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
感谢您的认真阅读。
如果你觉得有帮助，欢迎点赞支持！
不定期分享软件开发经验，欢迎关注作者, 一起交流软件开发：