原理

通过线程安全findAndModify 实现锁

实现

定义锁存储对象：

/**
 * mongodb 分布式锁
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
@Document(collection = "distributed-lock-doc")
public class LockDocument {
    @Id
    private String id;
    private long expireAt;
    private String token;
}

定义Lock API：

public interface LockService {

    String acquire(String key, long expiration);

    boolean release(String key, String token);

    boolean refresh(String key, String token, long expiration);
}

获取锁：

 	@Override
    public String acquire(String key, long expiration) {
        Query query = Query.query(Criteria.where("_id").is(key));
        String token = this.generateToken();
        Update update = new Update()
            .setOnInsert("_id", key)
            .setOnInsert("expireAt", System.currentTimeMillis() + expiration)
            .setOnInsert("token", token);

        FindAndModifyOptions options = new FindAndModifyOptions().upsert(true)
                                                                 .returnNew(true);
        LockDocument doc = mongoTemplate.findAndModify(query, update, options,
                                                       LockDocument.class);
        boolean locked = doc.getToken() != null && doc.getToken().equals(token);

        // 如果已过期
        if (!locked && doc.getExpireAt() < System.currentTimeMillis()) {
            DeleteResult deleted = this.mongoTemplate.remove(
                Query.query(Criteria.where("_id").is(key)
                                    .and("token").is(doc.getToken())
                                    .and("expireAt").is(doc.getExpireAt())),
                LockDocument.class);
            if (deleted.getDeletedCount() >= 1) {
                // 成功释放锁， 再次尝试获取锁
                return this.acquire(key, expiration);
            }
        }

        log.debug("Tried to acquire lock for key {} with token {} . Locked: {}",
                  key, token, locked);
        return locked ? token : null;
    }

原理：

• 先尝试upsert锁对象，如果成功且token一致，说明拿到锁
• 否则加锁失败
• 如果未拿到锁，但是锁已过期，尝试删除锁
  • 如果删除成功，再次尝试拿锁
  • 如果失败，说明锁可能已经续期了

释放和续期锁：

   @Override
   public boolean release(String key, String token) {
       Query query = Query.query(Criteria.where("_id").is(key)
                                         .and("token").is(token));
       DeleteResult deleted = mongoTemplate.remove(query, LockDocument.class);
       boolean released = deleted.getDeletedCount() == 1;
       if (released) {
           log.debug("Remove query successfully affected 1 record for key {} with token {}",
                     key, token);
       } else if (deleted.getDeletedCount() > 0) {
           log.error("Unexpected result from release for key {} with token {}, released {}",
                     key, token, deleted);
       } else {
           log.error("Remove query did not affect any records for key {} with token {}",
                     key, token);
       }

       return released;
   }

   @Override
   public boolean refresh(String key, String token,
                          long expiration) {
       Query query = Query.query(Criteria.where("_id").is(key)
                                         .and("token").is(token));
       Update update = Update.update("expireAt",
                                     System.currentTimeMillis() + expiration);
       UpdateResult updated =
           mongoTemplate.updateFirst(query, update, LockDocument.class);

       final boolean refreshed = updated.getModifiedCount() == 1;
       if (refreshed) {
           log.debug("Refresh query successfully affected 1 record for key {} " +
                     "with token {}", key, token);
       } else if (updated.getModifiedCount() > 0) {
           log.error("Unexpected result from refresh for key {} with token {}, " +
                     "released {}", key, token, updated);
       } else {
           log.warn("Refresh query did not affect any records for key {} with token {}. " +
                    "This is possible when refresh interval fires for the final time " +
                    "after the lock has been released",
                    key, token);
       }

       return refreshed;
   }

使用

private  LockService lockService;

private void tryAcquireLockAndSchedule() {
        while (!this.stopSchedule) {
            // 尝试拿锁
            this.token = this.lockService.acquire(SCHEDULER_LOCK, 20000);
            if (this.token != null) {
				// 拿到锁
            } else {
                // 等待LOCK_EXPIRATION, 再次尝试
                Thread.sleep(LOCK_EXPIRATION);
            }
        }
    }

• 先尝试拿锁，如果获取到token，说明拿锁成功
• 否则可以sleep一段时间后再拿锁

完整代码，可到github查看 https://github.com/jadepeng/docker-pipeline/blob/main/pipeline-master/src/main/java/com/github/jadepeng/pipeline/service/impl/MongoLockService.java

感谢您的认真阅读。

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