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Distributed Lock Library for JVM

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package com.coditory.sherlock;

import java.time.Instant;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static java.util.stream.Collectors.toList;

class InMemoryDistributedLockStorage {
    private static InMemoryDistributedLockStorage INSTANCE = new InMemoryDistributedLockStorage();

    static InMemoryDistributedLockStorage singleton() {
        return INSTANCE;
    }

    private final Map locks = new HashMap<>();

    synchronized public boolean acquire(LockRequest lockRequest, Instant now) {
        dropExpiredLocks(now);
        LockId id = lockRequest.getLockId();
        InMemoryDistributedLock lock = locks.get(id);
        if (lock == null) {
            locks.put(id, InMemoryDistributedLock.fromLockRequest(lockRequest, now));
            return true;
        }
        return false;
    }

    synchronized public boolean acquireOrProlong(LockRequest lockRequest, Instant now) {
        dropExpiredLocks(now);
        LockId id = lockRequest.getLockId();
        InMemoryDistributedLock lock = locks.get(id);
        if (lock == null || lock.isOwnedBy(lockRequest.getOwnerId())) {
            locks.put(id, InMemoryDistributedLock.fromLockRequest(lockRequest, now));
            return true;
        }
        return false;
    }

    synchronized public boolean forceAcquire(LockRequest lockRequest, Instant now) {
        dropExpiredLocks(now);
        LockId id = lockRequest.getLockId();
        locks.put(id, InMemoryDistributedLock.fromLockRequest(lockRequest, now));
        return true;
    }

    synchronized public boolean release(LockId lockId, Instant now, OwnerId ownerId) {
        dropExpiredLocks(now);
        InMemoryDistributedLock lock = locks.get(lockId);
        if (lock != null && lock.isOwnedBy(ownerId)) {
            locks.remove(lockId);
            return true;
        }
        return false;
    }

    synchronized public boolean forceRelease(LockId lockId, Instant now) {
        dropExpiredLocks(now);
        InMemoryDistributedLock lock = locks.get(lockId);
        if (lock != null) {
            locks.remove(lockId);
            return true;
        }
        return false;
    }

    synchronized public boolean forceReleaseAll(Instant now) {
        dropExpiredLocks(now);
        int size = locks.size();
        if (size > 0) {
            locks.clear();
            return true;
        }
        return false;
    }

    private void dropExpiredLocks(Instant now) {
        List expired = locks.values().stream()
                .filter(lock -> lock.isExpired(now))
                .map(InMemoryDistributedLock::getId)
                .collect(toList());
        expired.forEach(locks::remove);
    }
}