实现原理
先进先出队列是最常用的队列，使用Zookeeper实现先进先出队列就是在特定的目录下创建PERSISTENT_EQUENTIAL节点，创建成功时Watcher通知等待的队列，队列删除序列号最小的节点用以消费。此场景下Zookeeper的znode用于消息存储，znode存储的数据就是消息队列中的消息内容，SEQUENTIAL序列号就是消息的编号，按序取出即可。由于创建的节点是持久化的，所以不必担心队列消息的丢失问题。
队列（Queue）
分布式队列是通用的数据结构，为了在 Zookeeper 中实现分布式队列，首先需要指定一个 Znode 节点作为队列节点（queue node）， 各个分布式客户端通过调用 create() 函数向队列中放入数据，调用create()时节点路径名带"qn-"结尾，并设置顺序（sequence）节点标志。 由于设置了节点的顺序标志，新的路径名具有以下字符串模式："_path-to-queue-node_/qn-X"，X 是唯一自增号。需要从队列中获取数据/移除数据的客户端首先调用 getChildren() 函数，有数据则获取（获取数据后可以删除也可以不删），没有则在队列节点（queue node）上将 watch 设置为 true，等待触发并处理最小序号的节点（即从序号最小的节点中取数据）。
应用场景
Zookeeper队列不太适合要求高性能的场合，但可以在数据量不大的情况下考虑使用。比如已在项目中使用Zookeeper又需要小规模的队列应用，这时可以使用Zookeeper实现的队列;毕竟引进一个消息中间件会增加系统的复杂性和运维的压力。
详细代码
ZookeeperClient工具类
package org.massive.common;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* Created by Massive on 2016/12/18.
*/
public class ZooKeeperClient {
private static String connectionString = "localhost:2181";
private static int sessionTimeout = 10000;
public static ZooKeeper getInstance() throws IOException, InterruptedException {
//--------------------------------------------------------------
// 为避免连接还未完成就执行zookeeper的get/create/exists操作引起的（KeeperErrorCode = ConnectionLoss)
// 这里等Zookeeper的连接完成才返回实例
//--------------------------------------------------------------
final CountDownLatch connectedSignal = new CountDownLatch(1);
ZooKeeper zk = new ZooKeeper(connectionString, sessionTimeout, new Watcher() {
@Override
public void process(WatchedEvent event) {
if (event.getState() == Event.KeeperState.SyncConnected) {
connectedSignal.countDown();
} else if (event.getState() == Event.KeeperState.Expired) {
}
}
});
connectedSignal.await(sessionTimeout, TimeUnit.MILLISECONDS);
return zk;
}
public static int getSessionTimeout() {
return sessionTimeout;
}
public static void setSessionTimeout(int sessionTimeout) {
ZooKeeperClient.sessionTimeout = sessionTimeout;
}
}
ZooKeeperQueue
package org.massive.queue;
import org.apache.commons.lang3.RandomUtils;
import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;
import org.massive.common.ZooKeeperClient;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;
/**
* Created by Allen on 2016/12/22.
