ZooKeeper是一个分布式的，开放源码的分布式应用程序协调服务，提供的功能包括配置维护、名字服务、分布式同步、组服务等。
ZooKeeper会维护一个树形的数据结构，类似于Windows资源管理器目录，其中EPHEMERAL类型的节点会随着创建它的客户端断开而被删除，利用这个特性很容易实现软负载均衡。

基本原理是，每个应用的Server启动时创建一个EPHEMERAL节点，应用客户端通过读取节点列表获得可用服务器列表，并订阅节点事件，有Server宕机断开时触发事件，客户端监测到后把该Server从可用列表中删除。

来看示例，这里用了BIO模型编写了一个接收/应答的小程序用于演示效果，优点就是简单。为了方便后面的改造，客户端每次发送消息时都会读取服务器列表并从新建立连接。后边会看到只需要几十行代码即可改造为使用ZooKeeper的软负载模式。

Server代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;public class SimpleServer implements Runnable {public static void main(String[] args) throws IOException {int port = 18080;SimpleServer server = new SimpleServer(port);Thread thread = new Thread(server);thread.start();}private int port;public SimpleServer(int port) {this.port = port;}@Overridepublic void run() {ServerSocket server = null;try {server = new ServerSocket(port);System.out.println("Server started");Socket socket = null;while (true) {socket = server.accept();new Thread(new SimpleServerHandler(socket)).start();}} catch(IOException ex) {ex.printStackTrace();} finally {if (server != null) {try {server.close();} catch (IOException e) {}}}}
}class SimpleServerHandler implements Runnable {private Socket socket;public SimpleServerHandler(Socket socket) {this.socket = socket;}@Overridepublic void run() {BufferedReader in = null;PrintWriter out = null;try {in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));out = new PrintWriter(this.socket.getOutputStream(), true);String body = null;while (true) {body = in.readLine();if (body == null)break;System.out.println("Receive : " + body);out.println("Hello, " + body);}} catch (Exception e) {if (in != null) {try {in.close();} catch (IOException e1) {e1.printStackTrace();}}if (out != null) {out.close();}if (this.socket != null) {try {this.socket.close();} catch (IOException e1) {e1.printStackTrace();}this.socket = null;}}}
}

客户端代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;public class SimpleClient {private static List<String> servers = new ArrayList<>();public static void main(String[] args) {initServerList();SimpleClient client = new SimpleClient();BufferedReader console = new BufferedReader(new InputStreamReader(System.in));while (true) {String name;try {name = console.readLine();if("exit".equals(name)) {System.exit(0);}client.send(name);} catch (IOException e) {e.printStackTrace();}}}private static void initServerList() {servers.clear();servers.add("127.0.0.1:18080");}public static String getServer() {return servers.get(0);}public SimpleClient() {}public void send(String name) {String server = SimpleClient.getServer();String[] cfg = server.split(":");Socket socket = null;BufferedReader in = null;PrintWriter out = null;try {socket = new Socket(cfg[0], Integer.parseInt(cfg[1]));in = new BufferedReader(new InputStreamReader(socket.getInputStream()));out = new PrintWriter(socket.getOutputStream(), true);out.println(name);while(true) {String resp = in.readLine();if(resp == null)break;else if(resp.length() > 0) {System.out.println("Receive : " + resp);break;}}} catch (Exception e) {e.printStackTrace();} finally {if (out != null) {out.close();}if (in != null) {try {in.close();} catch (IOException e) {e.printStackTrace();}}if (socket != null) {try {socket.close();} catch (IOException e) {e.printStackTrace();}}}}
}

运行测试，服务器端输出截图：

客户端输出截图：

很好，一切运行正常。

接下来添加ZooKeeper部分。为了演示效果更好，修改一下ZooKeeper的配置文件，以便于服务器断开后能更快的被监测到。主要是减小Session的超时时间

zookeeper/conf/zoo.cfg

tickTime=2000
initLimit=2
syncLimit=5
dataDir=D:\\ZooKeeper\\zookeeper-3.4.8\\data
clientPort=2181
http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_maintenance
minSessionTimeout=2000
maxSessionTimeout=5000

在项目中添加zkclient的maven依赖

		<!-- http://mvnrepository.com/artifact/com.101tec/zkclient --><dependency><groupId>com.101tec</groupId><artifactId>zkclient</artifactId><version>0.8</version></dependency>

Server代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;import org.I0Itec.zkclient.ZkClient;public class SimpleServer implements Runnable {public static void main(String[] args) throws IOException {int port = 18080;SimpleServer server = new SimpleServer(port);Thread thread = new Thread(server);thread.start();}private int port;public SimpleServer(int port) {this.port = port;}private void regServer() {//向ZooKeeper注册当前服务器ZkClient client = new ZkClient("127.0.0.1:2181", 60000, 1000);String path = "/test/server" + port;if(client.exists(path))client.delete(path);client.createEphemeral(path, "127.0.0.1:" + port);}@Overridepublic void run() {ServerSocket server = null;try {server = new ServerSocket(port);regServer();System.out.println("Server started at " + port);Socket socket = null;while (true) {socket = server.accept();new Thread(new SimpleServerHandler(socket)).start();}} catch(IOException ex) {ex.printStackTrace();} finally {if (server != null) {try {server.close();} catch (IOException e) {}}}}
}
//SimpleServerHandler略

客户端代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;import org.I0Itec.zkclient.IZkChildListener;
import org.I0Itec.zkclient.ZkClient;public class SimpleClient {private static List<String> servers = new ArrayList<>();public static void main(String[] args) {initServerList();SimpleClient client = new SimpleClient();BufferedReader console = new BufferedReader(new InputStreamReader(System.in));while (true) {String name;try {name = console.readLine();if("exit".equals(name)) {System.exit(0);}client.send(name);} catch (IOException e) {e.printStackTrace();}}}private static void initServerList() {//启动时从ZooKeeper读取可用服务器String path = "/test";ZkClient zkClient = new ZkClient("127.0.0.1:2181", 60000, 1000);List<String> childs = zkClient.getChildren(path);servers.clear();for(String p : childs) {servers.add(zkClient.readData(path + "/" + p));}//订阅节点变化事件zkClient.subscribeChildChanges("/test", new IZkChildListener() {@Overridepublic void handleChildChange(String parentPath, List<String> currentChilds)throws Exception {System.out.println(String.format("[ZookeeperRegistry] service list change: path=%s, currentChilds=%s", parentPath, currentChilds.toString()));servers.clear();for(String p : currentChilds) {servers.add(zkClient.readData(path + "/" + p));}System.out.println("Servers: " + servers.toString());}});}public static String getServer() {return servers.get(new Random().nextInt(servers.size()));}//其他无变化, 略
}

分别启动Server和Client，然后修改Server的端口号，再启动一个实例，可以看到客户端检测到了这个新服务器的存在

在客户端发送一些消息，可以看到被随机的分发到两个Server上处理

接下来关闭其中一个Server，可以看到客户端几秒钟后监测到这个事件并自动删除了该服务器。

可以看到，基于ZooKeeper实现软负载均衡非常简单，与应用紧密结合，使用灵活。

源码下载