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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。
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。
select *
from account
where id = #{id}
update account
set deposit=#{deposit},
version = version + 1
where id = #{id}
and version = #{version}
select *
from account
where id = #{id} for
update
update account
set deposit=#{deposit}
where id = #{id}
select sum(deposit) from account;
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 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 transferThreads = new ArrayList<>();
private List> 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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