今天就跟大家聊聊有关java中怎么利用mongodb实现分布式锁，可能很多人都不太了解，为了让大家更加了解，小编给大家总结了以下内容，希望大家根据这篇文章可以有所收获。
原理
通过线程安全findAndModify 实现锁
实现
定义锁存储对象：
@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
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