Add 'spring.jpa.bootstrap' property

Add a `spring.jpa.bootstrap` property that can be set to `async`
to switch on background bootstrapping in classic JPA setup.

Closes gh-49733

module/spring-boot-hibernate/src/test/java/org/springframework/boot/hibernate/autoconfigure/HibernateJpaAutoConfigurationTests.java

@@ -67,6 +67,8 @@ import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.autoconfigure.TestAutoConfigurationPackage;
+import org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration;
+import org.springframework.boot.diagnostics.FailureAnalyzedException;
 import org.springframework.boot.flyway.autoconfigure.FlywayAutoConfiguration;
 import org.springframework.boot.hibernate.SpringImplicitNamingStrategy;
 import org.springframework.boot.hibernate.autoconfigure.HibernateJpaAutoConfigurationTests.JpaUsingApplicationListenerConfiguration.EventCapturingApplicationListener;
@@ -102,6 +104,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Primary;
 import org.springframework.context.event.ContextRefreshedEvent;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.jdbc.core.JdbcTemplate;
 import org.springframework.jdbc.support.SQLExceptionTranslator;
 import org.springframework.jdbc.support.SQLStateSQLExceptionTranslator;
@@ -247,6 +250,59 @@ class HibernateJpaAutoConfigurationTests {
 			.run((context) -> assertThat(context).doesNotHaveBean(OpenEntityManagerInViewInterceptor.class));
 	}
 
+	@Test
+	void whenBackgroundBootstrapingAndSingleAsyncTaksExecutorConfiguresBackgroundExecutor() {
+		this.contextRunner.withPropertyValues("spring.jpa.bootstrap=async")
+			.withUserConfiguration(SingleAsyncTaskExecutorConfiguration.class)
+			.run((context) -> assertThat(
+					context.getBean(LocalContainerEntityManagerFactoryBean.class).getBootstrapExecutor())
+				.isInstanceOf(SimpleAsyncTaskExecutor.class));
+	}
+
+	@Test
+	void whenBackgroundBootstrapingAndApplicationTaksExecutorConfiguresBackgroundExecutor() {
+		this.contextRunner.withPropertyValues("spring.jpa.bootstrap=async")
+			.withUserConfiguration(MultipleAsyncTaskExecutorsConfiguration.class,
+					ApplicationTaskExecutorConfiguration.class)
+			.run((context) -> assertThat(
+					context.getBean(LocalContainerEntityManagerFactoryBean.class).getBootstrapExecutor())
+				.isSameAs(context.getBean(TaskExecutionAutoConfiguration.APPLICATION_TASK_EXECUTOR_BEAN_NAME)));
+	}
+
+	@Test
+	void whenBackgroundBootstrapingAndMissingTaksExecutorThrowsException() {
+		this.contextRunner.withPropertyValues("spring.jpa.bootstrap=async")
+			.run((context) -> assertThat(context).getFailure()
+				.rootCause()
+				.isInstanceOf(FailureAnalyzedException.class)
+				.message()
+				.contains("bootstrap executor is required when 'spring.jpa.bootstrap' is set to 'async'"));
+	}
+
+	@Test
+	void whenBackgroundBootstrapingAndMultipleTaksExecutorThrowsException() {
+		this.contextRunner.withPropertyValues("spring.jpa.bootstrap=async")
+			.withUserConfiguration(MultipleAsyncTaskExecutorsConfiguration.class)
+			.run((context) -> assertThat(context).getFailure()
+				.rootCause()
+				.isInstanceOf(FailureAnalyzedException.class)
+				.message()
+				.contains("bootstrap executor is required when 'spring.jpa.bootstrap' is set to 'async'"));
+	}
+
+	@Test
+	void whenBackgroundBootstrapingAndCustomizedBackgroundExecutorThrowsException() {
+		this.contextRunner.withPropertyValues("spring.jpa.bootstrap=async")
+			.withBean(EntityManagerFactoryBuilderCustomizer.class, this::bootstrapExecutorCustomizer)
+			.run((context) -> assertThat(
+					context.getBean(LocalContainerEntityManagerFactoryBean.class).getBootstrapExecutor())
+				.isInstanceOf(SimpleAsyncTaskExecutor.class));
+	}
+
+	private EntityManagerFactoryBuilderCustomizer bootstrapExecutorCustomizer() {
+		return (builder) -> builder.setBootstrapExecutor(new SimpleAsyncTaskExecutor());
+	}
+
 	@Test
 	void customJpaProperties() {
 		this.contextRunner
@@ -1383,4 +1439,39 @@ class HibernateJpaAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class SingleAsyncTaskExecutorConfiguration {
+
+		@Bean
+		SimpleAsyncTaskExecutor exampleTaskExecutor() {
+			return new SimpleAsyncTaskExecutor();
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class MultipleAsyncTaskExecutorsConfiguration {
+
+		@Bean
+		SimpleAsyncTaskExecutor exampleTaskExecutor1() {
+			return new SimpleAsyncTaskExecutor();
+		}
+
+		@Bean
+		SimpleAsyncTaskExecutor exampleTaskExecutor2() {
+			return new SimpleAsyncTaskExecutor();
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class ApplicationTaskExecutorConfiguration {
+
+		@Bean
+		SimpleAsyncTaskExecutor applicationTaskExecutor() {
+			return new SimpleAsyncTaskExecutor();
+		}
+
+	}
+
 }

