本文介绍zk客户端curator的使用，本文主要介绍Curator recipes的使用
官方文档传送门
官方文档Curator recipes
参考：http://www.throwable.club/2018/12/16/zookeeper-curator-usage/
[https://blog.csdn.net/hosaos/article/details/88727817]
参考书籍：《从Paxos到ZooKeeper 分布式一致性原理与实践》
1. 分布式锁InterProcessMutex
在分布式系统中，经常要进行同步控制，这时候就会用到分布式锁，InterProcessMutex是ZK的分布式锁实现。
pom
<dependency>
<groupId>org.apache.curator</groupId>
<artifactId>curator-recipes</artifactId>
<version>2.13.0</version>
</dependency>
构造方法
/**
* @param client client：客户端
* @param path the path to lock：路径
*/
public InterProcessMutex(CuratorFramework client, String path)
{
this(client, path, new StandardLockInternalsDriver());
}
/**
* @param client client：客户端
* @param path the path to lock：路径
* @param driver lock driver：可自定义lock驱动实现分布式锁
*/
public InterProcessMutex(CuratorFramework client, String path, LockInternalsDriver driver)
{
this(client, path, LOCK_NAME, 1, driver);
}
主要方法
// 获取锁，若失败则阻塞直到成功，支持重入
public void acquire() throws Exception
// 获取锁，超时失败
public boolean acquire(long time, TimeUnit unit) throws Exception
// 释放锁
public void release() throws Exception
2. 生成订单号事例
模拟10个线程同时抢锁，抢锁成功的线程生成订单号，睡一秒后释放锁。
事例
import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import java.text.SimpleDateFormat;
import java.util.Date;
/**
* 分布式锁InterProcessMutex
*/
@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.DEFINED_PORT)
public class InterProcessMutexTests {
private CuratorFramework client;
/**
* 使用Fluent风格API创建
*/
@Before
public void client() {
// 重试策略
RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
client = CuratorFrameworkFactory.builder()
.connectString("localhost:2181")
.sessionTimeoutMs(5000) // 会话超时时间，默认60000ms
.connectionTimeoutMs(10000) // 连接超时时间，默认15000ms
.retryPolicy(retryPolicy) // 重试策略
// .namespace("base") // 名称空间，如果这里设置了名称空间，那么所有路径都将预先指定名称空间
.build();
client.start();
}
/**
* InterProcessMutex事例：生成订单号
*/
@Test
public void InterProcessMutex() throws Exception {
String lockPath = "/lock_path";
InterProcessMutex lock = new InterProcessMutex(client, lockPath);
// 模拟10个线程抢锁
for (int i = 0; i < 10; i++) {
new Thread(new TestThread(i, lock)).start();
}
Thread.sleep(Integer.MAX_VALUE);
}
static class TestThread implements Runnable {
private Integer n;
private InterProcessMutex lock;
public TestThread(Integer n, InterProcessMutex lock) {
this.n = n;
this.lock = lock;
}
@Override
public void run() {
try {
lock.acquire();
System.out.println("第" + n + "线程获取到了锁");
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss|SSS");
String orderNo = sdf.format(new Date());
System.out.println("生成的订单号是 : " + orderNo);
// 1秒后释放锁
Thread.sleep(1000);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
lock.release();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
控制台
原理
多个线程同时在指定路径下创建临时顺序节点，编号最小的获取锁，没抢到锁的线程会调用wait()方法等待被唤醒，唤醒操作是通过Watch实现的，每个节点监听前一个节点的删除事件。
如果path被删除，Watch中就会触发notifyAll()，唤醒其他线程继续抢锁。
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关键代码
获取锁，每个线程在path下面创建临时顺序节点
private boolean internalLockLoop(long startMillis, Long millisToWait, String ourPath) throws Exception
{
boolean haveTheLock = false;
boolean doDelete = false;
try
{
if ( revocable.get() != null )
{
client.getData().usingWatcher(revocableWatcher).forPath(ourPath);
}
// 自选获取锁
while ( (client.getState() == CuratorFrameworkState.STARTED) && !haveTheLock )
{
// 临时顺序节点集合，编号从小到大
List<String> children = getSortedChildren();
// 当前线程节点编号
String sequenceNodeName = ourPath.substring(basePath.length() + 1); // +1 to include the slash
// 根据当前节点编号得到当前节点在children中的index， 判断index是否 < maxLease，maxLease此处是1。也就是判断节点是否是最小的编号，只有最小的编号才能获取到锁。
PredicateResults predicateResults = driver.getsTheLock(client, children, sequenceNodeName, maxLeases);
if ( predicateResults.getsTheLock() )
{
haveTheLock = true;
}
else
{
// 前一个节点路径
String previousSequencePath = basePath + "/" + predicateResults.getPathToWatch();
synchronized(this)
{
try
{
// use getData() instead of exists() to avoid leaving unneeded watchers which is a type of resource leak
// 如果没抢到锁就监听前一个节点删除事件 client.getData().usingWatcher(watcher).forPath(previousSequencePath);
if ( millisToWait != null )
{
millisToWait -= (System.currentTimeMillis() - startMillis);
startMillis = System.currentTimeMillis();
// 判断超时
if ( millisToWait <= 0 )
{
// 如果超时，直接退出并且标记删除该节点
doDelete = true; // timed out - delete our node
break;
}
wait(millisToWait);
}
else
{
// 等待
wait();
}
}
catch ( KeeperException.NoNodeException e )
{
// it has been deleted (i.e. lock released). Try to acquire again
}
}
}
}
}
catch ( Exception e )
{
ThreadUtils.checkInterrupted(e);
doDelete = true;
throw e;
}
finally
{
if ( doDelete )
{
//如果标记删除，删除节点
deleteOurPath(ourPath);
}
}
return haveTheLock;
}
判断是否获取到锁
driver.getsTheLock(client, children, sequenceNodeName, maxLeases);
@Override
public PredicateResults getsTheLock(CuratorFramework client, List<String> children, String sequenceNodeName, int maxLeases) throws Exception
{
int ourIndex = children.indexOf(sequenceNodeName);
validateOurIndex(sequenceNodeName, ourIndex);
boolean getsTheLock = ourIndex < maxLeases;
String pathToWatch = getsTheLock ? null : children.get(ourIndex - maxLeases);
return new PredicateResults(pathToWatch, getsTheLock);
}
释放锁
就是删除节点，触发Watch，调用notifyAll()唤醒其他线程。
private void deleteOurPath(String ourPath) throws Exception {
try {
this.client.delete().guaranteed().forPath(ourPath);
} catch (NoNodeException var3) {
}
}
private synchronized void notifyFromWatcher() {
this.notifyAll();
}
源码地址
IT-CLOUD-ZOOKEEPER :zookeeper客户端Curator事例
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