ZK实现分布式事务锁代码及原理验证

先复习一下ZK实现分布式锁的原理：

每个客户端对某个方法加锁时，在zookeeper上的与该方法对应的指定节点的目录下，生成一个唯一的瞬时有序节点。 判断是否获取锁的方式很简单，只需要判断有序节点中序号最小的一个。 当释放锁的时候，只需将这个瞬时节点删除即可。同时，其可以避免服务宕机导致的锁无法释放，而产生的死锁问题。

通过代码验证是否生成了瞬时的有序节点

package com.jv.zookeeper.curator;

import java.util.concurrent.TimeUnit;

import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;
import org.apache.curator.retry.ExponentialBackoffRetry;

public class TestInterProcessMutex {
	public static void main(String[] args) throws Exception {
		RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
		CuratorFramework client = CuratorFrameworkFactory.newClient("192.168.245.101:2181", retryPolicy);
		client.start();
		InterProcessMutex lock = new InterProcessMutex(client, "/mylock");
		//lock.acquire(1000, TimeUnit.MILLISECONDS) 获取锁，超时时间为1000毫秒
		if ( lock.acquire(1000, TimeUnit.MILLISECONDS) ) 
		{
		    try 
		    {
		       System.out.println("得到锁，并执行");
		       //模拟线程需要执行很长时间，观察ZK中/mylock下的临时ZNODE情况
		       Thread.sleep(10000000);
		    }
		    finally
		    {
		        lock.release();
		        System.out.println("释放锁");
		    }
		}
	}
}

package com.jv.zookeeper.curator;

import java.util.concurrent.TimeUnit;

import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;
import org.apache.curator.retry.ExponentialBackoffRetry;

public class TestInterProcessMutex2 {
	public static void main(String[] args) throws Exception {
		RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
		CuratorFramework client = CuratorFrameworkFactory.newClient("192.168.245.101:2181", retryPolicy);
		client.start();
		InterProcessMutex lock = new InterProcessMutex(client, "/mylock");
		//将超时时间设置足够长，观察ZK中ZNODE的情况，以验证分布式锁的原理是否是使用建立临时顺序ZNODE实现的
		if ( lock.acquire(1000000, TimeUnit.MILLISECONDS) ) 
		{
		    try 
		    {
		       System.out.println("得到锁，并执行");
		       Thread.sleep(10000000);
		    }
		    finally
		    {
		        lock.release();
		        System.out.println("释放锁");
		    }
		}
	}
}

要把代码跑起来，在pom.xml中加入如下依赖

		<dependency>
			<groupId>org.apache.zookeeper</groupId>
			<artifactId>zookeeper</artifactId>
			<version>3.4.6</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.apache.curator/curator-recipes -->
		<dependency>
			<groupId>org.apache.curator</groupId>
			<artifactId>curator-recipes</artifactId>
			<version>4.0.0</version>
		</dependency>

先运行TestInterProcessMutex，在运行TestInterProcessMutex2

使用xshell或者securityCRT登录zookeeper主机

进入到zookeeper的安装目录/bin

./zkCli.sh

ls /mylock

可以看到确实生成了两个瞬时有序节点，并且序号小的客户端获得了锁

curator封装过后使用确实很方便

补充一点，curator还可以很方便的实现选举

LeaderSelectorListener listener = new LeaderSelectorListenerAdapter()
{
    public void takeLeadership(CuratorFramework client) throws Exception
    {
        // 这是你变成leader时执行的方法，你可以在这里执行leader的所有操作
        // 如果你想放弃leader,你必须退出此方法
    }
}

LeaderSelector selector = new LeaderSelector(client, path, listener);
selector.autoRequeue();  // not required, but this is behavior that you will probably expect
selector.start();