【ZooKeeper 系列】2. 用 Java 实现 ZooKeeper API 的调用

温馨提示：在这里我再次提个小要求，希望大家能习惯看官方文档，文档虽然是英文但用词都比较简单，基本都能看懂文档表达的意思。授之以鱼不如授之以渔的道理相信大家都明白，也希望通过猿人谷的这个 ZooKeeper 系列，让大家入门、到熟悉，举一反三后能精通 ZooKeeper。

在前一篇我们介绍了 ZooKeeper 单机版、伪集群和集群环境搭建，通过命令行的方式做了节点的创建、删除、更新、获取节点信息的测试。Zookeeper 的目的是为客户端构建复杂的协调功能提供简单、高效的核心 API，这一篇我们用 Java 通过 ZooKeeper 提供的 API 接口来实现这些增删改查的功能。

1 简介

org.apache.zookeeper.Zookeeper 是 ZooKeeper 客户端的主类，在官方文档（该系列文章以 v3.5.5 为主，v3.6.6 的 API Docs 还没有）中已明确说明（This is the main class of ZooKeeper client library.）。

This is the main class of ZooKeeper client library. To use a ZooKeeper service, an application must first instantiate an object of ZooKeeper class. All the iterations will be done by calling the methods of ZooKeeper class. The methods of this class are thread-safe unless otherwise noted.
Once a connection to a server is established, a session ID is assigned to the client. The client will send heart beats to the server periodically to keep the session valid.

创建一个 ZooKeeper 的实例来使用 org.apache.zookeeper.Zookeeper 里的方法，官方文档已经指出没有特别声明的话，ZooKeeper 类里的方法是 线程安全 的。客户端连接到 ZooKeeper 服务的时候，会给客户端分配一个会话 ID（session ID），客户端与服务端会通过心跳来保持会话有效。

org.apache.zookeeper.Zookeeper 里的方法非常多，就不一一列举了，只列几个增删改查的。

2 测试环境搭建

这里新建一个 Spring Boot 的项目来进行测试，新建 Spring Boot 项目的过程很简单，也不是这里的重点，就不做介绍了。

项目里会需要额外引入两个包来进行测试：

        <dependency>
            <groupId>org.apache.zookeeper</groupId>
            <artifactId>zookeeper</artifactId>
            <version>3.5.5</version>
        </dependency>

        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>5.5.2</version>
        </dependency>

3 API 测试

完整测试代码如下：

/**
 *  简单测试示例
 * @author 猿人谷
 * @date 2019/12/16
 */
public class ZooKeeperDemo {

    private static final Logger LOGGER = LoggerFactory.getLogger(ZooKeeperDemo.class);

    private static final int SESSION_TIME_OUT = 10000;
    // ZooKeeper服务的地址，如为集群，多个地址用逗号分隔
    private static final String CONNECT_STRING = "127.0.0.1:2181";
    private static final String ZNODE_PATH = "/zk_demo";
    private static final String ZNODE_PATH_PARENT = "/app1";
    private static final String ZNODE_PATH_CHILDREN = "/app1/app1_1";

    private ZooKeeper zk = null;

    @Before
    public void init() throws IOException {
        zk = new ZooKeeper(CONNECT_STRING, SESSION_TIME_OUT, new Watcher(){
            @Override
            public void process(WatchedEvent event) {
                System.out.println("已经触发了" + event.getType() + "事件！");
            }
        });

    }

    @Test
    public void testCreate() throws KeeperException, InterruptedException {
        zk.create(ZNODE_PATH, "anna2019".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    }

    @Test
    public void testCreateParentZnode() throws KeeperException, InterruptedException {
        zk.create(ZNODE_PATH_PARENT, "anna2019".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    }

    @Test
    public void testCreateChildrenZnode() throws KeeperException, InterruptedException {
        zk.create(ZNODE_PATH_CHILDREN, "anna2020".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    }

    @Test
    public void testGet() throws KeeperException, InterruptedException {
        byte[] data1 = zk.getData(ZNODE_PATH, false, null);
        byte[] data2 = zk.getData(ZNODE_PATH_PARENT, false, null);
        byte[] data3 = zk.getData(ZNODE_PATH_CHILDREN, false, null);
        LOGGER.info("{}的信息：{}", ZNODE_PATH, new String(data1) );
        LOGGER.info("{}的信息：{}", ZNODE_PATH_PARENT, new String(data2) );
        LOGGER.info("{}的信息：{}", ZNODE_PATH_CHILDREN, new String(data3) );
    }

