Restructure bootstrap classes to a foundational layer

Move bootstrap code from `org.springframework.boot` to
`org.springframework.boot.bootstrap` and make them a foundational
layer.

This move helps reduce `org.springframework.boot.context.config`
dependencies to `org.springframework.boot`.

See gh-47232

Author: philwebb

config/checkstyle/import-control.xml

@@ -13,7 +13,10 @@
 	<!-- Allow other imports -->
 	<allow pkg=".*" regex="true" />
 
-	<!-- Keep foundation packages away from SpringApplication package -->
+	<!-- Keep foundation packages away from 'org.springframework.boot' package -->
+	<subpackage name="bootstrap">
+		<disallow pkg="org.springframework.boot" exact-match="true"/>
+	</subpackage>
 	<subpackage name="ssl">
 		<disallow pkg="org.springframework.boot" exact-match="true"/>
 	</subpackage>

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/ConfigDataApplicationContextInitializer.java

@@ -16,8 +16,8 @@
 
 package org.springframework.boot.test.context;
 
-import org.springframework.boot.DefaultBootstrapContext;
 import org.springframework.boot.DefaultPropertiesPropertySource;
+import org.springframework.boot.bootstrap.DefaultBootstrapContext;
 import org.springframework.boot.context.config.ConfigData;
 import org.springframework.boot.context.config.ConfigDataEnvironmentPostProcessor;
 import org.springframework.boot.env.RandomValuePropertySource;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootContextLoader.java

@@ -30,13 +30,13 @@
 import org.springframework.beans.BeanUtils;
 import org.springframework.boot.ApplicationContextFactory;
 import org.springframework.boot.Banner;
-import org.springframework.boot.ConfigurableBootstrapContext;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.SpringApplication.AbandonedRunException;
 import org.springframework.boot.SpringApplicationHook;
 import org.springframework.boot.SpringApplicationRunListener;
 import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.boot.WebApplicationType;
+import org.springframework.boot.bootstrap.ConfigurableBootstrapContext;
 import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
 import org.springframework.boot.test.context.SpringBootTest.UseMainMethod;
 import org.springframework.boot.test.mock.web.SpringBootMockServletContext;

core/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -58,6 +58,9 @@
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.beans.factory.xml.XmlBeanDefinitionReader;
 import org.springframework.boot.Banner.Mode;
+import org.springframework.boot.bootstrap.BootstrapRegistry;
+import org.springframework.boot.bootstrap.BootstrapRegistryInitializer;
+import org.springframework.boot.bootstrap.DefaultBootstrapContext;
 import org.springframework.boot.context.properties.bind.Bindable;
 import org.springframework.boot.context.properties.bind.Binder;
 import org.springframework.boot.context.properties.source.ConfigurationPropertySources;

core/spring-boot/src/main/java/org/springframework/boot/SpringApplicationRunListener.java

@@ -20,6 +20,7 @@
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.boot.bootstrap.ConfigurableBootstrapContext;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.env.ConfigurableEnvironment;

core/spring-boot/src/main/java/org/springframework/boot/SpringApplicationRunListeners.java

@@ -23,6 +23,7 @@
 import org.apache.commons.logging.Log;
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.boot.bootstrap.ConfigurableBootstrapContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.metrics.ApplicationStartup;

core/spring-boot/src/main/java/org/springframework/boot/BootstrapContext.java renamed to core/spring-boot/src/main/java/org/springframework/boot/bootstrap/BootstrapContext.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.boot;
+package org.springframework.boot.bootstrap;
 
 import java.util.function.Supplier;
 
@@ -31,7 +31,7 @@
  * shared before the {@link ApplicationContext} is available.
  *
  * @author Phillip Webb
- * @since 2.4.0
+ * @since 4.0.0
  */
 public interface BootstrapContext {
 

core/spring-boot/src/main/java/org/springframework/boot/BootstrapContextClosedEvent.java renamed to core/spring-boot/src/main/java/org/springframework/boot/bootstrap/BootstrapContextClosedEvent.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.boot;
+package org.springframework.boot.bootstrap;
 
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -23,7 +23,7 @@
  * {@link ApplicationEvent} published by a {@link BootstrapContext} when it's closed.
  *
  * @author Phillip Webb
- * @since 2.4.0
+ * @since 4.0.0
  * @see BootstrapRegistry#addCloseListener(org.springframework.context.ApplicationListener)
  */
 public class BootstrapContextClosedEvent extends ApplicationEvent {

core/spring-boot/src/main/java/org/springframework/boot/BootstrapRegistry.java renamed to core/spring-boot/src/main/java/org/springframework/boot/bootstrap/BootstrapRegistry.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.boot;
+package org.springframework.boot.bootstrap;
 
 import java.util.function.Supplier;
 
@@ -42,7 +42,7 @@
  * use.
  *
  * @author Phillip Webb
- * @since 2.4.0
+ * @since 4.0.0
  * @see BootstrapContext
  * @see ConfigurableBootstrapContext
  */

core/spring-boot/src/main/java/org/springframework/boot/BootstrapRegistryInitializer.java renamed to core/spring-boot/src/main/java/org/springframework/boot/bootstrap/BootstrapRegistryInitializer.java

@@ -14,15 +14,14 @@
  * limitations under the License.
  */
 
-package org.springframework.boot;
+package org.springframework.boot.bootstrap;
 
 /**
  * Callback interface that can be used to initialize a {@link BootstrapRegistry} before it
  * is used.
  *
  * @author Phillip Webb
- * @since 2.4.5
- * @see SpringApplication#addBootstrapRegistryInitializer(BootstrapRegistryInitializer)
+ * @since 4.0.0
  * @see BootstrapRegistry
  */
 @FunctionalInterface