一。为何使用分布式锁?
当应用服务器数量超过1台，对相同数据的访问可能造成访问冲突（特别是写冲突）。单纯使用关系数据库比如MYSQL的应用可以借助于事务来实现锁，也可以使用版本号等实现乐观锁，最大的缺陷就是可用性降低（性能差）。对于GLEASY这种满足大规模并发访问请求的应用来说，使用数据库事务来实现数据库就有些捉襟见肘了。另外对于一些不依赖数据库的应用，比如分布式文件系统，为了保证同一文件在大量读写操作情况下的正确性，必须引入分布式锁来约束对同一文件的并发操作。二。对分布式锁的要求
1.高性能（分布式锁不能成为系统的性能瓶颈） 2.避免死锁（拿到锁的结点挂掉不会导致其它结点永远无法继续） 3.支持锁重入三。方案1，基于zookeeper的分布式锁
/*** DistributedLockUtil.java* 分布式锁工厂类，所有分布式请求都由该工厂类负责**/public class DistributedLockUtil { private static Object schemeLock = new Object(); private static Object mutexLock = new Object(); private static Map mutexLockMap = new ConcurrentHashMap(); private String schema; private Map cache = new ConcurrentHashMap (); private static Map instances = new ConcurrentHashMap(); public static DistributedLockUtil getInstance(String schema){ DistributedLockUtil u = instances.get(schema); if(u==null){ synchronized(schemeLock){ u = instances.get(schema); if(u == null){ u = new DistributedLockUtil(schema); instances.put(schema, u); } } } return u; } private DistributedLockUtil(String schema){ this.schema = schema; } private Object getMutex(String key){ Object mx = mutexLockMap.get(key); if(mx == null){ synchronized(mutexLock){ mx = mutexLockMap.get(key); if(mx==null){ mx = new Object(); mutexLockMap.put(key,mx); } } } return mx; } private DistributedReentrantLock getLock(String key){ DistributedReentrantLock lock = cache.get(key); if(lock == null){ synchronized(getMutex(key)){ lock = cache.get(key); if(lock == null){ lock = new DistributedReentrantLock(key,schema); cache.put(key, lock); } } } return lock; } public void reset(){ for(String s : cache.keySet()){ getLock(s).unlock(); } } /** * 尝试加锁 * 如果当前线程已经拥有该锁的话,直接返回false,表示不用再次加锁,此时不应该再调用unlock进行解锁 * * @param key * @return * @throws InterruptedException * @throws KeeperException */ public LockStat lock(String key) throws InterruptedException, KeeperException{ if(getLock(key).isOwner()){ return LockStat.NONEED; } getLock(key).lock(); return LockStat.SUCCESS; } public void clearLock(String key) throws InterruptedException, KeeperException{ synchronized(getMutex(key)){ DistributedReentrantLock l = cache.get(key); l.clear(); cache.remove(key); } } public void unlock(String key,LockStat stat) throws InterruptedException, KeeperException{ unlock(key,stat,false); } public void unlock(String key,LockStat stat,boolean keepalive) throws InterruptedException, KeeperException{ if(stat == null) return; if(LockStat.SUCCESS.equals(stat)){ DistributedReentrantLock lock = getLock(key); boolean hasWaiter = lock.unlock(); if(!hasWaiter && !keepalive){ synchronized(getMutex(key)){ lock.clear(); cache.remove(key); } } } } public static enum LockStat{ NONEED, SUCCESS }}
/***DistributedReentrantLock.java*本地线程之间锁争用，先使用虚拟机内部锁机制，减少结点间通信开销*/public class DistributedReentrantLock { private static final Logger logger = Logger.getLogger(DistributedReentrantLock.class); private ReentrantLock reentrantLock = new ReentrantLock(); private WriteLock writeLock; private long timeout = 3*60*1000; private final Object mutex = new Object(); private String dir; private String schema; private final ExitListener exitListener = new ExitListener(){ @Override public void execute() { initWriteLock(); } }; private synchronized void initWriteLock(){ logger.debug("初始化writeLock"); writeLock = new WriteLock(dir,new LockListener(){ @Override public void lockAcquired() { synchronized(mutex){ mutex.notify(); } } @Override public void lockReleased() { } },schema); if(writeLock != null && writeLock.zk != null){ writeLock.zk.addExitListener(exitListener); } synchronized(mutex){ mutex.notify(); } } public DistributedReentrantLock(String dir,String schema) { this.dir = dir; this.schema = schema; initWriteLock(); } public void lock(long timeout) throws InterruptedException, KeeperException { reentrantLock.lock();//多线程竞争时，先拿到第一层锁 try{ boolean res = writeLock.trylock(); if(!res){ synchronized(mutex){ mutex.wait(timeout); } if(writeLock == null || !writeLock.isOwner()){ throw new InterruptedException("锁超时"); } } }catch(InterruptedException e){ reentrantLock.unlock(); throw e; }catch(KeeperException e){ reentrantLock.unlock(); throw e; } } public void lock() throws InterruptedException, KeeperException { lock(timeout); } public void destroy() throws KeeperException { writeLock.unlock(); } public boolean unlock(){ if(!isOwner()) return false; try{ writeLock.unlock(); reentrantLock.unlock();//多线程竞争时，释放最外层锁 }catch(RuntimeException e){ reentrantLock.unlock();//多线程竞争时，释放最外层锁 throw e; } return reentrantLock.hasQueuedThreads(); } public boolean isOwner() { return reentrantLock.isHeldByCurrentThread() && writeLock.isOwner(); } public void clear() { writeLock.clear(); }}
