Amazon dynamodb DynamoDBMapper和事务序列

我正在看AWS文档中的示例代码。它似乎在将映射器放入事务之前先调用mapper.save。从我的角度来看，根据顺序，在创建事务之前，首先保存了对象。在调用save（）之前，我们不应该先将它们放入事务中吗

private static void testPutAndUpdateInTransactionWrite() {
        // Create new Forum item for S3 using save
        Forum s3Forum = new Forum();
        s3Forum.name = "S3 Forum";
        s3Forum.category = "Core Amazon Web Services";
        s3Forum.threads = 0;
        mapper.save(s3Forum);

        // Update Forum item for S3 and Create new Forum item for DynamoDB using transactionWrite
        s3Forum.category = "Amazon Web Services";
        Forum dynamodbForum = new Forum();
        dynamodbForum.name = "DynamoDB Forum";
        dynamodbForum.category = "Amazon Web Services";
        dynamodbForum.threads = 0;
        TransactionWriteRequest transactionWriteRequest = new TransactionWriteRequest();
        transactionWriteRequest.addUpdate(s3Forum);
        transactionWriteRequest.addPut(dynamodbForum);
        executeTransactionWrite(transactionWriteRequest);
    }