springboot+mybatis写一个交易

好的，下面是一个简单的 springboot + mybatis 实现交易的示例：

首先，我们需要定义交易实体类：

public class Transaction {
    private Long id;
    private Long userId;
    private BigDecimal amount;
    // ... 省略其它属性、getter和setter方法
}

然后，我们需要定义一个 DAO 接口和对应的 mapper.xml 文件来实现交易的增删改查操作：

public interface TransactionDao {
    int save(Transaction transaction);
    int delete(Long id);
    int update(Transaction transaction);
    Transaction getById(Long id);
    List<Transaction> getByUserId(Long userId);
}

<!-- TransactionMapper.xml -->
<mapper namespace="com.example.dao.TransactionDao">
    <insert id="save" parameterType="com.example.entity.Transaction">
        insert into transaction(user_id, amount) values(#{userId}, #{amount})
    </insert>
    <delete id="delete" parameterType="Long">
        delete from transaction where id = #{id}
    </delete>
    <update id="update" parameterType="com.example.entity.Transaction">
        update transaction set user_id = #{userId}, amount = #{amount} where id = #{id}
    </update>
    <select id="getById" parameterType="Long" resultType="com.example.entity.Transaction">
        select * from transaction where id = #{id}
    </select>
    <select id="getByUserId" parameterType="Long" resultType="com.example.entity.Transaction">
        select * from transaction where user_id = #{userId}
    </select>
</mapper>

接下来，我们需要编写一个 Service 层来封装 DAO 层的操作，并实现交易的业务逻辑：

@Service
public class TransactionService {
    @Autowired
    private TransactionDao transactionDao;

    @Transactional
    public void transfer(Long sourceUserId, Long targetUserId, BigDecimal amount) {
        // 扣除源用户的金额
        Transaction sourceTransaction = new Transaction();
        sourceTransaction.setUserId(sourceUserId);
        sourceTransaction.setAmount(amount.negate()); // 注意这里需要取负值
        transactionDao.save(sourceTransaction);

        // 增加目标用户的金额
        Transaction targetTransaction = new Transaction();
        targetTransaction.setUserId(targetUserId);
        targetTransaction.setAmount(amount);
        transactionDao.save(targetTransaction);
    }
}

最后，我们需要编写一个 Controller 层来处理 HTTP 请求：

@RestController
@RequestMapping("/transactions")
public class TransactionController {
    @Autowired
    private TransactionService transactionService;

    @PostMapping
    public void transfer(@RequestParam("sourceUserId") Long sourceUserId,
                         @RequestParam("targetUserId") Long targetUserId,
                         @RequestParam("amount") BigDecimal amount) {
        transactionService.transfer(sourceUserId, targetUserId, amount);
    }
}

这样，我们就完成了一个简单的交易系统。当用户 A 要向用户 B 转账时，通过访问 `/transactions` 接口，将源用户的金额扣除，并将目标用户的金额增加。