Java ZooKeeper是一個分布式的開源協調服務框架，提供了可靠的分布式協調服務。ZooKeeper作為一個文件系統，具有節點分級，監控和訂閱等特性，可以在分布式環境下實現消息發布/訂閱模式等應用。

在ZooKeeper中，訂閱和發布是非常常見的用例，ZooKeeper可以作為一個中介在客戶端之間傳遞消息。下面是一個使用Java ZooKeeper實現發布/訂閱的例子：

public class Subscriber implements Watcher {
private ZooKeeper zk;
private String zkPath = "/test";
private Listsubscribers = new ArrayList<>();
private String nodeName;
public Subscriber(String nodeName) throws IOException, KeeperException, InterruptedException {
this.nodeName = nodeName;
zk = new ZooKeeper("localhost:2181", 5000, this);
// 創建節點，假設為test
zk.create(zkPath, "".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
// 訂閱節點變化
zk.getChildren(zkPath, true);
}
@Override
public void process(WatchedEvent event) {
try {
// 節點變化，重新訂閱
if (event.getType().equals(Event.EventType.NodeChildrenChanged)) {
Listchildren = zk.getChildren(zkPath, true);
subscribers.clear();
for (String child : children) {
byte[] data = zk.getData(zkPath + "/" + child, false, null);
subscribers.add(new String(data));
}
System.out.println(nodeName + " subscriber list updated: " + subscribers);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
public class Publisher {
private ZooKeeper zk;
private String zkPath = "/test";
public Publisher() throws IOException, KeeperException, InterruptedException {
zk = new ZooKeeper("localhost:2181", 5000, null);
// 創建節點，假設為test
zk.create(zkPath, "".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
public void send(String content) throws KeeperException, InterruptedException {
// 創建節點，每次內容不同
zk.create(zkPath + "/message", content.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
}
}
public class Main {
public static void main(String[] args) throws InterruptedException, KeeperException, IOException {
// 創建發布者
Publisher publisher = new Publisher();
// 創建2個訂閱者
Subscriber subscriber1 = new Subscriber("subscriber1");
Subscriber subscriber2 = new Subscriber("subscriber2");
// 發布消息
publisher.send("message1");
publisher.send("message2");
Thread.sleep(10000);
// 關閉ZooKeeper連接
subscriber1.zk.close();
subscriber2.zk.close();
publisher.zk.close();
}
}

執行上面的代碼，可以看到訂閱者每當收到新消息時，會更新自己的訂閱列表。

可以使用Java ZooKeeper輕松實現發布/訂閱模式，這對于分布式應用來說是非常有用的。