    /**
     *  删除
     * @throws KeeperException
     * @throws InterruptedException
     */
    @Test
    public void testDelete() throws KeeperException, InterruptedException {
        // 指定要删除的版本，-1表示删除所有版本
        zk.delete(ZNODE_PATH, -1);
    }

    /**
     *  删除含有子节点
     * @throws KeeperException
     * @throws InterruptedException
     */
    @Test
    public void testDeleteHasChildrenZnode() throws KeeperException, InterruptedException {
        // 指定要删除的版本，-1表示删除所有版本
        zk.delete(ZNODE_PATH_PARENT, -1);
    }

    @Test
    public void testSet() throws KeeperException, InterruptedException {
        Stat stat = zk.setData(ZNODE_PATH, "yuanrengu".getBytes(), -1);
        LOGGER.info(stat.toString());
    }

}

上面有用到 @Before，简单说明下：

@BeforeClass – 表示在类中的任意 public static void 方法执行之前执行
@AfterClass – 表示在类中的任意 public static void 方法执行之后执行
@Before – 表示在任意使用 @Test 注解标注的 public void 方法执行之前执行
@After – 表示在任意使用 @Test 注解标注的 public void 方法执行之后执行
@Test – 使用该注解标注的 public void 方法会表示为一个测试方法

如果将 SESSION_TIME_OUT 设置的时间太短，会报 API 客户端异常：org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /zk_demo 。完整的报错信息如下：

09:33:52.139 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxnSocketNIO - Ignoring exception during shutdown input
java.net.SocketException: Socket is not connected
	at sun.nio.ch.Net.translateToSocketException(Net.java:123)
	at sun.nio.ch.Net.translateException(Net.java:157)
	at sun.nio.ch.Net.translateException(Net.java:163)
	at sun.nio.ch.SocketAdaptor.shutdownInput(SocketAdaptor.java:401)
	at org.apache.zookeeper.ClientCnxnSocketNIO.cleanup(ClientCnxnSocketNIO.java:198)
	at org.apache.zookeeper.ClientCnxn$SendThread.cleanup(ClientCnxn.java:1338)
	at org.apache.zookeeper.ClientCnxn$SendThread.cleanAndNotifyState(ClientCnxn.java:1276)
	at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1254)
Caused by: java.nio.channels.NotYetConnectedException: null
	at sun.nio.ch.SocketChannelImpl.shutdownInput(SocketChannelImpl.java:782)
	at sun.nio.ch.SocketAdaptor.shutdownInput(SocketAdaptor.java:399)
	... 4 common frames omitted
09:33:52.140 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxnSocketNIO - Ignoring exception during shutdown output
java.net.SocketException: Socket is not connected
	at sun.nio.ch.Net.translateToSocketException(Net.java:123)
	at sun.nio.ch.Net.translateException(Net.java:157)
	at sun.nio.ch.Net.translateException(Net.java:163)
	at sun.nio.ch.SocketAdaptor.shutdownOutput(SocketAdaptor.java:409)
	at org.apache.zookeeper.ClientCnxnSocketNIO.cleanup(ClientCnxnSocketNIO.java:205)
	at org.apache.zookeeper.ClientCnxn$SendThread.cleanup(ClientCnxn.java:1338)
	at org.apache.zookeeper.ClientCnxn$SendThread.cleanAndNotifyState(ClientCnxn.java:1276)
	at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1254)
Caused by: java.nio.channels.NotYetConnectedException: null
	at sun.nio.ch.SocketChannelImpl.shutdownOutput(SocketChannelImpl.java:799)
	at sun.nio.ch.SocketAdaptor.shutdownOutput(SocketAdaptor.java:407)
	... 4 common frames omitted

org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /zk_demo

	at org.apache.zookeeper.KeeperException.create(KeeperException.java:102)
	at org.apache.zookeeper.KeeperException.create(KeeperException.java:54)
	at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:2131)
	at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:2160)
	at com.yuanrengu.demo.ZooKeeperDemo.testGet(ZooKeeperDemo.java:48)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Disconnected from the target VM, address: '127.0.0.1:60454', transport: 'socket'

