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本文以轉賬操作為例，實現并測試樂觀鎖和悲觀鎖。
全部代碼：https://github.com/imcloudfloating/Lock_Demo
GitHub Page：https://cloudli.top
死鎖問題
當 A, B 兩個賬戶同時向對方轉賬時，會出現如下情況：
| 時刻 | 事務 1 (A 向 B 轉賬) | 事務 2 (B 向 A 轉賬) |
|---|---|---|
| T1 | Lock A | Lock B |
| T2 | Lock B (由于事務 2 已經 Lock A，等待) | Lock A (由于事務 1 已經 Lock B，等待) |
由于兩個事務都在等待對方釋放鎖，于是死鎖產生了，解決方案：按照主鍵的大小來加鎖，總是先鎖主鍵較小或較大的那行數據。
建立數據表并插入數據（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
悲觀鎖使用 select ... for update，樂觀鎖使用 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 方法上有個自定義注解 @Retry，這個用來實現樂觀鎖失敗后重試。
@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;
/**
* 轉賬操作，悲觀鎖
*
* @param fromId 扣款賬戶
* @param toId 收款賬戶
* @param value 金額
*/
@Transactional(isolation = Isolation.READ_COMMITTED)
public Result transferPessimistic(int fromId, int toId, BigDecimal value) {
Account from, to;
try {
// 先鎖 id 較大的那行，避免死鎖
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;
}
/**
* 轉賬操作，樂觀鎖
* @param fromId 扣款賬戶
* @param toId 收款賬戶
* @param value 金額
*/
@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;
// 先鎖 id 較大的那行，避免死鎖
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) {
// 失敗，拋出重試異常，執行重試
throw new RetryException("Transfer failed, retry.");
} else {
return Result.SUCCESS;
}
}
}
使用 Spring AOP 實現樂觀鎖失敗后重試
自定義注解 Retry
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Retry {
int value() default 3; // 重試次數
}
重試異常 RetryException
public class RetryException extends RuntimeException {
public RetryException(String message) {
super(message);
}
}
重試的切面類
tryAgain 方法使用了 @Around 注解（表示環繞通知），可以決定目標方法在何時執行，或者不執行，以及自定義返回結果。這里首先通過 ProceedingJoinPoint.proceed() 方法執行目標方法，如果拋出了重試異常，那么重新執行直到滿三次，三次都不成功則回滾并返回 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);
}
}
單元測試
用多個線程模擬并發轉賬，經過測試，悲觀鎖除了賬戶余額不足，或者數據庫連接不夠以及等待超時，全部成功；樂觀鎖即使加了重試，成功的線程也很少，500 個平均也就十幾個成功。
所以對于寫多讀少的操作，使用悲觀鎖，對于讀多寫少的操作，可以使用樂觀鎖。
完整代碼請見 Github：https://github.com/imcloudfloating/Lock_Demo。
@Slf4j
@SpringBootTest
@RunWith(SpringRunner.class)
class AccountServiceTest {
// 并發數
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));
}
}
/**
* 測試悲觀鎖
*/
@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));
}
/**
* 測試樂觀鎖
*/
@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));
}
/**
* 轉賬線程
*/
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
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持億速云。
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