ZooKeeper有一个针对Java和C的官方API绑定.ZooKeeper社区为大多数语言(.NET,python等)提供非官方API.使用ZooKeeper API,应用程序可以连接,交互,操作数据,协调,最后与ZooKeeper集合断开连接.
ZooKeeper API具有丰富的功能集,可以获得所有功能. ZooKeeper以简单安全的方式进行整合. ZooKeeper API提供同步和异步方法.
ZooKeeper集合和ZooKeeper API在各个方面完全相互补充,它以一种很好的方式使开发人员受益.让我们在本章讨论Java绑定.
与ZooKeeper集合交互的应用程序称为 ZooKeeper客户端或只是客户端.
Znode是ZooKeeper集合的核心组件,ZooKeeper API提供了一小组方法来使用ZooKeeper来操纵znode的所有细节ensemble.
客户应按照下面给出的步骤与ZooKeeper合奏进行清晰,干净的互动.
连接到ZooKeeper合奏. ZooKeeper集合为客户端分配会话ID.
定期向服务器发送心跳.否则,ZooKeeper集合将使会话ID到期,客户端需要重新连接.
只要会话ID处于活动状态,就获取/设置znodes.
完成所有任务后,断开与ZooKeeper集合的连接.如果客户端长时间处于非活动状态,则ZooKeeper集合将自动断开客户端连接.
让我们在本章中了解最重要的ZooKeeper API集. ZooKeeper API的核心部分是 ZooKeeper类.它提供了在其构造函数中连接ZooKeeper集合的选项,并具有以下方法 :
connect : 连接到ZooKeeper合奏
创建 : 创建一个znode
存在 : 检查znode是否存在及其信息
getData : 从特定的znode获取数据
setData : 在特定的znode中设置数据
getChildren : 获取特定znode中的所有子节点
删除 : 得到一个特定的znode及其所有孩子
关闭 : 关闭连接
ZooKeeper类通过其提供连接功能构造函数.构造函数的签名如下 :
ZooKeeper(String connectionString, int sessionTimeout, Watcher watcher)
其中,
connectionString : ZooKeeper合奏主持人.
sessionTimeout : 会话超时(以毫秒为单位).
观察者 : 实现"Watcher"界面的对象. ZooKeeper集合通过观察者对象返回连接状态.
让我们创建一个新的帮助类 ZooKeeperConnection 并添加方法连接. connect 方法创建一个ZooKeeper对象,连接到ZooKeeper集合,然后返回该对象.
这里 CountDownLatch 用于停止(等待)主进程,直到客户端连接ZooKeeper集合.
ZooKeeper集合通过观察者回调回复连接状态.一旦客户端连接ZooKeeper集合并且Watcher回调调用 CountDownLatch 的 countDown 方法释放锁, await 在主进程中.
这是连接ZooKeeper集合的完整代码.
// import java classes import java.io.IOException; import java.util.concurrent.CountDownLatch; // import zookeeper classes import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.AsyncCallback.StatCallback; import org.apache.zookeeper.KeeperException.Code; import org.apache.zookeeper.data.Stat; public class ZooKeeperConnection { // declare zookeeper instance to access ZooKeeper ensemble private ZooKeeper zoo; final CountDownLatch connectedSignal = new CountDownLatch(1); // Method to connect zookeeper ensemble. public ZooKeeper connect(String host) throws IOException,InterruptedException { zoo = new ZooKeeper(host,5000,new Watcher() { public void process(WatchedEvent we) { if (we.getState() == KeeperState.SyncConnected) { connectedSignal.countDown(); } } }); connectedSignal.await(); return zoo; } // Method to disconnect from zookeeper server public void close() throws InterruptedException { zoo.close(); } }
保存上面的代码,它将在下一节中用于连接ZooKeeper集合.
ZooKeeper类提供了 create method 来在ZooKeeper集合中创建一个新的znode. create 方法的签名如下 :
create(String path, byte[] data, List<ACL> acl, CreateMode createMode)
其中,
路径 : Znode路径.例如,/myapp1,/myapp2,/myapp1/mydata1,myapp2/mydata1/myanothersubdata
数据 : 要存储在指定的znode路径中的数据
acl : 要创建的节点的访问控制列表. ZooKeeper API提供了一个静态接口 ZooDefs.Ids 来获取一些基本的acl列表.例如,ZooDefs.Ids.OPEN_ACL_UNSAFE返回打开的znode的acl列表.
createMode : 节点的类型,短暂的,顺序的或两者兼而有之.这是 enum .