Process finished with exit code -1

起初以为是 ZooKeeper 服务部署有问题或服务没启动，经检查确认无误后，debug 调试发现，是 SESSION_TIME_OUT = 2000;设置的值太小，改为 10000 后，不再报错。

SESSION_TIME_OUT 是 会话超时时间，也就是当一个 zookeeper 超过该时间没有心跳，则认为该节点故障。所以，如果此值小于 zookeeper 的创建时间，则当 zookeeper 还未来得及创建连接，会话时间已到，因此抛出异常认为该节点故障。

3.1 创建会话

通过创建一个 ZooKeeper 实例来连接 ZooKeeper 服务器（详见 ZooKeeper 单机版、伪集群和集群环境搭建）。

官方提供了 10 种 ZooKeeper 构造方法和描述如下：

关于每种构造方法的英文描述用词都很简单，基本都能看的很明白，根据实际的应用场景选取相应的构造方法。

有传入参数中包括 sessionId 和 sessionPasswd 的构造方法，分别代表 会话ID 和 会话密钥。这两个参数能够唯一确定一个会话，同时客户端使用这两个参数可以 实现客户端会话复用，从而达到恢复会话的效果。具体使用方法是第一次连接上 ZooKeeper 服务器时，通过调用 ZooKeeper 对象实例的以下两个接口，即可获取当前会话的 ID 和密钥：long getSessionId(); byte[] getSessionPasswd();获取到这两个参数值之后，就可以在下次创建 ZooKeeper 对象实例的时候传入构造方法了。

选取几个典型的构造方法来带领大家解读下文档。

3.1.1 ZooKeeper(String connectString, int sessionTimeout, Watcher watcher)

public ZooKeeper(String connectString,
                 int sessionTimeout,
                 Watcher watcher)
          throws IOException
          
To create a ZooKeeper client object, the application needs to pass a connection string containing a comma separated list of host:port pairs, each corresponding to a ZooKeeper server.
Session establishment is asynchronous. This constructor will initiate connection to the server and return immediately - potentially (usually) before the session is fully established. The watcher argument specifies the watcher that will be notified of any changes in state. This notification can come at any point before or after the constructor call has returned.

The instantiated ZooKeeper client object will pick an arbitrary server from the connectString and attempt to connect to it. If establishment of the connection fails, another server in the connect string will be tried (the order is non-deterministic, as we random shuffle the list), until a connection is established. The client will continue attempts until the session is explicitly closed.

Added in 3.2.0: An optional "chroot" suffix may also be appended to the connection string. This will run the client commands while interpreting all paths relative to this root (similar to the unix chroot command).

Parameters:
connectString - comma separated host:port pairs, each corresponding to a zk server. e.g. "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002" If the optional chroot suffix is used the example would look like: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the client would be rooted at "/app/a" and all paths would be relative to this root - ie getting/setting/etc... "/foo/bar" would result in operations being run on "/app/a/foo/bar" (from the server perspective).
sessionTimeout - session timeout in milliseconds
watcher - a watcher object which will be notified of state changes, may also be notified for node events
Throws:
IOException - in cases of network failure
IllegalArgumentException - if an invalid chroot path is specified

有一点需要特别说明下，文档说客户端和服务端建立会话是 异步 的。构造方法会在处理完客户端初始化工作后立即返回，在通常情况下，此时并没有真正建立好一个可用的会话，此时在会话的生命周期中处于“CONNECTING”的状态。当该会话真正创建完毕后，ZooKeeper 服务端会向会话对应的客户端发送一个事件通知，以告知客户端，客户端只有在获取这个通知后，才算真正建立了会话。

实例化的 ZooKeeper 客户端对象将从 connectString 列举的服务器中 随机 选择一个服务器，并尝试连接到该服务器。如果建立连接失败，将尝试连接另一个服务器（顺序是不确定的，因为列举的服务器是随机洗牌的），直到建立连接。即客户端连接一个服务器失败，将继续尝试，直到会话显式关闭。

