chore(spanner): make previoustransactionId field final

Author: harshachinta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AsyncTransactionManagerImpl.java

@@ -88,7 +88,7 @@ private ApiFuture<TransactionContext> internalBeginAsync(boolean firstAttempt) {
       // Use the current transactionId if available, otherwise fallback to the previous aborted
       // transactionId.
       multiplexedSessionPreviousTransactionId =
-          txn.transactionId != null ? txn.transactionId : txn.previousTransactionId;
+          txn.transactionId != null ? txn.transactionId : txn.getPreviousTransactionId();
     }
 
     txn =

google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionManagerImpl.java

@@ -110,7 +110,7 @@ public TransactionContext resetForRetry() {
         // Use the current transactionId if available, otherwise fallback to the previous aborted
         // transactionId.
         multiplexedSessionPreviousTransactionId =
-            txn.transactionId != null ? txn.transactionId : txn.previousTransactionId;
+            txn.transactionId != null ? txn.transactionId : txn.getPreviousTransactionId();
       }
       txn =
           session.newTransaction(

google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunnerImpl.java

@@ -209,7 +209,7 @@ public void removeListener(Runnable listener) {
 
     volatile ByteString transactionId;
 
-    ByteString previousTransactionId;
+    final ByteString previousTransactionId;
 
     private CommitResponse commitResponse;
     private final Clock clock;
@@ -257,6 +257,10 @@ private void decreaseAsyncOperations() {
       }
     }
 
+    ByteString getPreviousTransactionId() {
+      return this.previousTransactionId;
+    }
+
     @Override
     public void close() {
       // Only mark the context as closed, but do not end the tracer span, as that is done by the
@@ -295,7 +299,7 @@ private void createTxnAsync(final SettableApiFuture<Void> res) {
       span.addAnnotation("Creating Transaction");
       final ApiFuture<ByteString> fut =
           session.beginTransactionAsync(
-              options, isRouteToLeader(), getTransactionChannelHint(), previousTransactionId);
+              options, isRouteToLeader(), getTransactionChannelHint(), getPreviousTransactionId());
       fut.addListener(
           () -> {
             try {
@@ -571,7 +575,8 @@ TransactionSelector getTransactionSelector() {
           if (tx == null) {
             return TransactionSelector.newBuilder()
                 .setBegin(
-                    SessionImpl.createReadWriteTransactionOptions(options, previousTransactionId))
+                    SessionImpl.createReadWriteTransactionOptions(
+                        options, getPreviousTransactionId()))
                 .build();
           } else {
             // Wait for the transaction to come available. The tx.get() call will fail with an
@@ -1138,7 +1143,7 @@ private <T> T runInternal(final TransactionCallable<T> txCallable) {
               // Use the current transactionId if available, otherwise fallback to the previous
               // transactionId.
               multiplexedSessionPreviousTransactionId =
-                  txn.transactionId != null ? txn.transactionId : txn.previousTransactionId;
+                  txn.transactionId != null ? txn.transactionId : txn.getPreviousTransactionId();
             }
 
             txn =

google-cloud-spanner/src/test/java/com/google/cloud/spanner/AsyncTransactionManagerImplTest.java

@@ -99,7 +99,7 @@ public void testRetryUsesPreviousTransactionIdOnMultiplexedSession() {
 
       // Mock the transaction object to contain transactionID=null and
       // previousTransactionId=mockPreviousTransactionId
-      transaction.previousTransactionId = mockPreviousTransactionId;
+      when(transaction.getPreviousTransactionId()).thenReturn(mockPreviousTransactionId);
       manager.resetForRetryAsync();
       // Verify that in the first retry attempt, the `previousTransactionId`
       // (mockPreviousTransactionId) is passed to the new transaction.

google-cloud-spanner/src/test/java/com/google/cloud/spanner/TransactionManagerImplTest.java

@@ -385,7 +385,7 @@ public void storePreviousTxnIdOnAbortForMultiplexedSession() {
     assertThrows(AbortedException.class, () -> manager.commit());
 
     txn = Mockito.mock(TransactionRunnerImpl.TransactionContextImpl.class);
-    txn.previousTransactionId = mockTransactionId;
+    when(txn.getPreviousTransactionId()).thenReturn(mockTransactionId);
     when(session.newTransaction(Options.fromTransactionOptions(), mockTransactionId))
         .thenReturn(txn);
     when(session.getIsMultiplexed()).thenReturn(true);