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SpringBoot整合MyBatis实现乐观锁和悲观锁的示例

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本文以转账操作为例，实现并测试乐观锁和悲观锁。

全部代码：https://github.com/imcloudfloating/Lock_Demo

GitHub Page：https://cloudli.top

死锁问题

当 A, B 两个账户同时向对方转账时，会出现如下情况：

时刻 事务 1 (A 向 B 转账) 事务 2 (B 向 A 转账)
T1 Lock A Lock B
T2 Lock B (由于事务 2 已经 Lock A，等待) Lock A (由于事务 1 已经 Lock B，等待)

由于两个事务都在等待对方释放锁，于是死锁产生了，解决方案：按照主键的大小来加锁，总是先锁主键较小或较大的那行数据。

建立数据表并插入数据（MySQL）

create table account
(
  id   int auto_increment
    primary key,
  deposit decimal(10, 2) default 0.00 not null,
  version int      default 0  not null
);

INSERT INTO vault.account (id, deposit, version) VALUES (1, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (2, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (3, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (4, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (5, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (6, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (7, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (8, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (9, 1000, 0);
INSERT INTO vault.account (id, deposit, version) VALUES (10, 1000, 0);

Mapper 文件

悲观锁使用 select ... for update，乐观锁使用 version 字段。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.cloud.demo.mapper.AccountMapper">
  <select id="selectById" resultType="com.cloud.demo.model.Account">
    select *
    from account
    where id = #{id}
  </select>
  <update id="updateDeposit" keyProperty="id" parameterType="com.cloud.demo.model.Account">
    update account
    set deposit=#{deposit},
      version = version + 1
    where id = #{id}
     and version = #{version}
  </update>
  <select id="selectByIdForUpdate" resultType="com.cloud.demo.model.Account">
    select *
    from account
    where id = #{id} for
    update
  </select>
  <update id="updateDepositPessimistic" keyProperty="id" parameterType="com.cloud.demo.model.Account">
    update account
    set deposit=#{deposit}
    where id = #{id}
  </update>
  <select id="getTotalDeposit" resultType="java.math.BigDecimal">
    select sum(deposit) from account;
  </select>
</mapper>

Mapper 接口

@Component
public interface AccountMapper {
  Account selectById(int id);
  Account selectByIdForUpdate(int id);
  int updateDepositWithVersion(Account account);
  void updateDeposit(Account account);
  BigDecimal getTotalDeposit();
}

Account POJO

@Data
public class Account {
  private int id;
  private BigDecimal deposit;
  private int version;
}

AccountService

在 transferOptimistic 方法上有个自定义注解 @Retry，这个用来实现乐观锁失败后重试。

@Slf4j
@Service
public class AccountService {

  public enum Result{
    SUCCESS,
    DEPOSIT_NOT_ENOUGH,
    FAILED,
  }

  @Resource
  private AccountMapper accountMapper;

  private BiPredicate<BigDecimal, BigDecimal> isDepositEnough = (deposit, value) -> deposit.compareTo(value) > 0;

  /**
   * 转账操作，悲观锁
   *
   * @param fromId 扣款账户
   * @param toId  收款账户
   * @param value 金额
   */
  @Transactional(isolation = Isolation.READ_COMMITTED)
  public Result transferPessimistic(int fromId, int toId, BigDecimal value) {
    Account from, to;

    try {
      // 先锁 id 较大的那行，避免死锁
      if (fromId > toId) {
        from = accountMapper.selectByIdForUpdate(fromId);
        to = accountMapper.selectByIdForUpdate(toId);
      } else {
        to = accountMapper.selectByIdForUpdate(toId);
        from = accountMapper.selectByIdForUpdate(fromId);
      }
    } catch (Exception e) {
      log.error(e.getMessage());
      TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();
      return Result.FAILED;
    }

    if (!isDepositEnough.test(from.getDeposit(), value)) {
      TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();
      log.info(String.format("Account %d is not enough.", fromId));
      return Result.DEPOSIT_NOT_ENOUGH;
    }

    from.setDeposit(from.getDeposit().subtract(value));
    to.setDeposit(to.getDeposit().add(value));

    accountMapper.updateDeposit(from);
    accountMapper.updateDeposit(to);

    return Result.SUCCESS;
  }

  /**
   * 转账操作，乐观锁
   * @param fromId 扣款账户
   * @param toId  收款账户
   * @param value 金额
   */
  @Retry
  @Transactional(isolation = Isolation.REPEATABLE_READ)
  public Result transferOptimistic(int fromId, int toId, BigDecimal value) {
    Account from = accountMapper.selectById(fromId),
        to = accountMapper.selectById(toId);

    if (!isDepositEnough.test(from.getDeposit(), value)) {
      TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();
      return Result.DEPOSIT_NOT_ENOUGH;
    }

    from.setDeposit(from.getDeposit().subtract(value));
    to.setDeposit(to.getDeposit().add(value));

    int r1, r2;

