource("classpath:/switch-messages.properties")
 	protected static class Config {

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -133,6 +133,7 @@ content into your application. Rather, pick only the properties that you need.
 	spring.messages.cache-duration= # Loaded resource bundle files cache duration. When not set, bundles are cached forever. If a duration suffix is not specified, seconds will be used.
 	spring.messages.encoding=UTF-8 # Message bundles encoding.
 	spring.messages.fallback-to-system-locale=true # Whether to fall back to the system Locale if no files for a specific Locale have been found.
+	spring.messages.reloadable=false # Whether to use a ReloadableResourceBundleMessageSource instead of the default ResourceBundleMessageSource. Recommended during development only.
 	spring.messages.use-code-as-default-message=false # Whether to use the message code as the default message instead of throwing a "NoSuchMessageException". Recommended during development only.
 
 	# OUTPUT