基于ZK實現分布式锁

引言

ZooKeeper是一个分布式的，开放源码的分布式应用程序协调服务，是Google的Chubby一个开源的实现，是Hadoop和Hbase的重要组件。它是一个为分布式应用提供一致性服务的软件，提供的功能包括：配置维护、域名服务、分布式同步、组服务等。

ZooKeeper的架构通过冗余服务实现高可用性。因此，如果第一次无应答，客户端就可以询问另一台ZooKeeper主机。ZooKeeper节点将它们的数据存储于一个分层的命名空间，非常类似于一个文件系统或一个前缀树结构。客户端可以在节点读写，从而以这种方式拥有一个共享的配置服务。更新是全序的。

基于ZooKeeper分布式锁的原理

1. 在zookeeper指定节点（locks）下创建临时顺序节点node_n
2. 获取locks下所有子节点children
3. 对子节点按节点自增序号从小到大排序
4. 判断本节点是不是第一个子节点，若是，则获取锁；若不是，则监听比该节点小的那个节点的删除事件
5. 若监听事件生效，则回到第二步重新进行判断，直到获取到锁

具体实现

下面就具体使用java和zookeeper实现分布式锁，操作zookeeper使用的是apache提供的zookeeper的包。

• 通过实现Watch接口，实现process(WatchedEvent event)方法来实施监控，使CountDownLatch来完成监控，在等待锁的时候使用CountDownLatch来计数，等到后进行countDown，停止等待，继续运行。
• 以下整体流程基本与上述描述流程一致，只是在监听的时候使用的是CountDownLatch来监听前一个节点。

package org.java.base.zookeeper;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

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;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;
import org.codehaus.jettison.json.JSONString;

import com.fasterxml.jackson.databind.util.JSONPObject;
import com.google.gson.JsonObject;


public class DistributedLock implements Lock, Watcher {
 private ZooKeeper zk = null;
 // 根节点
 private String ROOT_LOCK = "/locks";
 // 竞争的资源
 private String lockName;
 // 等待的前一个锁
 private String WAIT_LOCK;
 // 当前锁
 private String CURRENT_LOCK;
 // 计数器
 private CountDownLatch countDownLatch;
 private int sessionTimeout = 30000;
 private List<Exception> exceptionList = new ArrayList<Exception>();

 /**
 * 配置分布式锁
 * @param config 连接的url
 * @param lockName 竞争资源
 */
 public DistributedLock(String config, String lockName) {
 this.lockName = lockName;
 try {
 // 连接zookeeper
 zk = new ZooKeeper(config, sessionTimeout, this);
 Stat stat = zk.exists(ROOT_LOCK, false);
 if (stat == null) {
 // 如果根节点不存在，则创建根节点
 zk.create(ROOT_LOCK, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
 }
 } catch (IOException e) {
 e.printStackTrace();
 } catch (InterruptedException e) {
 e.printStackTrace();
 } catch (KeeperException e) {
 e.printStackTrace();
 }
 }

 // 节点监视器
 public void process(WatchedEvent event) {
 if (this.countDownLatch != null) {
 this.countDownLatch.countDown();
 }
 }

 public void lock() {
 if (exceptionList.size() > 0) {
 throw new LockException(exceptionList.get(0));
 }
 try {
 if (this.tryLock()) {
 System.out.println(Thread.currentThread().getName() + " " + lockName + "获得了锁");
 return;
 } else {
 // 等待锁
 waitForLock(WAIT_LOCK, sessionTimeout);
 }
 } catch (InterruptedException e) {
 e.printStackTrace();
 } catch (KeeperException e) {
 e.printStackTrace();
 }
 }

 public boolean tryLock() {
 try {
 String splitStr = "_lock_";
 if (lockName.contains(splitStr)) {
 throw new LockException("锁名有误");
 }
 // 创建临时有序节点
 CURRENT_LOCK = zk.create(ROOT_LOCK + "/" + lockName + splitStr, new byte[0],
 ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
 System.out.println(CURRENT_LOCK + " 已经创建");
 // 取所有子节点
 List<String> subNodes = zk.getChildren(ROOT_LOCK, false);
 // 取出所有lockName的锁
 List<String> lockObjects = new ArrayList<String>();
 for (String node : subNodes) {
 String _node = node.split(splitStr)[0];
 if (_node.equals(lockName)) {
 lockObjects.add(node);
 }
 }
 Collections.sort(lockObjects);
 System.out.println(Thread.currentThread().getName() + " 的锁是 " + CURRENT_LOCK);
 // 若当前节点为最小节点，则获取锁成功
 if (CURRENT_LOCK.equals(ROOT_LOCK + "/" + lockObjects.get(0))) {
 return true;
 }

 // 若不是最小节点，则找到自己的前一个节点
 String prevNode = CURRENT_LOCK.substring(CURRENT_LOCK.lastIndexOf("/") + 1);
 WAIT_LOCK = lockObjects.get(Collections.binarySearch(lockObjects, prevNode) - 1);
 } catch (InterruptedException e) {
 e.printStackTrace();
 } catch (KeeperException e) {
 e.printStackTrace();
 }
 return false;
 }

 public boolean tryLock(long timeout, TimeUnit unit) {
 try {
 if (this.tryLock()) {
 return true;
 }
 return waitForLock(WAIT_LOCK, timeout);
 } catch (Exception e) {
 e.printStackTrace();
 }
 return false;
 }

 // 等待锁
 private boolean waitForLock(String prev, long waitTime) throws KeeperException, InterruptedException {
 Stat stat = zk.exists(ROOT_LOCK + "/" + prev, true);

 if (stat != null) {
 System.out.println(Thread.currentThread().getName() + "等待锁 " + ROOT_LOCK + "/" + prev);
 this.countDownLatch = new CountDownLatch(1);
 // 计数等待，若等到前一个节点消失，则precess中进行countDown，停止等待，获取锁
 this.countDownLatch.await(waitTime, TimeUnit.MILLISECONDS);
 this.countDownLatch = null;
 System.out.println(Thread.currentThread().getName() + " 等到了锁");
 }
 return true;
 }

 public void unlock() {
 try {
 System.out.println("释放锁 " + CURRENT_LOCK);
 zk.delete(CURRENT_LOCK, -1);
 CURRENT_LOCK = null;
 zk.close();
 } catch (InterruptedException e) {
 e.printStackTrace();
 } catch (KeeperException e) {
 e.printStackTrace();
 }
 }

 public Condition newCondition() {
 return null;
 }

 public void lockInterruptibly() throws InterruptedException {
 this.lock();
 }


 public class LockException extends RuntimeException {
 private static final long serialVersionUID = 1L;
 public LockException(String e){
 super(e);
 }
 public LockException(Exception e){
 super(e);
 }
 }
}

测试类

package org.java.base.zookeeper;
/**
 * 这段代码有一个问题，看大家能否发现？
 * @author Liuhaihua
 *
 */
public class Test {
 static int n = 500;
 
 public static void secskill() {
 System.out.println(--n);
 }

 public static void main(String[] args) {
 
 Runnable runnable = new Runnable() {
 public void run() {
 DistributedLock lock = null;
 try {
 lock = new DistributedLock("10.152.17.73:2181", "test1");
 lock.lock();
 secskill();
 System.out.println(Thread.currentThread().getName() + "正在运行");
 } finally {
 if (lock != null) {
 lock.unlock();
 }
 }
 }
 };

 for (int i = 0; i < 10; i++) {
 Thread t = new Thread(runnable);
 t.start();
 }
 }
}