基于ZooKeeper实现分布式锁

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分布式锁的流程

• 在zookeeper指定节点（locks）下创建临时顺序节点node_n
• 获取locks下所有子节点children
• 对子节点按节点自增序号从小到大排序
• 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
• 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。

/*** @author chenhongshan* @date 2020/9/4*/
import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;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;/*** Created by liuyang on 2017/4/20.*/
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 exceptiOnList= new ArrayList();/*** 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zookeeperzk = 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 subNodes = zk.getChildren(ROOT_LOCK, false);// 取出所有lockName的锁List lockObjects = new ArrayList();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);}}
}

测试代码：

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("127.0.0.1:2181", "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();}}
}

运行结果：