Propagate read-only status through setDefaultReadOnly(true) for JPA

This involves a new ResourceTransactionDefinition variant that JpaTransactionManager indicates a transaction-local EntityManager on. HibernateJpaDialect uses this indicator for hard read-only behavior.

Issue: SPR-16956

spring-orm/src/main/java/org/springframework/orm/jpa/JpaTransactionManager.java

@@ -48,6 +48,7 @@ import org.springframework.transaction.TransactionSystemException;
 import org.springframework.transaction.support.AbstractPlatformTransactionManager;
 import org.springframework.transaction.support.DefaultTransactionStatus;
 import org.springframework.transaction.support.DelegatingTransactionDefinition;
+import org.springframework.transaction.support.ResourceTransactionDefinition;
 import org.springframework.transaction.support.ResourceTransactionManager;
 import org.springframework.transaction.support.TransactionSynchronizationManager;
 import org.springframework.util.Assert;
@@ -398,12 +399,7 @@ public class JpaTransactionManager extends AbstractPlatformTransactionManager
 			// Delegate to JpaDialect for actual transaction begin.
 			final int timeoutToUse = determineTimeout(definition);
 			Object transactionData = getJpaDialect().beginTransaction(em,
-					new DelegatingTransactionDefinition(definition) {
-						@Override
-						public int getTimeout() {
-							return timeoutToUse;
-						}
-					});
+					new JpaTransactionDefinition(definition, timeoutToUse, txObject.isNewEntityManagerHolder()));
 			txObject.setTransactionData(transactionData);
 
 			// Register transaction timeout.
@@ -741,6 +737,35 @@ public class JpaTransactionManager extends AbstractPlatformTransactionManager
 	}
 
 
+	/**
+	 * JPA-specific transaction definition to be passed to {@link JpaDialect#beginTransaction}.
+	 * @since 5.1
+	 */
+	private static class JpaTransactionDefinition extends DelegatingTransactionDefinition
+			implements ResourceTransactionDefinition {
+
+		private final int timeout;
+
+		private final boolean localResource;
+
+		public JpaTransactionDefinition(TransactionDefinition targetDefinition, int timeout, boolean localResource) {
+			super(targetDefinition);
+			this.timeout = timeout;
+			this.localResource = localResource;
+		}
+
+		@Override
+		public int getTimeout() {
+			return this.timeout;
+		}
+
+		@Override
+		public boolean isLocalResource() {
+			return this.localResource;
+		}
+	}
+
+
 	/**
 	 * Holder for suspended resources.
 	 * Used internally by {@code doSuspend} and {@code doResume}.

spring-orm/src/main/java/org/springframework/orm/jpa/vendor/HibernateJpaDialect.java

@@ -67,6 +67,7 @@ import org.springframework.orm.jpa.JpaSystemException;
 import org.springframework.transaction.InvalidIsolationLevelException;
 import org.springframework.transaction.TransactionDefinition;
 import org.springframework.transaction.TransactionException;
+import org.springframework.transaction.support.ResourceTransactionDefinition;
 import org.springframework.util.Assert;
 import org.springframework.util.ReflectionUtils;
 
@@ -165,6 +166,15 @@ public class HibernateJpaDialect extends DefaultJpaDialect {
 
 		// Adapt flush mode and store previous isolation level, if any.
 		FlushMode previousFlushMode = prepareFlushMode(session, definition.isReadOnly());
+		if (definition instanceof ResourceTransactionDefinition &&
+				((ResourceTransactionDefinition) definition).isLocalResource()) {
+			// As of 5.1, we explicitly optimize for a transaction-local EntityManager,
+			// aligned with native HibernateTransactionManager behavior.
+			previousFlushMode = null;
+			if (definition.isReadOnly()) {
+				session.setDefaultReadOnly(true);
+			}
+		}
 		return new SessionTransactionData(session, previousFlushMode, preparedCon, previousIsolationLevel);
 	}