Zookeeper 能保证数据的强一致性，用户任何时候都可以相信集群中每个节点的数据都是相同的。一个用户创建一个节点作为锁，另一个用户检测该节点，如果存在，代表别的用户已经锁住，如果不存在，则可以创建一个节点，代表拥有一个锁。
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本篇内容包括：Demo 概述、代码实现、测试结果
Zookeeper 能保证数据的强一致性，用户任何时候都可以相信集群中每个节点的数据都是相同的。一个用户创建一个节点作为锁，另一个用户检测该节点，如果存在，代表别的用户已经锁住，如果不存在，则可以创建一个节点，代表拥有一个锁。
分布式锁本质，就是多个资源竞争者对一份资源的排他占有
<dependency>
<groupId>org.apache.zookeepergroupId>
<artifactId>zookeeperartifactId>
<version>3.7.0version>
dependency>
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
public class ConnectionWatcher implements Watcher {
private final CountDownLatch connectedSignal = new CountDownLatch(1);
private static final int SESSION_TIMEOUT = 5000;
protected ZooKeeper zk;
public void connect(String hosts) throws IOException, InterruptedException {
zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
connectedSignal.await();
}
@Override
public void process(WatchedEvent event) {
if (event.getState() == Event.KeeperState.SyncConnected) {
connectedSignal.countDown();
}
}
public void close() throws InterruptedException {
zk.close();
}
}
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.List;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.data.Stat;
public class ActiveKeyValueStore extends ConnectionWatcher {
private static final Charset CHARSET = StandardCharsets.UTF_8;
int state = 0;
/**
* 写入节点数据
*
* @param path 节点地址
* @param value 数据值
* @throws InterruptedException 中断异常
* @throws KeeperException ZooKeeper异常
*/
public void write(String path, String value) throws InterruptedException, KeeperException {
Stat stat = zk.exists(path, false);
if (stat == null) {
if (value == null) {
zk.create(path, null,
ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
} else {
zk.create(path, value.getBytes(CHARSET),
ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} else {
if (value == null) {
zk.setData(path, null, -1);
} else {
zk.setData(path, value.getBytes(CHARSET), -1);
}
}
}
public boolean lock(String path, String name) throws InterruptedException, KeeperException {
boolean flag = tryLock(path, name);
if (flag) {
state++;
}
return flag;
}
public boolean tryLock(String path, String name) throws InterruptedException, KeeperException {
String lockPath = path + "/" + name;
zk.create(lockPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
List<String> waits = readChildren(path, null);
Collections.sort(waits);
if (waits.get(0).equals(name)) {
return true;
}
CountDownLatch latch = new CountDownLatch(1);
for (int i = 0; i < waits.size(); i++) {
String cur = waits.get(i);
if (!cur.equalsIgnoreCase(name)) {
continue;
}
String prePath = path + "/" + waits.get(i - 1);
zk.exists(prePath, new Watcher() {
@Override
public void process(WatchedEvent event) {
latch.countDown();
}
});
break;
}
latch.await();
return true;
}
public boolean unlock(String path, String name) {
if (state > 1) {
state--;
return true;
}
String lockPath = path + "/" + name;
try {
Stat stat = zk.exists(lockPath, false);
int version = stat.getVersion();
zk.delete(lockPath, version);
state--;
return true;
} catch (Exception e) {
System.out.println("unlock:" + lockPath + " ,exception,");
}
return false;
}
/**
* 获取所有子节点
*
* @param path 节点地址
* @param watcher watcher
* @return 所有子节点
* @throws InterruptedException 中断异常
* @throws KeeperException ZooKeeper异常
*/
public List<String> readChildren(String path, Watcher watcher) throws InterruptedException, KeeperException {
List<String> childrens = null;
if (watcher == null) {
childrens = zk.getChildren(path, false);
} else {
childrens = zk.getChildren(path, watcher, null);
}
return childrens;
}
}
import lombok.SneakyThrows;
import org.apache.zookeeper.KeeperException;
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
public class ZkLock {
/**
* 开启的线程数，模拟多客户端操作
*/
private static final int CLIENTS_NUM = 3;
private final ActiveKeyValueStore store;
public ZkLock(String hosts) throws IOException, InterruptedException {
//定义一个类
store = new ActiveKeyValueStore();
//连接Zookeeper
store.connect(hosts);
}
public static void testLock() {
//线程计数器控制业务的执行
final CountDownLatch countDownLatch = new CountDownLatch(CLIENTS_NUM);
for (int i = 0; i < CLIENTS_NUM; i++) {
new Thread() {
@Override
public void run() {
}
}.start();
}
try {
// 堵塞线程，任务执行完后释放
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
String hosts = "localhost:2181";
ZkLock zkLock = new ZkLock(hosts);
// 创建父节点
zkLock.store.write("/lock4", "父亲节点");
//
CountDownLatch latch = new CountDownLatch(CLIENTS_NUM);
for (int i = 0; i < CLIENTS_NUM; i++) {
int finalI = i;
new Thread() {
@SneakyThrows
@Override
public void run() {
String name = "Thread-" + String.valueOf(finalI);
zkLock.store.lock("/lock4", name);
TimeUnit.SECONDS.sleep(2);
System.out.println("线程-" + name + "执行完毕");
latch.countDown();
zkLock.store.unlock("/lock4", name);
}
}.start();
}
latch.await();
System.out.println("end ...");
}
}
ZkLock 代码测试结果如下：
线程-Thread-0执行完毕
线程-Thread-1执行完毕
线程-Thread-2执行完毕
end ...
通过 ZkLock 打印的信息可以看出，已经成功模拟实现分布式锁