Apache Curator Leader选举 简单示例

版本依赖：

<dependency>
	<groupId>org.apache.curator</groupId>
	<artifactId>curator-recipes</artifactId>
	<version>2.10.0</version>
</dependency>

示例一：

package com.zk.leader;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.leader.LeaderLatch;
import org.apache.curator.retry.ExponentialBackoffRetry;

/**
 * 必须启动LeaderLatch: leaderLatch.start();
 * 一旦启动， LeaderLatch会和其它使用相同latch path的其它LeaderLatch交涉，然后随机的选择其中一个作为leader。 
 * 你可以随时查看一个给定的实例是否是leader:
 */
public class Example1
{
	private static final String PATH = "/leader";
	private static final int COUNT = 5;

	public static void main(String[] args) throws Exception
	{
		CuratorFramework client = CuratorFrameworkFactory.newClient("192.168.1.100:2181", new ExponentialBackoffRetry(1000, 3));
		client.start();

		List<LeaderLatch> list = new ArrayList<>();

		for (int i = 1; i <= COUNT; i++)
		{
			LeaderLatch example = new LeaderLatch(client, PATH, "Client #" + i);
			example.start();
			list.add(example);
		}

		TimeUnit.SECONDS.sleep(2);

		LeaderLatch leader = null;
		for (LeaderLatch ll : list)
		{
			if (ll.hasLeadership())
			{
				leader = ll;
			}
			System.out.println(ll.getId() + "\t" + ll.hasLeadership());
		}
		;

		TimeUnit.SECONDS.sleep(2);
		
		list.remove(leader);
		leader.close();
		
		TimeUnit.SECONDS.sleep(2);

		System.out.println("========================");
		
		for (LeaderLatch ll : list)
		{
			System.out.println(ll.getId() + "\t" + ll.hasLeadership());
		}

		for (LeaderLatch ll : list)
		{
			ll.close();
		}

		client.close();
	}
}

输出如下：

Client #1	false
Client #2	false
Client #3	true
Client #4	false
Client #5	false
========================
Client #1	false
Client #2	false
Client #4	false
Client #5	true

示例二：

package com.zk.leader;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.leader.LeaderSelector;
import org.apache.curator.framework.recipes.leader.LeaderSelectorListenerAdapter;
import org.apache.curator.retry.ExponentialBackoffRetry;

/**
 * 与LeaderLatch相比， 通过LeaderSelectorListener可以对领导权进行控制， 在适当的时候释放领导权，这样每个节点都有可能获得领导权。 
 * 而LeaderLatch一根筋到死， 除非调用close方法，否则它不会释放领导权。
 */
public class Example2
{
	private static final String PATH = "/leader";
	private static final int COUNT = 5;
	
	public static void main(String[] args) throws Exception
	{
		CuratorFramework client = CuratorFrameworkFactory.newClient("192.168.1.100:2181", new ExponentialBackoffRetry(1000, 3));
		client.start();
		
		List<MyLeaderSelectorListenerAdapter> list = new ArrayList<>();
		
		for (int i = 1; i <= COUNT; i++)
		{
			MyLeaderSelectorListenerAdapter example = new MyLeaderSelectorListenerAdapter(client, PATH, "Client #" + i);
			list.add(example);
		}
		
		TimeUnit.SECONDS.sleep(20);
		
		for (MyLeaderSelectorListenerAdapter listener : list)
		{
			listener.close();
		}
		
		client.close();
	}
}
class MyLeaderSelectorListenerAdapter extends LeaderSelectorListenerAdapter
{
	private final String name;
    private final LeaderSelector leaderSelector;
    
    public MyLeaderSelectorListenerAdapter(CuratorFramework client, String path, String name)
    {
    	this.name = name;
    	leaderSelector = new LeaderSelector(client, path, this);
    	
    	//保证在此实例释放领导权之后还可能获得领导权。
    	leaderSelector.autoRequeue();
    	
    	leaderSelector.start();
    }
    
    public void close()
    {
    	leaderSelector.close();
    }
    
    /**
     * 你可以在takeLeadership进行任务的分配等等，并且不要返回，如果你想要要此实例一直是leader的话可以加一个死循环。
     * 一旦此方法执行完毕之后，就会重新选举
     */
	public void takeLeadership(CuratorFramework client) throws Exception
	{
		System.out.println(name + " 当选为leader");
		TimeUnit.SECONDS.sleep(2);
	}
}

输出如下：

Client #1 当选为leader
Client #4 当选为leader
Client #2 当选为leader
Client #3 当选为leader
Client #5 当选为leader
Client #1 当选为leader
Client #4 当选为leader
Client #2 当选为leader
Client #3 当选为leader
Client #5 当选为leader

以上是使用curator实现leader的两种方法