    // 先锁 id 较大的那行，避免死锁
    if (from.getId() > to.getId()) {
      r1 = accountMapper.updateDepositWithVersion(from);
      r2 = accountMapper.updateDepositWithVersion(to);
    } else {
      r2 = accountMapper.updateDepositWithVersion(to);
      r1 = accountMapper.updateDepositWithVersion(from);
    }

    if (r1 < 1 || r2 < 1) {
      // 失败，抛出重试异常，执行重试
      throw new RetryException("Transfer failed, retry.");
    } else {
      return Result.SUCCESS;
    }
  }
}

使用 Spring AOP 实现乐观锁失败后重试

自定义注解 Retry

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Retry {
  int value() default 3; // 重试次数
}

重试异常 RetryException

public class RetryException extends RuntimeException {
  public RetryException(String message) {
    super(message);
  }
}

重试的切面类

tryAgain 方法使用了 @Around 注解（表示环绕通知），可以决定目标方法在何时执行，或者不执行，以及自定义返回结果。这里首先通过 ProceedingJoinPoint.proceed() 方法执行目标方法，如果抛出了重试异常，那么重新执行直到满三次，三次都不成功则回滚并返回 FAILED。

@Slf4j
@Aspect
@Component
public class RetryAspect {

  @Pointcut("@annotation(com.cloud.demo.annotation.Retry)")
  public void retryPointcut() {

  }

  @Around("retryPointcut() && @annotation(retry)")
  @Transactional(isolation = Isolation.READ_COMMITTED)
  public Object tryAgain(ProceedingJoinPoint joinPoint, Retry retry) throws Throwable {
    int count = 0;
    do {
      count++;
      try {
        return joinPoint.proceed();
      } catch (RetryException e) {
        if (count > retry.value()) {
          log.error("Retry failed!");
          TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();
          return AccountService.Result.FAILED;
        }
      }
    } while (true);
  }
}

单元测试

用多个线程模拟并发转账，经过测试，悲观锁除了账户余额不足，或者数据库连接不够以及等待超时，全部成功；乐观锁即使加了重试，成功的线程也很少，500 个平均也就十几个成功。

所以对于写多读少的操作，使用悲观锁，对于读多写少的操作，可以使用乐观锁。

完整代码请见 Github：https://github.com/imcloudfloating/Lock_Demo。

@Slf4j
@SpringBootTest
@RunWith(SpringRunner.class)
class AccountServiceTest {

  // 并发数
  private static final int COUNT = 500;

  @Resource
  AccountMapper accountMapper;

  @Resource
  AccountService accountService;

  private CountDownLatch latch = new CountDownLatch(COUNT);
  private List<Thread> transferThreads = new ArrayList<>();
  private List<Pair<Integer, Integer>> transferAccounts = new ArrayList<>();

  @BeforeEach
  void setUp() {
    Random random = new Random(currentTimeMillis());
    transferThreads.clear();
    transferAccounts.clear();

    for (int i = 0; i < COUNT; i++) {
      int from = random.nextInt(10) + 1;
      int to;
      do{
        to = random.nextInt(10) + 1;
      } while (from == to);
      transferAccounts.add(new Pair<>(from, to));
    }
  }

  /**
   * 测试悲观锁
   */
  @Test
  void transferByPessimisticLock() throws Throwable {
    for (int i = 0; i < COUNT; i++) {
      transferThreads.add(new Transfer(i, true));
    }
    for (Thread t : transferThreads) {
      t.start();
    }
    latch.await();

    Assertions.assertEquals(accountMapper.getTotalDeposit(),
        BigDecimal.valueOf(10000).setScale(2, RoundingMode.HALF_UP));
  }

  /**
   * 测试乐观锁
   */
  @Test
  void transferByOptimisticLock() throws Throwable {
    for (int i = 0; i < COUNT; i++) {
      transferThreads.add(new Transfer(i, false));
    }
    for (Thread t : transferThreads) {
      t.start();
    }
    latch.await();

    Assertions.assertEquals(accountMapper.getTotalDeposit(),
        BigDecimal.valueOf(10000).setScale(2, RoundingMode.HALF_UP));
  }

  /**
   * 转账线程
   */
  class Transfer extends Thread {
    int index;
    boolean isPessimistic;

    Transfer(int i, boolean b) {
      index = i;
      isPessimistic = b;
    }

    @Override
    public void run() {
      BigDecimal value = BigDecimal.valueOf(
          new Random(currentTimeMillis()).nextFloat() * 100
      ).setScale(2, RoundingMode.HALF_UP);

      AccountService.Result result = AccountService.Result.FAILED;
      int fromId = transferAccounts.get(index).getKey(),
          toId = transferAccounts.get(index).getValue();
      try {
        if (isPessimistic) {
          result = accountService.transferPessimistic(fromId, toId, value);
        } else {
          result = accountService.transferOptimistic(fromId, toId, value);
        }
      } catch (Exception e) {
        log.error(e.getMessage());
      } finally {
        if (result == AccountService.Result.SUCCESS) {
          log.info(String.format("Transfer %f from %d to %d success", value, fromId, toId));
        }
        latch.countDown();
      }
    }
  }
}

MySQL 配置

innodb_rollback_on_timeout='ON'
max_connections=1000
innodb_lock_wait_timeout=500
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