分布式锁系列文章
分布式锁（1) ----- 介绍和基于数据库的分布式锁
分布式锁（2) ----- 基于redis的分布式锁
分布式锁（3) ----- 基于zookeeper的分布式锁
思路
利用zookeeper的临时有序节点和watch实现，思路如下：
- 获取锁的请求在锁的根目录下调用create()创建临时有序节点
- 在锁的根目录下调用getChildren()获取所有子节点
- 判断步骤1创建的临时有序节点是否是所有子节点中最小的一个，是则获取锁，结束获取
- 否则对排在该节点的前一个节点调用exists()判断节点是否存在，并设置watch监听节点的删除事件
- 如果步骤4返回节点不存在转到步骤2，否则等待节点删除再转到步骤2
注意：
- 创建临时节点，是在请求锁的客户端挂了，节点会自动删除
- 创建有序节点，是为了防止群体效应，只需监听前一个节点的删除事件，不用监听所有节点的删除事件
- 该思路实现的是公平锁
实现
java实现
public boolean tryLock(long waitTime){
try {
//1.创建临时有序节点
myZNode = zk.create(root + "/" + lockName + "-", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
return waitForLock(waitTime);
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
return false;
}
private boolean waitForLock(long watiTime) {
long start = System.currentTimeMillis();
try {
while (System.currentTimeMillis() - start < watiTime) {
//2.获取子节点
List<String> children = zk.getChildren(root, false);
List<String> lockNodes = new ArrayList<>();
for (String s : children) {
if (s.startsWith(lockName)) {
lockNodes.add(s);
}
}
Collections.sort(lockNodes);
//3.判断是否最小节点
if (myZNode.equals(root + "/" + lockNodes.get(0))) {
return true;
}
//4.监听前一个节点删除事件
String seq = myZNode.substring(myZNode.lastIndexOf('/') + 1);
String waitNode = lockNodes.get(Collections.binarySearch(lockNodes, seq) - 1);
CountDownLatch latch = new CountDownLatch(1);
Stat stat = zk.exists(root + "/" + waitNode, watchedEvent -> {
if (watchedEvent.getType() == Watcher.Event.EventType.NodeDeleted) {
latch.countDown();
}
});
if (stat != null) {
latch.await(watiTime - System.currentTimeMillis() + start, TimeUnit.MILLISECONDS);
}
}
}catch (Exception e){
deleteNode();
e.printStackTrace();
}
deleteNode();
return false;
}
public boolean unlock() {
return deleteNode();
}
private boolean deleteNode(){
try {
zk.delete(myZNode, -1);
return true;
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
}
return false;
}测试代码
private static class ZoookeeperLockTest implements Runnable {
private CyclicBarrier barrier;
ZoookeeperLockTest(CyclicBarrier barrier) {
this.barrier = barrier;
}
@Override
public void run() {
try {
CountDownLatch latch = new CountDownLatch(1);
ZooKeeper zk = new ZooKeeper("127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183", 5000, WatchedEvent -> {
latch.countDown();
});
latch.await();
try {
DistributeLock lock = new ZookeeperLock(zk, "lock");
barrier.await();
lock.tryLock(Integer.MAX_VALUE);
try {
System.out.println(Thread.currentThread().getName() + "get lock");
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + "get unlock");
} finally {
lock.unlock();
}
} finally {
zk.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}缺点与解决方案
没有实现重入。可以使用threadlocal保存重入次数，每次解锁-1，到0时才删除节点
curator
curator是apache的顶级开源项目，实现了分布式锁及队列、选举等多种高级功能。同时优化了zookeeper原生api的很多问题，如支持自动连接，递归创建删除节点等等。
private static class CuratorLockTest implements Runnable {
private CyclicBarrier barrier;
CuratorLockTest(CyclicBarrier barrier) {
this.barrier = barrier;
}
@Override
public void run() {
RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
CuratorFramework client = CuratorFrameworkFactory.builder()
.connectString("127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183")
.sessionTimeoutMs(5000)
.retryPolicy(retryPolicy)
.build();
client.start();
InterProcessLock lock = new InterProcessMutex(client, "/locks/curator-lock");
try{
lock.acquire();
try {
System.out.println(Thread.currentThread().getName() + "get lock");
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + "get unlock");
}finally {
lock.release();
}
} catch (Exception e) {
e.printStackTrace();
}finally {
client.close();
}
}
}参考资料
http://zookeeper.apache.org/doc/r3.4.13/recipes.html#sc_recipes_Locks
https://www.cnblogs.com/seesun2012/p/9214653.html