从 3.2.0版本 开始添加了可选的"chroot"后缀，意思就是可将“chroot”加在 connectString 中列举的服务器后面，即客户端连上 ZooKeeper 服务器后，所有对 ZooKeeper 的操作都会基于这个 根目录。

对参数做下简要说明：

参数	描述
connectString	指定 ZooKeeper 服务器列表，有英文逗号分隔的 host：port 字符串组成，如"127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002"。可以指定客户端连上 connectString 中服务器后的根目录，如 "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" ，`对ZooKeeper的操作都会基于/app/a这个根目录，即创建路径为"/foo/bar"的节点，实际该节点的路径为"/app/a/foo/bar"` 。
sessionTimeout	会话的超时事件，以“毫秒”为单位的整型值。在一个会话周期内，ZooKeeper 客户端和服务器之间会通过心跳检测机制来维持会话的有效性，一旦在 sessionTimeout 时间内没有进行有效的心跳检测，会话就会失效。
watcher	ZooKeeper 允许客户端在构造方法中传入一个接口 Watcher（org.apache.zookeeper.Watcher）的实现类对象来作为默认的 Watch 事件通知器。该参数也可以设置为 `null`，表明不需要设置默认的 Watch 处理器。

3.1.2 ZooKeeper(String connectString, int sessionTimeout, Watcher watcher, boolean canBeReadOnly)

public ZooKeeper(String connectString,
                 int sessionTimeout,
                 Watcher watcher,
                 boolean canBeReadOnly)
          throws IOException
          
To create a ZooKeeper client object, the application needs to pass a connection string containing a comma separated list of host:port pairs, each corresponding to a ZooKeeper server.
Session establishment is asynchronous. This constructor will initiate connection to the server and return immediately - potentially (usually) before the session is fully established. The watcher argument specifies the watcher that will be notified of any changes in state. This notification can come at any point before or after the constructor call has returned.

The instantiated ZooKeeper client object will pick an arbitrary server from the connectString and attempt to connect to it. If establishment of the connection fails, another server in the connect string will be tried (the order is non-deterministic, as we random shuffle the list), until a connection is established. The client will continue attempts until the session is explicitly closed.

Added in 3.2.0: An optional "chroot" suffix may also be appended to the connection string. This will run the client commands while interpreting all paths relative to this root (similar to the unix chroot command).

Parameters:
connectString - comma separated host:port pairs, each corresponding to a zk server. e.g. "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002" If the optional chroot suffix is used the example would look like: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the client would be rooted at "/app/a" and all paths would be relative to this root - ie getting/setting/etc... "/foo/bar" would result in operations being run on "/app/a/foo/bar" (from the server perspective).
sessionTimeout - session timeout in milliseconds
watcher - a watcher object which will be notified of state changes, may also be notified for node events
canBeReadOnly - (added in 3.4) whether the created client is allowed to go to read-only mode in case of partitioning. Read-only mode basically means that if the client can't find any majority servers but there's partitioned server it could reach, it connects to one in read-only mode, i.e. read requests are allowed while write requests are not. It continues seeking for majority in the background.
Throws:
IOException - in cases of network failure
IllegalArgumentException - if an invalid chroot path is specified

这个构造方法跟上个方法非常相似，只是从 3.4版本 开始多了一个 canBeReadOnly 参数，用于标识当前会话是否支持“read-only”模式（只读模式）。默认情况下，在 ZooKeeper 集群中，一个节点如果和集群中过半及以上节点失去网络连接（建立不了连接），那么这个机器将不再处理客户端请求（包括读写请求）。但是在某些使用场景下，当 ZooKeeper 服务器发生此类故障的时候，还是希望 ZooKeeper 服务器能够提供读服务（写服务肯定无法提供），这就是 ZooKeeper 的 “read-only”模式。但客户端可以连接某一分区的服务器，它将以只读模式连接到其中一个服务器，允许读取请求，而不允许写入请求，然后继续在后台寻找更多数的服务器(这一句我描述的不够简练精准)。

3.2 新增

public String create(String path,
                     byte[] data,
                     List<ACL> acl,
                     CreateMode createMode)
              throws KeeperException,
                     InterruptedException
                     
Create a node with the given path. The node data will be the given data, and node acl will be the given acl.
The flags argument specifies whether the created node will be ephemeral or not.

