zookeeper下的分布式锁

师傅你而

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;
}
}