让我们创建一个新的Java应用程序来检查 create ZooKeeper API的功能.创建文件 ZKCreate.java .在main方法中,创建一个 ZooKeeperConnection 类型的对象,并调用 connect 方法连接到ZooKeeper集合.
连接方法将返回ZooKeeper对象 zk .现在,使用自定义路径和数据调用 zk 对象的创建方法.
创建znode的完整程序代码如下 :
import java.io.IOException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.ZooDefs; public class ZKCreate { // create static instance for zookeeper class. private static ZooKeeper zk; // create static instance for ZooKeeperConnection class. private static ZooKeeperConnection conn; // Method to create znode in zookeeper ensemble public static void create(String path, byte[] data) throws KeeperException,InterruptedException { zk.create(path, data, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT); } public static void main(String[] args) { // znode path String path = "/MyFirstZnode"; // Assign path to znode // data in byte array byte[] data = "My first zookeeper app".getBytes(); // Declare data try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); create(path, data); // Create the data to the specified path conn.close(); } catch (Exception e) { System.out.println(e.getMessage()); //Catch error message } } }
编译并执行应用程序后,将在ZooKeeper集合中创建具有指定数据的znode.您可以使用ZooKeeper CLI zkCli.sh 检查它.
cd /path/to/zookeeper bin/zkCli.sh >>> get /MyFirstZnode
ZooKeeper类提供 exists方法来检查znode的存在.它返回给我如果指定的znode存在,则为znode的数据元素. 存在方法的签名如下 :
exists(String path, boolean watcher)
其中,
路径 : Znode路径
观察者 : boolean值指定是否观察指定的znode
让我们创建一个新的Java应用程序来检查"存在"功能ZooKeeper API.创建文件"ZKExists.java".在main方法中,使用"ZooKeeperConnection"对象创建ZooKeeper对象"zk".然后,用自定义"path"调用"zk"对象的"exists"方法.完整列表如下:
import java.io.IOException; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.data.Stat; public class ZKExists { private static ZooKeeper zk; private static ZooKeeperConnection conn; // Method to check existence of znode and its status, if znode is available. public static Stat znode_exists(String path) throws KeeperException,InterruptedException { return zk.exists(path, true); } public static void main(String[] args) throws InterruptedException,KeeperException { String path = "/MyFirstZnode"; // Assign znode to the specified path try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); Stat stat = znode_exists(path); // Stat checks the path of the znode if(stat != null) { System.out.println("Node exists and the node version is " + stat.getVersion()); } else { System.out.println("Node does not exists"); } } catch(Exception e) { System.out.println(e.getMessage()); // Catches error messages } } }
编译并执行应用程序后,您将获得低于输出.
Node exists and the node version is 1.
ZooKeeper类提供 getData 方法来获取指定znode及其状态附加的数据. getData 方法的签名如下 :
getData(String path, Watcher watcher, Stat stat)
其中,
路径 : Znode路径.
观察者 : 类型观察者的回调函数.当指定znode的数据发生变化时,ZooKeeper集合将通过Watcher回调通知.这是一次性通知.
stat : 返回znode的元数据.
让我们创建一个新的Java应用程序来理解 getData 的功能ZooKeeper API.创建文件 ZKGetData.java .在main方法中,使用 ZooKeeperConnection 对象创建ZooKeeper对象 zk .然后,使用自定义路径调用zk对象的 getData 方法.
这是从指定节点获取数据的完整程序代码 :
import java.io.IOException; import java.util.concurrent.CountDownLatch; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.data.Stat; public class ZKGetData { private static ZooKeeper zk; private static ZooKeeperConnection conn; public static Stat znode_exists(String path) throws KeeperException,InterruptedException { return zk.exists(path,true); } public static void main(String[] args) throws InterruptedException, KeeperException { String path = "/MyFirstZnode"; final CountDownLatch connectedSignal = new CountDownLatch(1); try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); Stat stat = znode_exists(path); if(stat != null) { byte[] b = zk.getData(path, new Watcher() { public void process(WatchedEvent we) { if (we.getType() == Event.EventType.None) { switch(we.getState()) { case Expired: connectedSignal.countDown(); break; } } else { String path = "/MyFirstZnode"; try { byte[] bn = zk.getData(path, false, null); String data = new String(bn, "UTF-8"); System.out.println(data); connectedSignal.countDown(); } catch(Exception ex) { System.out.println(ex.getMessage()); } } } }, null); String data = new String(b, "UTF-8"); System.out.println(data); connectedSignal.await(); } else { System.out.println("Node does not exists"); } } catch(Exception e) { System.out.println(e.getMessage()); } } }
编译并执行应用程序后,您将获得以下输出
My first zookeeper app
并且应用程序将等待来自ZooKeeper合奏.使用ZooKeeper CLI zkCli.sh 更改指定znode的数据.
cd /path/to/zookeeper bin/zkCli.sh >>> set /MyFirstZnode Hello
现在,应用程序将打印以下输出并退出./p>
Hello
ZooKeeper类提供 setData 方法来修改指定znode中附加的数据. setData 方法的签名如下 :
setData(String path, byte[] data, int version)
其中,
路径 : Znode路径
数据 : 要存储在指定的znode路径中的数据.
