基于Redis实现分布式锁

一. 基本原理

1. 基于Redis的setnx命令，如果设置成功，则表示当前进程(线程)获取锁成功，否则说明有其他进程已经获取了锁，需进行等待
2. setnx 的key为需要加锁的资源名称(实例中为方法名)，value为业务唯一的id
3. 加锁时需制定锁的过期时间，避免锁未正常释放而导致的死锁问题
4. 加锁时需设置等待超时时间，避免等待时间过长导致系统性能下降
5. 释放锁时，与Redis中的value与当前的业务id进行对比，符合才执行释放操作

二. 代码实现

1. 加锁的核心操作

   /**
    * @Auther: ZhangShenao
    * @Date: 2019/3/8 10:29
    * @Description:分布式锁信息
    */
   @Getter
   @Setter
   public class LockInfo {
       //业务唯一id,作为锁的value
       private String transactionId;
   
       //锁的key
       private String lockKey;
   
       //重试次数
       private int retryTimes;
   
       public static LockInfo valueOf(String transactionId, String lockKey) {
           LockInfo info = new LockInfo();
           info.transactionId = transactionId;
           info.lockKey = lockKey;
           info.retryTimes = 0;
           return info;
       }
   
       public void addRetryTimes(){
           ++retryTimes;
       }
   }
   

   /**
    * @Auther: ZhangShenao
    * @Date: 2019/3/8 10:26
    * @Description:基于Redis实现的分布式锁
    */
   @Component
   public class RedisLock {
       @Autowired
       private StringRedisTemplate redisTemplate;
   
       private ValueOperations<String, String> opsForValue;
   
       private ThreadLocal<String> localTransactionId = new ThreadLocal<>();
   
       @PostConstruct
       private void init() {
           opsForValue = redisTemplate.opsForValue();
       }
   
       public Optional<LockInfo> tryLock(String lockKey, long timeoutMillis, long expireSeconds) throws InterruptedException {
           Assert.isTrue(timeoutMillis > 0L, "timeout must be positive!!");
           Assert.isTrue(expireSeconds > 0L, "expire must be positive!!");
   
           String transactionId = UUID.randomUUID().toString();
           localTransactionId.set(transactionId);
           LockInfo lockInfo = LockInfo.valueOf(transactionId, lockKey);
   
           for (long startTime = System.currentTimeMillis(); System.currentTimeMillis() - startTime < timeoutMillis; lockInfo.addRetryTimes()) {
               Boolean success = opsForValue.setIfAbsent(lockKey, transactionId);
               if (Boolean.TRUE.equals(success)) {
                   redisTemplate.expire(lockKey, expireSeconds, TimeUnit.SECONDS);
                   System.err.println(String.format("Thread fetch lock. transactionId: %s, lockKey: %s, retryTimes: %s", lockInfo.getTransactionId(), lockKey, lockInfo.getRetryTimes()));
                   return Optional.of(lockInfo);
               }
               Thread.sleep(20L);
           }
   
           //timeout
           System.err.println(String.format("Thread fetch lock timeout!! transactionId: %s, lockKey: %s", lockInfo.getTransactionId(), lockKey));
           return Optional.empty();
   
       }
   
       public void release(String lockKey) {
           String transactionId = Optional.ofNullable(opsForValue.get(lockKey)).orElseThrow(() -> new IllegalStateException("Lock Expired!! key: " + lockKey));
           if (!transactionId.equalsIgnoreCase(localTransactionId.get())) {
               throw new IllegalStateException("Thread try to release lock, but lock is not held by this thread!! threadId: " + Thread.currentThread().getId() + ", lockKey: " + lockKey);
           }
           redisTemplate.delete(lockKey);
           System.err.println(String.format("Thread release lock. transactionId: %s, lockKey: %s", transactionId, lockKey));
       }
   }
   

2. 基于Spring，开发自定义注解和切面，简化对方法加锁的操作

   /**
    * @Auther: ZhangShenao
    * @Date: 2019/3/8 14:13
    * @Description:自定义Lock,标记在方法上,用于指定哪些方法需要自动加Redis锁
    */
   @Target(value = {ElementType.METHOD})
   @Retention(value = RetentionPolicy.RUNTIME)
   public @interface Lock {
       String lockKey() default "";
   
       long expireSeconds();
   
       long timeoutSeconds();
   }
   
   

   import org.aspectj.lang.ProceedingJoinPoint;
   import org.aspectj.lang.annotation.Around;
   import org.aspectj.lang.annotation.Aspect;
   import org.aspectj.lang.reflect.MethodSignature;
   import org.springframework.beans.factory.annotation.Autowired;
   import org.springframework.stereotype.Component;
   import org.springframework.util.StringUtils;
   import java.util.Optional;
   
