Revise nullability of Transaction[Callback|Operations|Operator]

This commit revises the nullability declarations in TransactionCallback,
TransactionOperations, and TransactionalOperator.

Closes gh-35561

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionAspectSupport.java

@@ -412,7 +412,7 @@ public abstract class TransactionAspectSupport implements BeanFactoryAware, Init
 
 			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
 			try {
-				result = cpptm.execute(txAttr, status -> {
+				result = cpptm.<@Nullable Object> execute(txAttr, status -> {
 					TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
 					try {
 						Object retVal = invocation.proceedWithInvocation();

spring-tx/src/main/java/org/springframework/transaction/reactive/TransactionCallback.java

@@ -16,6 +16,7 @@
 
 package org.springframework.transaction.reactive;
 
+import org.jspecify.annotations.Nullable;
 import org.reactivestreams.Publisher;
 
 import org.springframework.transaction.ReactiveTransaction;
@@ -36,7 +37,7 @@ import org.springframework.transaction.ReactiveTransaction;
  * @see TransactionalOperator
  */
 @FunctionalInterface
-public interface TransactionCallback<T> {
+public interface TransactionCallback<T extends @Nullable Object> {
 
 	/**
 	 * Gets called by {@link TransactionalOperator} within a transactional context.

spring-tx/src/main/java/org/springframework/transaction/reactive/TransactionalOperator.java

@@ -16,6 +16,7 @@
 
 package org.springframework.transaction.reactive;
 
+import org.jspecify.annotations.Nullable;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -59,7 +60,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	default <T> Flux<T> transactional(Flux<T> flux) {
+	default <T extends @Nullable Object> Flux<T> transactional(Flux<T> flux) {
 		return execute(it -> flux);
 	}
 
@@ -70,7 +71,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	default <T> Mono<T> transactional(Mono<T> mono) {
+	default <T extends @Nullable Object> Mono<T> transactional(Mono<T> mono) {
 		return execute(it -> mono).singleOrEmpty();
 	}
 
@@ -85,7 +86,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	<T> Flux<T> execute(TransactionCallback<T> action) throws TransactionException;
+	<T extends @Nullable Object> Flux<T> execute(TransactionCallback<T> action) throws TransactionException;
 
 
 	// Static builder methods

spring-tx/src/main/java/org/springframework/transaction/support/CallbackPreferringPlatformTransactionManager.java

@@ -55,7 +55,7 @@ public interface CallbackPreferringPlatformTransactionManager extends PlatformTr
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	<T> @Nullable T execute(@Nullable TransactionDefinition definition, TransactionCallback<T> callback)
+	<T extends @Nullable Object> T execute(@Nullable TransactionDefinition definition, TransactionCallback<T> callback)
 			throws TransactionException;
 
 }

spring-tx/src/main/java/org/springframework/transaction/support/TransactionCallback.java

@@ -36,7 +36,7 @@ import org.springframework.transaction.TransactionStatus;
  * @see CallbackPreferringPlatformTransactionManager
  */
 @FunctionalInterface
-public interface TransactionCallback<T> {
+public interface TransactionCallback<T extends @Nullable Object> {
 
 	/**
 	 * Gets called by {@link TransactionTemplate#execute} within a transactional context.
@@ -53,6 +53,6 @@ public interface TransactionCallback<T> {
 	 * @see TransactionTemplate#execute
 	 * @see CallbackPreferringPlatformTransactionManager#execute
 	 */
-	@Nullable T doInTransaction(TransactionStatus status);
+	T doInTransaction(TransactionStatus status);
 
 }

spring-tx/src/main/java/org/springframework/transaction/support/TransactionCallbackWithoutResult.java

@@ -29,7 +29,7 @@ import org.springframework.transaction.TransactionStatus;
  * @since 28.03.2003
  * @see TransactionTemplate
  */
-public abstract class TransactionCallbackWithoutResult implements TransactionCallback<Object> {
+public abstract class TransactionCallbackWithoutResult implements TransactionCallback<@Nullable Object> {
 
 	@Override
 	public final @Nullable Object doInTransaction(TransactionStatus status) {

spring-tx/src/main/java/org/springframework/transaction/support/TransactionOperations.java

@@ -46,7 +46,7 @@ public interface TransactionOperations {
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 * @see #executeWithoutResult(Consumer)
 	 */
-	<T> @Nullable T execute(TransactionCallback<T> action) throws TransactionException;
+	<T extends @Nullable Object> T execute(TransactionCallback<T> action) throws TransactionException;
 
 	/**
 	 * Execute the action specified by the given {@link Runnable} within a transaction.
@@ -64,7 +64,7 @@ public interface TransactionOperations {
 	 * @see TransactionCallbackWithoutResult
 	 */
 	default void executeWithoutResult(Consumer<TransactionStatus> action) throws TransactionException {
-		execute(status -> {
+		this.<@Nullable Object> execute(status -> {
 			action.accept(status);
 			return null;
 		});

spring-tx/src/main/java/org/springframework/transaction/support/TransactionTemplate.java

@@ -124,7 +124,7 @@ public class TransactionTemplate extends DefaultTransactionDefinition
 
 
 	@Override
-	public <T> @Nullable T execute(TransactionCallback<T> action) throws TransactionException {
+	public <T extends @Nullable Object> T execute(TransactionCallback<T> action) throws TransactionException {
 		Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");
 
 		if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager cpptm) {

spring-tx/src/main/java/org/springframework/transaction/support/WithoutTransactionOperations.java

@@ -41,7 +41,7 @@ final class WithoutTransactionOperations implements TransactionOperations {
 
 
 	@Override
-	public <T> @Nullable T execute(TransactionCallback<T> action) throws TransactionException {
+	public <T extends @Nullable Object> T execute(TransactionCallback<T> action) throws TransactionException {
 		return action.doInTransaction(new SimpleTransactionStatus(false));
 	}