一:分布式互斥锁
??? 分布式锁主要用来在分布式系统中同步共享资源的访问,在zookeeper中,并没有像JAVA里一样有Synchronized或者是ReentrantLock机制来实现锁机制,但是在zookeeper中,实现起来更简单：
? 我们可以讲将zk的一个数据节点代表一个锁,当多个客户端同时调用create()节点创建节点的时候,zookeeper会保证只会有一个客户端创建成功,那么我们就可以让这个创建成功的客户端让其持有锁,而其它的客户端则注册Watcher监听
? 当持有锁的客户端释放锁后,监听的客户端就会收到Watcher通知,然后再去试图获取锁,这样反复即可。
二：大概流程
三 :代码示例
package com.travelsky.pss.react.zookeeper.lock;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;
/**
* 基于zookeeper实现互斥锁
*
* @author tanjie
*
*/
public class DistributedClient {
private static final int SESSION_TIMEOUT = 5000;
private String hosts = "192.168.8.88:2181,192.168.8.88:2182,192.168.8.88:2183";
private String groupNode = "locks";
private String subNode = "sub";
private ZooKeeper zk;
// 当前client创建的子节点
private volatile String thisPath;
// 当前client等待的子节点
private volatile String waitPath;
private CountDownLatch latch = new CountDownLatch(1);
/**
* 连接zookeeper
*
* @param countDownLatch
*/
public void connectZookeeper(final CountDownLatch countDownLatch)
throws Exception {
zk = new ZooKeeper(hosts, SESSION_TIMEOUT, new Watcher() {
@Override
public void process(WatchedEvent event) {
try {
if (event.getState() == KeeperState.SyncConnected) {
latch.countDown();
}
// 发生了waitPath的删除事件
/**
* 假设某个client在获得锁之前挂掉了, 由于client创建的节点是ephemeral类型的,
* 因此这个节点也会被删除, 从而导致排在这个client之后的client提前获得了锁.
* 此时会存在多个client同时访问共享资源. 如何解决这个问题呢? 可以在接到删除通知的时候,
* 进行一次确认, 确认当前的thisPath是否真的是列表中最小的节点.
*/
if (event.getType() == EventType.NodeDeleted
&& event.getPath().equals(waitPath)) {
// 确认thisPath是否真的是列表中的最小节点
List<String> childrenNodes = zk.getChildren("/"
+ groupNode, false);
String thisNode = thisPath
.substring(("/" + groupNode + "/").length());
// 排序
Collections.sort(childrenNodes);
int index = childrenNodes.indexOf(thisNode);
if (index == 0) {
// 确实是最小节点
doSomething(countDownLatch);
} else {
// 说明waitPath是由于出现异常而挂掉的
// 更新waitPath
waitPath = "/" + groupNode + "/"
+ childrenNodes.get(index - 1);
// 重新注册监听, 并判断此时waitPath是否已删除
if (zk.exists(waitPath, true) == null) {
doSomething(countDownLatch);
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
});
// 等待连接建立
latch.await();
if (null != zk) {
Stat stat = zk.exists("/" + groupNode, false);
if (null == stat) {
zk.create("/" + groupNode, null, Ids.OPEN_ACL_UNSAFE,
CreateMode.PERSISTENT);
}
}
// 创建子节点
thisPath = zk.create("/" + groupNode + "/" + subNode, null,
Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
Thread.sleep(10);
List<String> childrenNodes = zk.getChildren("/" + groupNode, false);
// 列表中只有一个子节点, 那肯定就是thisPath, 说明client获得锁
if (childrenNodes.size() == 1) {
doSomething(countDownLatch);
} else {
String thisNode = thisPath.substring(("/" + groupNode + "/")
.length());
Collections.sort(childrenNodes);
int index = childrenNodes.indexOf(thisNode);
if (index == 0) {
// index == 0, 说明thisNode在列表中最小, 当前client获得锁
doSomething(countDownLatch);
} else {
// 获得排名比thisPath前1位的节点
this.waitPath = "/" + groupNode + "/"
+ childrenNodes.get(index - 1);
// 在waitPath上注册监听器, 当waitPath被删除时, zookeeper会回调监听器的process方法
zk.getData(waitPath, true, new Stat());
}
}
}
private void doSomething(CountDownLatch countDownLatch) throws Exception {
try {
System.out.println("当前线程:" + Thread.currentThread().getName()
+ "获得了锁:" + thisPath);
//模拟获得锁的线程执行相应逻辑
Thread.sleep(2000);
} finally {
System.out.println("当前线程:" + Thread.currentThread().getName()
+ "已经释放了锁,让其它客户端有机会去获取," + thisPath);
// 将thisPath删除, 监听thisPath的client将获得通知,相当于释放锁
zk.delete(this.thisPath, -1);
countDownLatch.countDown();
zk.close();
}
}
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(20);
final CountDownLatch latch = new CountDownLatch(20);
for (int i = 0; i < 20; i++) {
executorService.submit(new Runnable() {
@Override
public void run() {
try {
DistributedClient lock = new DistributedClient();
lock.connectZookeeper(latch);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (null != executorService) {
executorService.shutdown();
}
}
}
}
??? 可能的运行结果如下:
?
