怎么理解Redis中的分布式锁
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Redis 分布式锁
大家项目中都会使用到分布式锁把，通常用来做数据的有序操作场景，比如一笔订单退款（如果可以退多次的情况）。或者用户多端下单。【相关推荐：Redis视频教程】
Maven 依赖
我主要是基于 Spring-Boot 2.1.2 + Jedis 进行实现
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.2.RELEASE</version>
</parent>
<groupId>cn.edu.cqvie</groupId>
<artifactId>redis-lock</artifactId>
<version>1.0-SNAPSHOT</version>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<java.version>1.8</java.version>
<redis.version>2.9.0</redis.version>
<spring-test.version>5.0.7</spring-test.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-redis</artifactId>
</dependency>
<dependency>
<groupId>redis.clients</groupId>
<artifactId>jedis</artifactId>
<version>${redis.version}</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-logging</artifactId>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>log4j-over-slf4j</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>配置文件
application.properties 配置文件内容如下：
spring.redis.host=127.0.0.1
spring.redis.port=6379
spring.redis.password=
spring.redis.timeout=30000
spring.redis.jedis.pool.max-active=8
spring.redis.jedis.pool.min-idle=2
spring.redis.jedis.pool.max-idle=4
logging.level.root=INFO接口定义
接口定义，对于锁我们核心其实就连个方法 lock 和 unlock.
public interface RedisLock {
long TIMEOUT_MILLIS = 30000;
int RETRY_MILLIS = 30000;
long SLEEP_MILLIS = 10;
boolean tryLock(String key);
boolean lock(String key);
boolean lock(String key, long expire);
boolean lock(String key, long expire, long retryTimes);
boolean unlock(String key);
}分布式锁实现
我的实现方式是通过 setnx 方式实现了，如果存在 tryLock 逻辑的话，会通过 自旋 的方式重试
// AbstractRedisLock.java 抽象类
public abstract class AbstractRedisLock implements RedisLock {
@Override
public boolean lock(String key) {
return lock(key, TIMEOUT_MILLIS);
}
@Override
public boolean lock(String key, long expire) {
return lock(key, TIMEOUT_MILLIS, RETRY_MILLIS);
}
}
// 具体实现
@Component
public class RedisLockImpl extends AbstractRedisLock {
private Logger logger = LoggerFactory.getLogger(getClass());
@Autowired
private RedisTemplate<String, String> redisTemplate;
private ThreadLocal<String> threadLocal = new ThreadLocal<String>();
private static final String UNLOCK_LUA;
static {
StringBuilder sb = new StringBuilder();
sb.append("if redis.call(\"get\",KEYS[1]) == ARGV[1] ");
sb.append("then ");
sb.append(" return redis.call(\"del\",KEYS[1]) ");
sb.append("else ");
sb.append(" return 0 ");
sb.append("end ");
UNLOCK_LUA = sb.toString();
}
@Override
public boolean tryLock(String key) {
return tryLock(key, TIMEOUT_MILLIS);
}
public boolean tryLock(String key, long expire) {
try {
return !StringUtils.isEmpty(redisTemplate.execute((RedisCallback<String>) connection -> {
JedisCommands commands = (JedisCommands) connection.getNativeConnection();
String uuid = UUID.randomUUID().toString();
threadLocal.set(uuid);
return commands.set(key, uuid, "NX", "PX", expire);
}));
} catch (Throwable e) {
logger.error("set redis occurred an exception", e);
}
return false;
}
@Override
public boolean lock(String key, long expire, long retryTimes) {
boolean result = tryLock(key, expire);
while (!result && retryTimes-- > 0) {
try {
logger.debug("lock failed, retrying...{}", retryTimes);
Thread.sleep(SLEEP_MILLIS);
} catch (InterruptedException e) {
return false;
}
result = tryLock(key, expire);
}
return result;
}
@Override
public boolean unlock(String key) {
try {
List<String> keys = Collections.singletonList(key);
List<String> args = Collections.singletonList(threadLocal.get());
Long result = redisTemplate.execute((RedisCallback<Long>) connection -> {
Object nativeConnection = connection.getNativeConnection();
if (nativeConnection instanceof JedisCluster) {
return (Long) ((JedisCluster) nativeConnection).eval(UNLOCK_LUA, keys, args);
}
if (nativeConnection instanceof Jedis) {
return (Long) ((Jedis) nativeConnection).eval(UNLOCK_LUA, keys, args);
}
return 0L;
});
return result != null && result > 0;
} catch (Throwable e) {
logger.error("unlock occurred an exception", e);
}
return false;
}
}测试代码
最后再来看看如何使用吧. (下面是一个模拟秒杀的场景)
@RunWith(SpringRunner.class)
@SpringBootTest
public class RedisLockImplTest {
private Logger logger = LoggerFactory.getLogger(getClass());
@Autowired
private RedisLock redisLock;
@Autowired
private StringRedisTemplate redisTemplate;
private ExecutorService executors = Executors.newScheduledThreadPool(8);
@Test
public void lock() {
// 初始化库存
redisTemplate.opsForValue().set("goods-seckill", "10");
List<Future> futureList = new ArrayList<>();
for (int i = 0; i < 100; i++) {
futureList.add(executors.submit(this::seckill));
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// 等待结果，防止主线程退出
futureList.forEach(action -> {
try {
action.get();
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
});
}
public int seckill() {
String key = "goods";
try {
redisLock.lock(key);
int num = Integer.valueOf(Objects.requireNonNull(redisTemplate.opsForValue().get("goods-seckill")));
if (num > 0) {
redisTemplate.opsForValue().set("goods-seckill", String.valueOf(--num));
logger.info("秒杀成功，剩余库存：{}", num);
} else {
logger.error("秒杀失败，剩余库存：{}", num);
}
return num;
} catch (Throwable e) {
logger.error("seckill exception", e);
} finally {
redisLock.unlock(key);
}
return 0;
}
}总结
本文是 Redis 锁的一种简单的实现方式，基于 jedis 实现了锁的重试操作。
但是缺点还是有的，不支持锁的自动续期，锁的重入，以及公平性（目前通过自旋的方式实现，相当于是非公平的方式）。
“怎么理解Redis中的分布式锁”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站，小编将为大家输出更多高质量的实用文章！
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