module/spring-boot-jpa/src/main/java/org/springframework/boot/jpa/autoconfigure/JpaBaseConfiguration.java

@@ -40,6 +40,7 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplicat
 import org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.boot.jpa.EntityManagerFactoryBuilder;
+import org.springframework.boot.jpa.autoconfigure.JpaProperties.Bootstrap;
 import org.springframework.boot.persistence.autoconfigure.EntityScanPackages;
 import org.springframework.boot.transaction.autoconfigure.TransactionManagerCustomizers;
 import org.springframework.context.annotation.Bean;
@@ -127,6 +128,10 @@ public abstract class JpaBaseConfiguration {
 		@Nullable AsyncTaskExecutor bootstrapExecutor = determineBootstrapExecutor(taskExecutors);
 		EntityManagerFactoryBuilder builder = new EntityManagerFactoryBuilder(jpaVendorAdapter,
 				this::buildJpaProperties, persistenceUnitManager.getIfAvailable(), null, bootstrapExecutor);
+		if (this.properties.getBootstrap() == Bootstrap.ASYNC) {
+			builder.requireBootstrapExecutor(
+					new PropertyBasedRequiredBackgroundBootstrapping("spring.jpa.bootstrap", "async"));
+		}
 		customizers.orderedStream().forEach((customizer) -> customizer.customize(builder));
 		return builder;
 	}

module/spring-boot-jpa/src/main/java/org/springframework/boot/jpa/autoconfigure/JpaProperties.java

@@ -77,6 +77,11 @@ public class JpaProperties {
 	 */
 	private @Nullable Boolean openInView;
 
+	/**
+	 * Bootstrap method to use.
+	 */
+	private Bootstrap bootstrap = Bootstrap.DEFAULT;
+
 	public Map<String, String> getProperties() {
 		return this.properties;
 	}
@@ -129,4 +134,31 @@ public class JpaProperties {
 		this.openInView = openInView;
 	}
 
+	public Bootstrap getBootstrap() {
+		return this.bootstrap;
+	}
+
+	public void setBootstrap(Bootstrap bootstrap) {
+		this.bootstrap = bootstrap;
+	}
+
+	/**
+	 * Bootstrap methods that can be used with JPA.
+	 */
+	public enum Bootstrap {
+
+		/**
+		 * Default JPA bootstrapping.
+		 */
+		DEFAULT,
+
+		/**
+		 * Asynchronous JPA bootstrapping. The ApplicationContext must either have a
+		 * single AsyncTaskExecutor bean, or an AsyncTaskExecutor bean named
+		 * 'applicationTaskExecutor'.
+		 */
+		ASYNC
+
+	}
+
 }