Enforce JPA bootstrap failure for incomplete Hibernate Validator setup

Closes gh-30549

spring-orm/spring-orm.gradle

@@ -11,6 +11,7 @@ dependencies {
 	optional("org.eclipse.persistence:org.eclipse.persistence.jpa")
 	optional("org.hibernate:hibernate-core-jakarta")
 	optional("jakarta.servlet:jakarta.servlet-api")
+	optional("jakarta.validation:jakarta.validation-api")
 	testImplementation(project(":spring-core-test"))
 	testImplementation(testFixtures(project(":spring-beans")))
 	testImplementation(testFixtures(project(":spring-context")))

spring-orm/src/main/java/org/springframework/orm/jpa/persistenceunit/DefaultPersistenceUnitManager.java

@@ -31,6 +31,8 @@ import jakarta.persistence.PersistenceException;
 import jakarta.persistence.SharedCacheMode;
 import jakarta.persistence.ValidationMode;
 import jakarta.persistence.spi.PersistenceUnitInfo;
+import jakarta.validation.NoProviderFoundException;
+import jakarta.validation.Validation;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
@@ -48,6 +50,7 @@ import org.springframework.jdbc.datasource.lookup.DataSourceLookup;
 import org.springframework.jdbc.datasource.lookup.JndiDataSourceLookup;
 import org.springframework.jdbc.datasource.lookup.MapDataSourceLookup;
 import org.springframework.lang.Nullable;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ResourceUtils;
 
@@ -100,6 +103,9 @@ public class DefaultPersistenceUnitManager
 	public static final String ORIGINAL_DEFAULT_PERSISTENCE_UNIT_NAME = "default";
 
 
+	private static final boolean beanValidationPresent = ClassUtils.isPresent(
+			"jakarta.validation.Validation", DefaultPersistenceUnitManager.class.getClassLoader());
+
 	protected final Log logger = LogFactory.getLog(getClass());
 
 	private String[] persistenceXmlLocations = new String[] {DEFAULT_PERSISTENCE_XML_LOCATION};
@@ -449,7 +455,7 @@ public class DefaultPersistenceUnitManager
 				pui.setPersistenceUnitRootUrl(determineDefaultPersistenceUnitRootUrl());
 			}
 
-			// Override DataSource and cache/validation mode
+			// Override DataSource and shared cache mode
 			if (pui.getJtaDataSource() == null && this.defaultJtaDataSource != null) {
 				pui.setJtaDataSource(this.defaultJtaDataSource);
 			}
@@ -459,9 +465,16 @@ public class DefaultPersistenceUnitManager
 			if (this.sharedCacheMode != null) {
 				pui.setSharedCacheMode(this.sharedCacheMode);
 			}
+
+			// Override validation mode or pre-resolve provider detection
 			if (this.validationMode != null) {
 				pui.setValidationMode(this.validationMode);
 			}
+			else if (pui.getValidationMode() == ValidationMode.AUTO) {
+				pui.setValidationMode(
+						beanValidationPresent && BeanValidationDelegate.isValidationProviderPresent() ?
+						ValidationMode.CALLBACK : ValidationMode.NONE);
+			}
 
 			// Initialize persistence unit ClassLoader
 			if (this.loadTimeWeaver != null) {
@@ -697,4 +710,21 @@ public class DefaultPersistenceUnitManager
 		return pui;
 	}
 
+
+	/**
+	 * Inner class to avoid a hard dependency on the Bean Validation API at runtime.
+	 */
+	private static class BeanValidationDelegate {
+
+		public static boolean isValidationProviderPresent() {
+			try {
+				Validation.byDefaultProvider().configure();
+				return true;
+			}
+			catch (NoProviderFoundException ex) {
+				return false;
+			}
+		}
+	}
+
 }