/***WriteLock.java*基于zk的锁实现*一个最简单的场景如下：*1.结点A请求加锁，在特定路径下注册自己（会话自增结点)，得到一个ID号1*2.结点B请求加锁，在特定路径下注册自己（会话自增结点)，得到一个ID号2*3.结点A获取所有结点ID，判断出来自己是最小结点号，于是获得锁*4.结点B获取所有结点ID，判断出来自己不是最小结点，于是监听小于自己的最大结点（结点A）变更事件*5.结点A拿到锁，处理业务，处理完，释放锁（删除自己）*6.结点B收到结点A变更事件，判断出来自己已经是最小结点号，于是获得锁。*/public class WriteLock extends ZkPrimative { private static final Logger LOG = Logger.getLogger(WriteLock.class); private final String dir; private String id; private LockNode idName; private String ownerId; private String lastChildId; private byte[] data = {0x12, 0x34}; private LockListener callback; public WriteLock(String dir,String schema) { super(schema,true); this.dir = dir; } public WriteLock(String dir,LockListener callback,String schema) { this(dir,schema); this.callback = callback; } public LockListener getLockListener() { return this.callback; } public void setLockListener(LockListener callback) { this.callback = callback; } public synchronized void unlock() throws RuntimeException { if(zk == null || zk.isClosed()){ return; } if (id != null) { try { zk.delete(id, -1); } catch (InterruptedException e) { LOG.warn("Caught: " + e, e); //set that we have been interrupted. Thread.currentThread().interrupt(); } catch (KeeperException.NoNodeException e) { // do nothing } catch (KeeperException e) { LOG.warn("Caught: " + e, e); throw (RuntimeException) new RuntimeException(e.getMessage()). initCause(e); }finally { if (callback != null) { callback.lockReleased(); } id = null; } } } private class LockWatcher implements Watcher { public void process(WatchedEvent event) { LOG.debug("Watcher fired on path: " + event.getPath() + " state: " + event.getState() + " type " + event.getType()); try { trylock(); } catch (Exception e) { LOG.warn("Failed to acquire lock: " + e, e); } } } private void findPrefixInChildren(String prefix, ZooKeeper zookeeper, String dir) throws KeeperException, InterruptedException { List names = zookeeper.getChildren(dir, false); for (String name : names) { if (name.startsWith(prefix)) { id = dir + "/" + name; if (LOG.isDebugEnabled()) { LOG.debug("Found id created last time: " + id); } break; } } if (id == null) { id = zookeeper.create(dir + "/" + prefix, data, acl, EPHEMERAL_SEQUENTIAL); if (LOG.isDebugEnabled()) { LOG.debug("Created id: " + id); } } } public void clear() { if(zk == null || zk.isClosed()){ return; } try { zk.delete(dir, -1); } catch (Exception e) { LOG.error("clear error: " + e,e); } } public synchronized boolean trylock() throws KeeperException, InterruptedException { if(zk == null){ LOG.info("zk 是空"); return false; } if (zk.isClosed()) { LOG.info("zk 已经关闭"); return false; } ensurePathExists(dir); LOG.debug("id:"+id); do { if (id == null) { long sessionId = zk.getSessionId(); String prefix = "x-" + sessionId + "-"; idName = new LockNode(id); LOG.debug("idName:"+idName); } if (id != null) { List names = zk.getChildren(dir, false); if (names.isEmpty()) { LOG.warn("No children in: " + dir + " when we've just " + "created one! Lets recreate it..."); id = null; } else { SortedSet sortedNames = new TreeSet (); for (String name : names) { sortedNames.add(new LockNode(dir + "/" + name)); } ownerId = sortedNames.first().getName(); LOG.debug("all:"+sortedNames); SortedSet lessThanMe = sortedNames.headSet(idName); LOG.debug("less than me:"+lessThanMe); if (!lessThanMe.isEmpty()) { LockNode lastChildName = lessThanMe.last(); lastChildId = lastChildName.getName(); if (LOG.isDebugEnabled()) { LOG.debug("watching less than me node: " + lastChildId); } Stat stat = zk.exists(lastChildId, new LockWatcher()); if (stat != null) { return Boolean.FALSE; } else { LOG.warn("Could not find the" + " stats for less than me: " + lastChildName.getName()); } } else { if (isOwner()) { if (callback != null) { callback.lockAcquired(); } return Boolean.TRUE; } } } } } while (id == null); return Boolean.FALSE; } public String getDir() { return dir; } public boolean isOwner() { return id != null && ownerId != null && id.equals(ownerId); } public String getId() { return this.id; }}
使用本方案实现的分布式锁，可以很好地解决锁重入的问题，而且使用会话结点来避免死锁；性能方面，根据笔者自测结果，加锁解锁各一次算是一个操作，本方案实现的分布式锁，TPS大概为2000-3000，性能比较一般；