柚子快報邀請碼778899分享：zookeeper實現(xiàn)分布式鎖

http://yzkb.51969.com/

zookeeper提供了分布式環(huán)境下的一致性解決方案。借此我們可以實現(xiàn)分布式鎖，而zookeeper實現(xiàn)的分布式鎖可以有兩種形式

1.分布式公平鎖

公平鎖就是指按照申請的順序來發(fā)放鎖，按照申請順序分為1,2,3,4執(zhí)行順序也是1,2,3,4

我們可以利用zookeeper的臨時順序節(jié)點來實現(xiàn)分布式公平鎖，順序節(jié)點來實現(xiàn)公平鎖，順序節(jié)點會在節(jié)點id的最后維護一個值來保證順序，

如果一個服務(wù)搶到鎖后宕機沒有釋放鎖那么就會陷入死鎖，導(dǎo)致服務(wù)停擺，

redis實現(xiàn)分布式防止這種情況的方法是設(shè)置過期時間，并通過看門狗進行無限續(xù)期，如果服務(wù)宕機，看門狗不能續(xù)期，該記錄就失效，失效后鎖釋放。

zookeeper的方法是使用臨時節(jié)點，臨時節(jié)點在服務(wù)與zookeeper結(jié)束會話時接自動刪除。此時鎖自動釋放。

當(dāng)一個服務(wù)想獲取鎖時，新建一個節(jié)點，如果自身不是順序最小的那個節(jié)點就監(jiān)聽自己的前一個節(jié)點（序號比自己小1的）。如果前一個節(jié)點被刪除了，就獲得鎖。

package org.example;

import org.apache.zookeeper.*;

import org.apache.zookeeper.data.Stat;

import java.io.IOException;

import java.util.Collections;

import java.util.List;

import java.util.concurrent.CountDownLatch;

public class DistributedLock {

private static final String ZOOKEEPER_CONNECTION_STRING = "localhost:2181";

private static final int SESSION_TIMEOUT = 5000;

private static final String LOCK_ROOT = "/locks";

private static final String LOCK_NAME = "lock-";

private ZooKeeper zooKeeper;

private String lockPath;

private String currentLock;

public DistributedLock() throws IOException, InterruptedException, KeeperException {

this.zooKeeper = new ZooKeeper(ZOOKEEPER_CONNECTION_STRING, SESSION_TIMEOUT, null);

ensureRootPath();

}

private void ensureRootPath() throws KeeperException, InterruptedException {

Stat stat = zooKeeper.exists(LOCK_ROOT, false);

if (stat == null) {

zooKeeper.create(LOCK_ROOT, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

}

}

public void lock() throws KeeperException, InterruptedException {

currentLock = zooKeeper.create(LOCK_ROOT + "/" + LOCK_NAME, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);

while (true) {

List children = zooKeeper.getChildren(LOCK_ROOT, false);

Collections.sort(children);

int index = children.indexOf(currentLock.substring(LOCK_ROOT.length() + 1));

if (index == 0) {

return;

} else {

String prevLock = LOCK_ROOT + "/" + children.get(index - 1);

final CountDownLatch latch = new CountDownLatch(1);

zooKeeper.exists(prevLock, new Watcher() {

@Override

public void process(WatchedEvent event) {

if (event.getType() == Event.EventType.NodeDeleted) {

latch.countDown();

}

}

});

latch.await();

}

}

}

public void unlock() throws KeeperException, InterruptedException {

zooKeeper.delete(currentLock, -1);

}

public void close() throws InterruptedException {

zooKeeper.close();

}

public static void main(String[] args) throws IOException, InterruptedException, KeeperException {

DistributedLock distributedLock = new DistributedLock();

distributedLock.lock();

System.out.println("搶到鎖了");

// 執(zhí)行臨界區(qū)代碼

distributedLock.unlock();

System.out.println();

distributedLock.close();

}

}

仍然可能出現(xiàn)死鎖，同一個客戶端多次擁有鎖可能出現(xiàn)死鎖。如果需要實現(xiàn)分布式鎖可以使用Curator框架的InterProcessMutex。他自己維護了一個lockData來確保鎖的可重入，解鎖死鎖。

2.分布式非公平鎖

非公平鎖指搶鎖不按照順序，一直搶誰運氣好就誰拿下。多個服務(wù)一起創(chuàng)建節(jié)點，誰創(chuàng)建成功誰擁有鎖，非順序節(jié)點不允許同名，所以只有一個服務(wù)可以擁有鎖。其他服務(wù)監(jiān)聽該節(jié)點，該節(jié)點被刪除即鎖被釋放時，其余服務(wù)再一起爭搶鎖。

package org.example;

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 java.io.IOException;

import java.util.concurrent.CountDownLatch;

public class Main {

public static final int NUM = 1;

private static final String ZK_ADDRESS = "127.0.0.1:2181";

private static final String LOCK_PATH = "/lock";

public static void main(String[] args) throws IOException, InterruptedException, KeeperException {

ZooKeeper zooKeeper = new ZooKeeper(ZK_ADDRESS, 1000, null);

while (true) {

if (lock(zooKeeper)) {

System.out.println(NUM + "號搶鎖成功");

Thread.sleep(100);

unlock(zooKeeper);

} else {

System.out.println(NUM + "號搶鎖失敗");

}

}

}

public static boolean lock(ZooKeeper zooKeeper) throws InterruptedException, KeeperException {

try {

zooKeeper.create(LOCK_PATH, String.valueOf(NUM).getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

return true;

} catch (KeeperException.NodeExistsException e) {

final CountDownLatch latch = new CountDownLatch(1);

zooKeeper.exists(LOCK_PATH, new Watcher() {

@Override

public void process(WatchedEvent event) {

if (event.getType() == Event.EventType.NodeDeleted) {

latch.countDown();

}

}

});

latch.await();

return lock(zooKeeper);

}

}

public static void unlock(ZooKeeper zooKeeper) throws InterruptedException, KeeperException {

zooKeeper.delete(LOCK_PATH, -1);

}

}

同樣仍可能出現(xiàn)死鎖。

柚子快報邀請碼778899分享：zookeeper實現(xiàn)分布式鎖

http://yzkb.51969.com/

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