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;
}
}
注意这里的lock，可能会失败，会尝试多次，每次失败后会Sleep一段时间。
PS：官方的代码有个小bug，id和ownerId应该都是全路径，即id = dir + “/” + name;原代码在findPrefixInChildren里有问题。
用于辅助节点大小顺序排序的类：
package org.apache.zookeeper.recipes.lock;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* Represents an ephemeral znode name which has an ordered sequence number and
* can be sorted in order
*
*/
class ZNodeName implements Comparable<ZNodeName> {
private final String name;
private String prefix;
private int sequence = -1;
private static final Logger LOG = LoggerFactory.getLogger(ZNodeName.class);
public ZNodeName(String name) {
if (name == null) {
throw new NullPointerException("id cannot be null");
}
this.name = name;
this.prefix = name;
int idx = name.lastIndexOf('-');
if (idx >= 0) {
this.prefix = name.substring(0, idx);
try {
this.sequence = Integer.parseInt(name.substring(idx + 1));
// If an exception occurred we misdetected a sequence suffix,
// so return -1.
} catch (NumberFormatException e) {
LOG.info("Number format exception for " + idx, e);
} catch (ArrayIndexOutOfBoundsException e) {
LOG.info("Array out of bounds for " + idx, e);
}
}
}
@Override
public String toString() {
return name.toString();
}
@Ove