基於ZK實現分散式鎖

引言

ZooKeeper是一個分散式的，開放原始碼的分散式應用程式協調服務，是Google的Chubby一個開源的實現，是Hadoop和Hbase的重要元件。它是一個為分散式應用提供一致性服務的軟體，提供的功能包括：配置維護、域名服務、分散式同步、組服務等。

ZooKeeper的架構通過冗餘服務實現高可用性。因此，如果第一次無應答，客戶端就可以詢問另一臺ZooKeeper主機。ZooKeeper節點將它們的資料儲存於一個分層的名稱空間，非常類似於一個檔案系統或一個字首樹結構。客戶端可以在節點讀寫，從而以這種方式擁有一個共享的配置服務。更新是全序的。

基於ZooKeeper分散式鎖的原理

1. 在zookeeper指定節點（locks）下建立臨時順序節點node_n
2. 獲取locks下所有子節點children
3. 對子節點按節點自增序號從小到大排序
4. 判斷本節點是不是第一個子節點，若是，則獲取鎖；若不是，則監聽比該節點小的那個節點的刪除事件
5. 若監聽事件生效，則回到第二步重新進行判斷，直到獲取到鎖

具體實現

下面就具體使用java和zookeeper實現分散式鎖，操作zookeeper使用的是apache提供的zookeeper的包。

• 通過實現Watch介面，實現process(WatchedEvent event)方法來實施監控，使CountDownLatch來完成監控，在等待鎖的時候使用CountDownLatch來計數，等到後進行countDown，停止等待，繼續執行。
• 以下整體流程基本與上述描述流程一致，只是在監聽的時候使用的是CountDownLatch來監聽前一個節點。

package org.java.base.zookeeper;
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;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;
import org.codehaus.jettison.json.JSONString;

import com.fasterxml.jackson.databind.util.JSONPObject;
import com.google.gson.JsonObject;


public class DistributedLock implements Lock, Watcher {
 private ZooKeeper zk = null;
 // 根節點
 private String ROOT_LOCK = "/locks";
 // 競爭的資源
 private String lockName;
 // 等待的前一個鎖
 private String WAIT_LOCK;
 // 當前鎖
 private String CURRENT_LOCK;
 // 計數器
 private CountDownLatch countDownLatch;
 private int sessionTimeout = 30000;
 private List<Exception> exceptionList = new ArrayList<Exception>();

 /**
 * 配置分散式鎖
 * @param config 連線的url
 * @param lockName 競爭資源
 */
 public DistributedLock(String config, String lockName) {
 this.lockName = lockName;
 try {
 // 連線zookeeper
 zk = new ZooKeeper(config, sessionTimeout, this);
 Stat stat = zk.exists(ROOT_LOCK, false);
 if (stat == null) {
 // 如果根節點不存在，則建立根節點
 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();
 }
 }

 // 節點監視器
 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 + "獲得了鎖");
 return;
 } else {
 // 等待鎖
 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("鎖名有誤");
 }
 // 建立臨時有序節點
 CURRENT_LOCK = zk.create(ROOT_LOCK + "/" + lockName + splitStr, new byte[0],
 ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
 System.out.println(CURRENT_LOCK + " 已經建立");
 // 取所有子節點
 List<String> subNodes = zk.getChildren(ROOT_LOCK, false);
 // 取出所有lockName的鎖
 List<String> lockObjects = new ArrayList<String>();
 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() + " 的鎖是 " + CURRENT_LOCK);
 // 若當前節點為最小節點，則獲取鎖成功
 if (CURRENT_LOCK.equals(ROOT_LOCK + "/" + lockObjects.get(0))) {
 return true;
 }

 // 若不是最小節點，則找到自己的前一個節點
 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;
 }

 // 等待鎖
 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() + "等待鎖 " + ROOT_LOCK + "/" + prev);
 this.countDownLatch = new CountDownLatch(1);
 // 計數等待，若等到前一個節點消失，則precess中進行countDown，停止等待，獲取鎖
 this.countDownLatch.await(waitTime, TimeUnit.MILLISECONDS);
 this.countDownLatch = null;
 System.out.println(Thread.currentThread().getName() + " 等到了鎖");
 }
 return true;
 }

 public void unlock() {
 try {
 System.out.println("釋放鎖 " + 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);
 }
 }
}複製程式碼

測試類

package org.java.base.zookeeper;
/**
 * 這段程式碼有一個問題，看大家能否發現？
 * @author Liuhaihua
 *
 */
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("10.152.17.73:2181", "test1");
 lock.lock();
 secskill();
 System.out.println(Thread.currentThread().getName() + "正在執行");
 } finally {
 if (lock != null) {
 lock.unlock();
 }
 }
 }
 };

 for (int i = 0; i < 10; i++) {
 Thread t = new Thread(runnable);
 t.start();
 }
 }
}複製程式碼