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
- 在zookeeper指定節點(locks)下建立臨時順序節點node_n
- 獲取locks下所有子節點children
- 對子節點按節點自增序號從小到大排序
- 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
- 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