*/
public class ZooKeeperQueue {
private ZooKeeper zk;
private int sessionTimeout;
private static byte[] ROOT_QUEUE_DATA = {0x12,0x34};
private static String QUEUE_ROOT = "/QUEUE";
private String queueName;
private String queuePath;
private Object mutex = new Object();
public ZooKeeperQueue(String queueName) throws IOException, KeeperException, InterruptedException {
this.queueName = queueName;
this.queuePath = QUEUE_ROOT + "/" + queueName;
this.zk = ZooKeeperClient.getInstance();
this.sessionTimeout = zk.getSessionTimeout();
//----------------------------------------------------
// 确保队列根目录/QUEUE和当前队列的目录的存在
//----------------------------------------------------
ensureExists(QUEUE_ROOT);
ensureExists(queuePath);
}
public byte[] consume() throws InterruptedException, KeeperException, UnsupportedEncodingException {
List<String> nodes = null;
byte[] returnVal = null;
Stat stat = null;
do {
synchronized (mutex) {
nodes = zk.getChildren(queuePath, new ProduceWatcher());
//----------------------------------------------------
// 如果没有消息节点，等待生产者的通知
//----------------------------------------------------
if (nodes == null || nodes.size() == 0) {
mutex.wait();
} else {
SortedSet<String> sortedNode = new TreeSet<String>();
for (String node : nodes) {
sortedNode.add(queuePath + "/" + node);
}
//----------------------------------------------------
// 消费队列里序列号最小的消息
//----------------------------------------------------
String first = sortedNode.first();
returnVal = zk.getData(first, false, stat);
zk.delete(first, -1);
System.out.print(Thread.currentThread().getName() + " ");
System.out.print("consume a message from queue：" + first);
System.out.println(", message data is: " + new String(returnVal,"UTF-8"));
return returnVal;
}
}
} while (true);
}
class ProduceWatcher implements Watcher {
@Override
public void process(WatchedEvent event) {
//----------------------------------------------------
// 生产一条消息成功后通知一个等待线程
//----------------------------------------------------
synchronized (mutex) {
mutex.notify();
}
}
}
public void produce(byte[] data) throws KeeperException, InterruptedException, UnsupportedEncodingException {
//----------------------------------------------------
// 确保当前队列目录存在
// example: /QUEUE/queueName
//----------------------------------------------------
ensureExists(queuePath);
String node = zk.create(queuePath + "/", data,
ZooDefs.Ids.OPEN_ACL_UNSAFE,
CreateMode.PERSISTENT_SEQUENTIAL);
System.out.print(Thread.currentThread().getName() + " ");
System.out.print("produce a message to queue：" + node);
System.out.println(" , message data is: " + new String(data,"UTF-8"));
}
public void ensureExists(String path) {
try {
Stat stat = zk.exists(path, false);
if (stat == null) {
zk.create(path, ROOT_QUEUE_DATA, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
String queueName = "test";
final ZooKeeperQueue queue = new ZooKeeperQueue(queueName);
for (int i = 0; i < 10; i++) {
new Thread(new Runnable() {
@Override
public void run() {
try {
queue.consume();
System.out.println("--------------------------------------------------------");
System.out.println();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
}
}).start();
}
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(RandomUtils.nextInt(100 * i, 200 * i));
queue.produce(("massive" + i).getBytes());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
}
}
},"Produce-thread").start();
}
}
测试
运行main方法，本机器的某次输出结果
Produce-thread produce a message to queue：/QUEUE/test/0000000000 , message data is: massive0 Thread-8 consume a message from queue：/QUEUE/test/0000000000, message data is: massive0 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000001 , message data is: massive1 Thread-6 consume a message from queue：/QUEUE/test/0000000001, message data is: massive1 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000002 , message data is: massive2 Thread-3 consume a message from queue：/QUEUE/test/0000000002, message data is: massive2 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000003 , message data is: massive3 Thread-0 consume a message from queue：/QUEUE/test/0000000003, message data is: massive3 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000004 , message data is: massive4 Thread-5 consume a message from queue：/QUEUE/test/0000000004, message data is: massive4 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000005 , message data is: massive5 Thread-2 consume a message from queue：/QUEUE/test/0000000005, message data is: massive5 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000006 , message data is: massive6 Thread-4 consume a message from queue：/QUEUE/test/0000000006, message data is: massive6 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000007 , message data is: massive7 Thread-9 consume a message from queue：/QUEUE/test/0000000007, message data is: massive7 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000008 , message data is: massive8 Thread-7 consume a message from queue：/QUEUE/test/0000000008, message data is: massive8 -------------------------------------------------------- Produce-thread produce a message to queue：/QUEUE/test/0000000009 , message data is: massive9 Thread-1 consume a message from queue：/QUEUE/test/0000000009, message data is: massive9
总结
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