Java tutorial
/* * * This is a simple Content Management System (CMS) * Copyright (C) 2011 Imran M Yousuf (imyousuf@smartitengineering.com) * * This program is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program. If not, see <http://www.gnu.org/licenses/>. */ package com.smartitengineering.cms.spi.lock.impl.distributed; import com.smartitengineering.cms.api.factory.write.Lock; import com.smartitengineering.cms.spi.lock.Key; import java.util.concurrent.TimeUnit; import java.util.concurrent.locks.ReentrantLock; import org.apache.commons.lang.StringUtils; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooDefs.Ids; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.data.Stat; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * * @author imyousuf */ public class ZKLock implements Lock, Watcher, LockTimeoutListener { protected final ZKConfig config; protected final Key key; protected String localLockId; protected final ReentrantLock lock = new ReentrantLock(); protected final transient Logger logger = LoggerFactory.getLogger(getClass()); protected ZKLock(ZKConfig config, Key key) { this.config = config; this.key = key; } public boolean isLockOwned() { lock.lock(); try { return StringUtils.isNotBlank(localLockId); } finally { lock.unlock(); } } public void lock() { boolean locked = false; try { locked = tryLock(1, TimeUnit.DAYS); } catch (Exception ex) { logger.warn(ex.getMessage(), ex); } if (!locked) { throw new IllegalStateException("Could not attain lock!"); } } public boolean tryLock() { try { return tryLock(-1, TimeUnit.DAYS); } catch (Exception ex) { logger.warn(ex.getMessage(), ex); return false; } } public boolean tryLock(long time, TimeUnit unit) throws InterruptedException { if (logger.isDebugEnabled()) { logger.debug("Attempting to attain lock with wait option of " + time + " " + unit.name()); } if (isLockOwned()) { return true; } lock.lock(); try { final long waitInMilliSeconds = time > 0 ? TimeUnit.MILLISECONDS.convert(time, unit) : time; final long start = System.currentTimeMillis(); String lockId = config.getRegistrar().lock(key, this, waitInMilliSeconds); final long availableMillisForRemoteLock = waitInMilliSeconds - (System.currentTimeMillis() - start); return tryRemoteLock(lockId, availableMillisForRemoteLock); } finally { lock.unlock(); } } protected boolean tryRemoteLock(String lockId, final long availableMillisForRemoteLock) throws IllegalStateException, InterruptedException { final LocalLockRegistrar registrar = config.getRegistrar(); final ZooKeeper keeper = config.getZooKeeper(); final String node = getNode(); if (logger.isDebugEnabled()) { logger.debug("Attained local lock " + lockId); } try { if (StringUtils.isNotBlank(lockId)) { keeper.create(node, org.apache.commons.codec.binary.StringUtils.getBytesUtf8(config.getNodeId()), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL); keeper.exists(node, this); localLockId = lockId; return true; } else { return false; } } catch (KeeperException ke) { if (ke.code() == KeeperException.Code.NODEEXISTS) { logger.debug("Lock alrady exists!"); if (availableMillisForRemoteLock > 0) { synchronized (ZKLock.this) { try { keeper.exists(node, new Watcher() { public void process(WatchedEvent event) { if (event.getType().equals(Event.EventType.NodeDeleted)) { synchronized (ZKLock.this) { ZKLock.this.notifyAll(); } } } }); } catch (Exception ex) { logger.error("Could not attach watcher", ex); } final long remoteStart = System.currentTimeMillis(); ZKLock.this.wait(availableMillisForRemoteLock); return tryRemoteLock(lockId, availableMillisForRemoteLock - (System.currentTimeMillis() - remoteStart)); } } else { registrar.unlock(key, lockId); return false; } } else { logger.error(ke.getMessage(), ke); throw new IllegalStateException(ke); } } catch (Exception ex) { registrar.unlock(key, lockId); logger.error(ex.getMessage(), ex); throw new IllegalStateException(ex); } } public void unlock() { if (isLockOwned()) { lock.lock(); try { ZooKeeper keeper = config.getZooKeeper(); final String node = getNode(); Stat stat = keeper.exists(node, false); String nodeId = org.apache.commons.codec.binary.StringUtils .newStringUtf8(keeper.getData(node, false, stat)); if (config.getNodeId().equals(nodeId)) { keeper.delete(node, stat.getVersion()); config.getRegistrar().unlock(key, localLockId); } else { logger.error( "Lock is owned but remote lock is not owned! System inconsistency! Releasing local lock."); config.getRegistrar().unlock(key, localLockId); throw new IllegalStateException( "Lock is owned but remote lock is not owned! Released local lock!"); } localLockId = null; } catch (Exception ex) { logger.error(ex.getMessage(), ex); throw new IllegalStateException(ex); } finally { lock.unlock(); } } } public void process(WatchedEvent event) { if (!event.getType().equals(Event.EventType.NodeDeleted)) { logger.warn("Remote lock changed unexpectedly! This may cause system inconsistency"); } } public void lockTimedOut(Key key) { unlock(); } protected String getNode() { return new StringBuilder(config.getRootNode()).append('/').append(key.getKeyStringRep()).toString(); } }