当前线程:pool-1-thread-16获得了锁:/locks/sub0000000053 当前线程:pool-1-thread-16已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000053 当前线程:pool-1-thread-20-EventThread获得了锁:/locks/sub0000000054 当前线程:pool-1-thread-20-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000054 当前线程:pool-1-thread-5-EventThread获得了锁:/locks/sub0000000055 当前线程:pool-1-thread-5-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000055 当前线程:pool-1-thread-2-EventThread获得了锁:/locks/sub0000000056 当前线程:pool-1-thread-2-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000056 当前线程:pool-1-thread-6-EventThread获得了锁:/locks/sub0000000057 当前线程:pool-1-thread-6-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000057 当前线程:pool-1-thread-10-EventThread获得了锁:/locks/sub0000000058 当前线程:pool-1-thread-10-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000058 当前线程:pool-1-thread-3-EventThread获得了锁:/locks/sub0000000059 当前线程:pool-1-thread-3-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000059 当前线程:pool-1-thread-11-EventThread获得了锁:/locks/sub0000000060 当前线程:pool-1-thread-11-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000060 当前线程:pool-1-thread-7-EventThread获得了锁:/locks/sub0000000061 当前线程:pool-1-thread-7-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000061 当前线程:pool-1-thread-13-EventThread获得了锁:/locks/sub0000000062 当前线程:pool-1-thread-13-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000062 当前线程:pool-1-thread-15-EventThread获得了锁:/locks/sub0000000063 当前线程:pool-1-thread-15-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000063 当前线程:pool-1-thread-1-EventThread获得了锁:/locks/sub0000000064 当前线程:pool-1-thread-1-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000064 当前线程:pool-1-thread-18-EventThread获得了锁:/locks/sub0000000065 当前线程:pool-1-thread-18-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000065 当前线程:pool-1-thread-4-EventThread获得了锁:/locks/sub0000000066 当前线程:pool-1-thread-4-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000066 当前线程:pool-1-thread-19-EventThread获得了锁:/locks/sub0000000067 当前线程:pool-1-thread-19-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000067 当前线程:pool-1-thread-14-EventThread获得了锁:/locks/sub0000000068 当前线程:pool-1-thread-14-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000068 当前线程:pool-1-thread-9-EventThread获得了锁:/locks/sub0000000069 当前线程:pool-1-thread-9-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000069 当前线程:pool-1-thread-8-EventThread获得了锁:/locks/sub0000000070 当前线程:pool-1-thread-8-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000070 当前线程:pool-1-thread-12-EventThread获得了锁:/locks/sub0000000071 当前线程:pool-1-thread-12-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000071 当前线程:pool-1-thread-17-EventThread获得了锁:/locks/sub0000000072 当前线程:pool-1-thread-17-EventThread已经释放了锁,让其它客户端有机会去获取,/locks/sub0000000072
? 可以看到，节点的获取实现了分布式互斥。
原文：http://090508tanjie.iteye.com/blog/2291865