我想写一本书 springboot 根据借方/贷方交易更新钱包余额的代码。我有两张table。钱包和交易。我正在运行一个测试套件，它运行100个并行事务（50个借方和50个贷方）。大约50%的交易失败，并出现以下错误，而且wallettable中的钱包余额与transactiontable中存储的交易不匹配
com.mysql.jdbc.exceptions.jdbc4.mysqltransactionrollbackexception:获取锁时发现死锁；尝试重新启动事务
我无法理解以下原因1）为什么死锁2）为什么钱包余额与成功存储的交易数量不匹配。我正在使用mysql innodb作为数据库

@Transactional(isolation = Isolation.SERIALIZABLE)
  public Transaction saveTransaction(String walletId, Txn txn) {
    Optional<Wallet> byId = walletRepo.findById(Integer.parseInt(walletId));
    if (!byId.isPresent()) {
      throw new WalletNotFoundException();
    }
    Wallet wallet = byId.get();
    Transaction transaction = new Transaction();
    transaction.setAmount(txn.getAmount());
    transaction.setType(txn.getType());
    transaction.setWallet(wallet);
    BigDecimal balance = applyTransactionToWallet(txn, wallet.getBalance());
    Transaction save = transactionRepo.save(transaction);
    wallet.setBalance(balance);
    return save;
  }

  public Optional<Wallet> getWallet(String walletId) {
    Optional<Wallet> byId = walletRepo.findById(Integer.parseInt(walletId));
    return byId;
  }

  private BigDecimal applyTransactionToWallet(Txn txn, BigDecimal amount) {
    if (txn.getType() == TransactionType.CREDIT) {
      return amount.add(txn.getAmount());
    }
    return amount.subtract(txn.getAmount());
  }