版本 : 当前版本的znode.每当数据发生变化时,ZooKeeper都会更新znode的版本号.
现在让我们创建一个新的Java应用程序来理解 ZooData ZooKeeper API的功能.创建文件 ZKSetData.java .在main方法中,使用 ZooKeeperConnection 对象创建ZooKeeper对象 zk .然后,使用指定的路径,新数据和节点版本调用 zk 对象的 setData 方法.
这是用于修改指定znode中附加数据的完整程序代码.
import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import java.io.IOException; public class ZKSetData { private static ZooKeeper zk; private static ZooKeeperConnection conn; // Method to update the data in a znode. Similar to getData but without watcher. public static void update(String path, byte[] data) throws KeeperException,InterruptedException { zk.setData(path, data, zk.exists(path,true).getVersion()); } public static void main(String[] args) throws InterruptedException,KeeperException { String path= "/MyFirstZnode"; byte[] data = "Success".getBytes(); //Assign data which is to be updated. try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); update(path, data); // Update znode data to the specified path } catch(Exception e) { System.out.println(e.getMessage()); } } }
编译并执行应用程序后,指定znode的数据将被更改并且可以使用ZooKeeper CLI检查它, zkCli.sh .
cd/path/to/zookeeper bin/zkCli.sh >>> get/MyFirstZnode
ZooKeeper类提供了 getChildren 方法来获取特定znode的所有子节点. getChildren 方法的签名如下 :
getChildren(String path,Watcher watcher)
其中,
路径 : Znode路径.
观察者 : "Watcher"类型的回调函数. ZooKeeper集合将在指定的znode被删除或者创建/删除znode下的子节点时通知.这是一次性通知.
import java.io.IOException; import java.util.*; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.Watcher.Event.KeeperState; import org.apache.zookeeper.data.Stat; public class ZKGetChildren { private static ZooKeeper zk; private static ZooKeeperConnection conn; // Method to check existence of znode and its status, if znode is available. public static Stat znode_exists(String path) throws KeeperException,InterruptedException { return zk.exists(path,true); } public static void main(String[] args) throws InterruptedException,KeeperException { String path = "/MyFirstZnode"; // Assign path to the znode try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); Stat stat = znode_exists(path); // Stat checks the path if(stat!= null) { //"getChildren" method- get all the children of znode.It has two args, path and watch List <String> children = zk.getChildren(path, false); for(int i = 0; i < children.size(); i++) System.out.println(children.get(i)); //Print children's } else { System.out.println("Node does not exists"); } } catch(Exception e) { System.out.println(e.getMessage()); } } }
在运行程序之前,让我们创建两个/MyFirstZnode 的子节点使用ZooKeeper CLI, zkCli.sh .
cd /path/to/zookeeper bin/zkCli.sh >>> create /MyFirstZnode/myfirstsubnode Hi >>> create /MyFirstZnode/mysecondsubmode Hi
现在,编译并运行程序将输出上面创建的znodes.
myfirstsubnode mysecondsubnode
ZooKeeper类提供了 delete 方法来删除指定的Z序节点. 删除方法的签名如下 :
delete(String path,int version)
其中,
路径 : Znode路径.
版本 : 当前版本的znode.
让我们创建一个新的Java应用程序来理解删除的功能. ZooKeeper API.创建文件 ZKDelete.java .在main方法中,使用 ZooKeeperConnection 对象创建ZooKeeper对象 zk .然后,使用指定的路径和节点版本调用 zk 对象的删除方法.
删除znode的完整程序代码如下 :
import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.KeeperException; public class ZKDelete { private static ZooKeeper zk; private static ZooKeeperConnection conn; // Method to check existence of znode and its status, if znode is available. public static void delete(String path) throws KeeperException,InterruptedException { zk.delete(path,zk.exists(path,true).getVersion()); } public static void main(String[] args) throws InterruptedException,KeeperException { String path = "/MyFirstZnode"; //Assign path to the znode try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); delete(path); //delete the node with the specified path } catch(Exception e) { System.out.println(e.getMessage()); // catches error messages } } }