zookeeper是hadoop下面的一个子项目, 用来进行分布式系统之间的相互协调。
在zookeeper源码包的recipe目录下有一个互斥锁lock的实现范例,笔者对其简要包装,以便看起来更为明了:
package org.apache.zookeeper.recipes.lock; import java.io.IOException; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooKeeper; public class DistributedLock { private WriteLock lock; private String lockPath = "/lock"; private ZooKeeper zooKeeper ; public DistributedLock(ZooKeeper zooKeeper){ this.zooKeeper = zooKeeper; } /** * 获得锁 * * Author: chenkangxian * * Last Modification Time: 2012-4-6 * * @return 获得锁是否成功 */ public boolean lock(){ lock = new WriteLock(zooKeeper, lockPath, null); try { while (true) { if (lock.lock()) { return true; } } } catch (KeeperException e) { e.printStackTrace(); return false; } catch (InterruptedException e) { e.printStackTrace(); return false; } } /** * 解锁 * * Author: chenkangxian * * Last Modification Time: 2012-4-6 * */ public void unlock(){ lock.unlock(); } public static void main(String args[]){ try { Watcher wh=new Watcher(){ public void process(org.apache.zookeeper.WatchedEvent event) { System.out.println(event.toString()); } }; ZooKeeper zooKeeper = new ZooKeeper("localhost:2181", 20000, wh); final DistributedLock distributedLock = new DistributedLock(zooKeeper); for(int i = 0; i < 100 ; i ++){ Thread thread = new Thread(new Runnable(){ @Override public void run() { if(distributedLock.lock()){ System.out.println("获得锁---------------"); } distributedLock.unlock(); } }); } Thread.sleep(2000*1000); } catch (IOException e) { e.printStackTrace(); } catch (InterruptedException e) { e.printStackTrace(); } } }
WirteLock实现:
package org.apache.zookeeper.recipes.lock; import org.apache.log4j.Logger; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import static org.apache.zookeeper.CreateMode.EPHEMERAL_SEQUENTIAL; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.data.ACL; import org.apache.zookeeper.data.Stat; import java.util.List; import java.util.SortedSet; import java.util.TreeSet; public class WriteLock extends ProtocolSupport { private static final Logger LOG = Logger.getLogger(WriteLock.class); private final String dir; private String id; private ZNodeName idName; private String ownerId; private String lastChildId; private byte[] data = {0x12, 0x34}; private LockListener callback; private LockZooKeeperOperation zop; /** * zookeeper contructor for writelock * @param zookeeper zookeeper client instance * @param dir the parent path you want to use for locking * @param acls the acls that you want to use for all the paths, * if null world read/write is used. */ public WriteLock(ZooKeeper zookeeper, String dir, List<ACL> acl) { super(zookeeper); this.dir = dir; if (acl != null) { setAcl(acl); } this.zop = new LockZooKeeperOperation(); } /** * zookeeper contructor for writelock with callback * @param zookeeper the zookeeper client instance * @param dir the parent path you want to use for locking * @param acl the acls that you want to use for all the paths * @param callback the call back instance */ public WriteLock(ZooKeeper zookeeper, String dir, List<ACL> acl, LockListener callback) { this(zookeeper, dir, acl); this.callback = callback; } /** * return the current locklistener * @return the locklistener */ public LockListener getLockListener() { return this.callback; } /** * register a different call back listener * @param callback the call back instance */ public void setLockListener(LockListener callback) { this.callback = callback; } /** * Removes the lock or associated znode if * you no longer require the lock. this also * removes your request in the queue for locking * in case you do not already hold the lock. * @throws RuntimeException throws a runtime exception * if it cannot connect to zookeeper. */ public synchronized void unlock() throws RuntimeException { if (!isClosed() && id != null) { // we don't need to retry this operation in the case of failure // as ZK will remove ephemeral files and we don't wanna hang // this process when closing if we cannot reconnect to ZK try { ZooKeeperOperation zopdel = new ZooKeeperOperation() { public boolean execute() throws KeeperException, InterruptedException { zookeeper.delete(id, -1); return Boolean.TRUE; } }; zopdel.execute(); } 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; } } } /** * the watcher called on * getting watch while watching * my predecessor */ private class LockWatcher implements Watcher { public void process(WatchedEvent event) { // lets either become the leader or watch the new/updated node LOG.debug("Watcher fired on path: " + event.getPath() + " state: " + event.getState() + " type " + event.getType()); try { lock(); } catch (Exception e) { LOG.warn("Failed to acquire lock: " + e, e); } } } /** * a zoookeeper operation that is mainly responsible * for all the magic required for locking. */ private class LockZooKeeperOperation implements ZooKeeperOperation { /** find if we have been created earler if not create our node * * @param prefix the prefix node * @param zookeeper teh zookeeper client * @param dir the dir paretn * @throws KeeperException * @throws InterruptedException */ private void findPrefixInChildren(String prefix, ZooKeeper zookeeper, String dir) throws KeeperException, InterruptedException { List<String> names = zookeeper.getChildren(dir, false); for (String name : names) { if (name.startsWith(prefix)) { id = name; if (LOG.isDebugEnabled()) { LOG.debug("Found id created last time: " + id); } break; } } if (id == null) { id = zookeeper.create(dir + "/" + prefix, data, getAcl(), EPHEMERAL_SEQUENTIAL); if (LOG.isDebugEnabled()) { LOG.debug("Created id: " + id); } } } /** * the command that is run and retried for actually * obtaining the lock * @return if the command was successful or not */ public boolean execute() throws KeeperException, InterruptedException { do { if (id == null) { long sessionId = zookeeper.getSessionId(); String prefix = "x-" + sessionId + "-"; // lets try look up the current ID if we failed // in the middle of creating the znode findPrefixInChildren(prefix, zookeeper, dir); idName = new ZNodeName(id); } if (id != null) { List<String> names = zookeeper.getChildren(dir, false); if (names.isEmpty()) { LOG.warn("No children in: " + dir + " when we've just " + "created one! Lets recreate it..."); // lets force the recreation of the id id = null; } else { // lets sort them explicitly (though they do seem to come back in order ususally :) SortedSet<ZNodeName> sortedNames = new TreeSet<ZNodeName>(); for (String name : names) { sortedNames.add(new ZNodeName(dir + "/" + name)); } ownerId = sortedNames.first().getName(); SortedSet<ZNodeName> lessThanMe = sortedNames.headSet(idName); if (!lessThanMe.isEmpty()) { ZNodeName lastChildName = lessThanMe.last(); lastChildId = lastChildName.getName(); if (LOG.isDebugEnabled()) { LOG.debug("watching less than me node: " + lastChildId); } Stat stat = zookeeper.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; } }; /** * Attempts to acquire the exclusive write lock returning whether or not it was * acquired. Note that the exclusive lock may be acquired some time later after * this method has been invoked due to the current lock owner going away. */ public synchronized boolean lock() throws KeeperException, InterruptedException { if (isClosed()) { return false; } ensurePathExists(dir); return (Boolean) retryOperation(zop); } /** * return the parent dir for lock * @return the parent dir used for locks. */ public String getDir() { return dir; } /** * Returns true if this node is the owner of the * lock (or the leader) */ public boolean isOwner() { return id != null && ownerId != null && id.equals(ownerId); } /** * return the id for this lock * @return the id for this lock */ public String getId() { return this.id; } }