悲观锁是数据库系统中使用的一种并发控制机制。它可以防止多个事务同时更新相同的数据。
Spring悲观锁应用：
@SpringBootApplication
public class SpringPessimisticLockApplication {
public static void main(String[] args) {
SpringApplication.run(SpringPessimisticLockApplication.class, args);
}
@Bean
public ApplicationRunner runner(BalanceRepository balanceRepository) {
return args -> {
Balance balance = Balance.builder().
owner("mert")
.balance(0)
.build();
balanceRepository.save(balance);
};
}
}
|
应用程序启动时，运行程序 Bean 会将 BalanceRepository 作为参数，并将Balance余额保存到数据库中。@Entity
@Table
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Data
public class Balance {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
private int balance;
private String owner;
@Version
private int version;
}
@RestController
@RequestMapping("/api/v1/balances")
public class BalanceController {
private final BalanceService balanceService;
public BalanceController(BalanceService balanceService) {
this.balanceService = balanceService;
}
@GetMapping
public String incrementBalance() {
balanceService.incrementBalance();
return "balance incremented.";
}
}
@Service
@Slf4j
public class BalanceService {
private final BalanceRepository balanceRepository;
public BalanceService(BalanceRepository balanceRepository) {
this.balanceRepository = balanceRepository;
}
public void incrementBalance(){
Balance balance = balanceRepository.findBalanceByOwner("mert")
.orElseThrow(() -> new EntityNotFoundException("Balance not found"));
log.info("balance incrementing..");
balance.setBalance(balance.getBalance() + 1);
balanceRepository.save(balance);
}
}
@Repository
public interface BalanceRepository extends JpaRepository<Balance, Integer> {
Optional<Balance> findBalanceByOwner(String owner);
}
|
源码
启动应用，发送并发 HTTP 请求
ab -n 100 -c 10 host.docker.internal:8080/api/v1/balances
|
Complete Requests: 100
Failed Requests: 73
余额应为 100，但数据不一致。
Exception: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction
org.hibernate.StaleObjectStateException：该异常是一种机制，用于防止一个事务覆盖另一个事务的更改，从而确保数据的一致性。
解决方案
要处理这种情况，通常需要实施某种形式的并发控制，如乐观锁或悲观锁。
本文将使用悲观锁定。
悲观锁定的缺点：
在BalanceRepository加入锁：
@Repository
public interface BalanceRepository extends JpaRepository<Balance, Integer> {
@Lock(LockModeType.PESSIMISTIC_WRITE)
Optional<Balance> findBalanceByOwner(String owner);
}
|
@Lock(LockModeType.PESSIMISTIC_WRITE)：此注解用于为方法指定锁定策略。它会阻止其他事务同时写入相同的记录，直到锁被释放。
再次测试，发送100 请求
Complete Requests: 100
Failed Requests: 0
还有错误：
jakarta.persistence.TransactionRequiredException：当您试图执行需要活动事务的操作，但当前没有事务正在进行时，通常会出现该异常。
Exception: jakarta.persistence.TransactionRequiredException: Query requires transaction be in progress, but no transaction is known to be in progress
解决方案
在服务方法中添加 @Transactional 注解。
@Service
@Slf4j
public class BalanceService {
private final BalanceRepository balanceRepository;
public BalanceService(BalanceRepository balanceRepository) {
this.balanceRepository = balanceRepository;
}
@Transactional
public void incrementBalance(){
Balance balance = balanceRepository.findBalanceByOwner("mert")
.orElseThrow(() -> new EntityNotFoundException("Balance not found"));
log.info("balance incrementing..");
balance.setBalance(balance.getBalance() + 1);
balanceRepository.save(balance);
}
}
|
再次测试，发送2000 请求
Complete Requests: 2000
Failed Requests: 0