   /**
    * @Auther: ZhangShenao
    * @Date: 2019/3/8 14:05
    * @Description:Redis锁切面,拦截标记了@Lock注解的方法,为其执行自动加锁和释放锁的操作
    */
   @Aspect
   @Component
   public class RedisLockAspect {
       @Autowired
       private RedisLock redisLock;
   
       @Around("@annotation(lock)")
       public Object addLockForMethods(ProceedingJoinPoint point, Lock lock) throws Throwable {
           Optional<LockInfo> lockInfo = Optional.empty();
           String lockKey = generateLockKey(point, lock);
           Object result = null;
           try {
               lockInfo = redisLock.tryLock(lockKey, lock.expireSeconds(), lock.timeoutSeconds());
               result = point.proceed();
               return result;
           } catch (Exception e) {
               e.printStackTrace();
           } finally {
               lockInfo.ifPresent(l -> redisLock.release(lockKey));
           }
           return result;
       }
   
       private String generateLockKey(ProceedingJoinPoint point, Lock lock) {
           if (StringUtils.hasText(lock.lockKey())) {
               return lock.lockKey();
           }
           MethodSignature signature = (MethodSignature) point.getSignature();
           String methodName = signature.getMethod().getName();
           String className = signature.getMethod().getDeclaringClass().getCanonicalName();
           return methodName + "-" + className;
       }
   }
   
   

3. 测试

/**
 * @Auther: ZhangShenao
 * @Date: 2019/3/8 14:28
 * @Description:
 */
@Service
public class DemoService {
    @Lock(expireSeconds = 60L, timeoutSeconds = 5L)
    public String demoTask() {
        return "Do demo task...";
    }
}

/**
 * @Auther: ZhangShenao
 * @Date: 2019/3/8 13:49
 * @Description:
 */
@SpringBootApplication
@Import(RedisConfig.class)
@EnableAspectJAutoProxy
public class RedisLockApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(RedisLockApplication.class, args);
        DemoService demoService = applicationContext.getBean(DemoService.class);

        ExecutorService executorService = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 10; i++) {
            System.err.println(demoService.demoTask());
        }

        executorService.shutdown();
    }
}

结果：

Thread fetch lock. transactionId: 47aa449b-e0b7-407f-b861-52b06668f36e, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 47aa449b-e0b7-407f-b861-52b06668f36e, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: e5ce6b1e-2b02-40a3-aa2b-4ffb00e43f92, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: e5ce6b1e-2b02-40a3-aa2b-4ffb00e43f92, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 9bde0756-614f-4a97-b57e-87b6dcabf763, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 9bde0756-614f-4a97-b57e-87b6dcabf763, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 81154275-05de-401d-8f83-1a68ad0f7c3f, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 81154275-05de-401d-8f83-1a68ad0f7c3f, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 1005e5b7-d1b8-4934-8e31-ceafa2fed891, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 1005e5b7-d1b8-4934-8e31-ceafa2fed891, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 56eb7b87-dd23-41d3-becc-2058b3b113c1, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 56eb7b87-dd23-41d3-becc-2058b3b113c1, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: b4d46d03-341b-486d-aa1d-878ec56c1272, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: b4d46d03-341b-486d-aa1d-878ec56c1272, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 8e48df53-a430-422e-90ee-bf90d5f2d455, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 8e48df53-a430-422e-90ee-bf90d5f2d455, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: 1a6963bc-869c-498c-95a1-279eb2b201e3, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: 1a6963bc-869c-498c-95a1-279eb2b201e3, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...
Thread fetch lock. transactionId: ff78fe9a-9c49-44e2-b5f7-b7dccd3b792c, lockKey: demoTask-william.redis.lock.DemoService, retryTimes: 0
Thread release lock. transactionId: ff78fe9a-9c49-44e2-b5f7-b7dccd3b792c, lockKey: demoTask-william.redis.lock.DemoService
Do demo task...

可以看到，通过加锁，线程以同步方式顺序执行。

三. 待改进

1. 加锁时和setnx操作和expire操作应该保证原子性，可以考虑在程序中执行Lua脚本