An ephemeral node will be removed by the ZooKeeper automatically when the session associated with the creation of the node expires.

The flags argument can also specify to create a sequential node. The actual path name of a sequential node will be the given path plus a suffix "i" where i is the current sequential number of the node. The sequence number is always fixed length of 10 digits, 0 padded. Once such a node is created, the sequential number will be incremented by one.

If a node with the same actual path already exists in the ZooKeeper, a KeeperException with error code KeeperException.NodeExists will be thrown. Note that since a different actual path is used for each invocation of creating sequential node with the same path argument, the call will never throw "file exists" KeeperException.

If the parent node does not exist in the ZooKeeper, a KeeperException with error code KeeperException.NoNode will be thrown.

An ephemeral node cannot have children. If the parent node of the given path is ephemeral, a KeeperException with error code KeeperException.NoChildrenForEphemerals will be thrown.

This operation, if successful, will trigger all the watches left on the node of the given path by exists and getData API calls, and the watches left on the parent node by getChildren API calls.

If a node is created successfully, the ZooKeeper server will trigger the watches on the path left by exists calls, and the watches on the parent of the node by getChildren calls.

The maximum allowable size of the data array is 1 MB (1,048,576 bytes). Arrays larger than this will cause a KeeperExecption to be thrown.

Parameters:
path - the path for the node
data - the initial data for the node
acl - the acl for the node
createMode - specifying whether the node to be created is ephemeral and/or sequential
Returns:
the actual path of the created node
Throws:
KeeperException - if the server returns a non-zero error code
KeeperException.InvalidACLException - if the ACL is invalid, null, or empty
InterruptedException - if the transaction is interrupted
IllegalArgumentException - if an invalid path is specified

Talk is cheap. Show me the code.这里我们不瞎 BB，直接上官方文档。官方文档是不是很容易看懂，而且解释的非常清楚（而且稍显啰嗦的感觉）？

这里简单列下文档中的几个关键点：

按指定路径和节点形式创建，可指定节点为持久节点、临时节点等。
这里要说下 CreateMode,大家可能都说 ZooKeeper 只有 4 种形式的节点（持久、临时、持久顺序、临时顺序），看文档的话，其实是有 7种 形式的。

public enum CreateMode {
   PERSISTENT(0, false, false, false, false),
   PERSISTENT_SEQUENTIAL(2, false, true, false, false),
   EPHEMERAL(1, true, false, false, false),
   EPHEMERAL_SEQUENTIAL(3, true, true, false, false),
   CONTAINER(4, false, false, true, false),
   PERSISTENT_WITH_TTL(5, false, false, false, true),
   PERSISTENT_SEQUENTIAL_WITH_TTL(6, false, true, false, true);
}

PERSISTENT：持久节点（也有叫永久节点的），不会随着会话的结束而自动删除。
PERSISTENT_SEQUENTIAL：带单调递增序号的持久节点，不会随着会话的结束而自动删除。
EPHEMERAL：临时节点，会随着会话的结束而自动删除。
EPHEMERAL_SEQUENTIAL：带单调递增序号的临时节点，会随着会话的结束而自动删除。
CONTAINER：容器节点，用于 Leader、Lock 等特殊用途，当容器节点不存在任何子节点时，容器将成为服务器在将来某个时候删除的候选节点。
PERSISTENT_WITH_TTL：带 TTL（time-to-live，存活时间）的持久节点，节点在 TTL 时间之内没有得到更新并且没有子节点，就会被自动删除。
PERSISTENT_SEQUENTIAL_WITH_TTL：带 TTL（time-to-live，存活时间）和单调递增序号的持久节点，节点在 TTL 时间之内没有得到更新并且没有子节点，就会被自动删除。

如果指令路径和版本的节点已经存在，则会抛出一个 KeeperException 异常。
临时节点不能有子节点。如果给临时节点创建子节点会抛 KeeperException 异常。
临时节点的生命周期与客户端会话绑定。一旦客户端会话失效（客户端与 Zookeeper 连接断开不一定会话失效），那么这个客户端创建的所有临时节点都会被移除。
byte[] data允许的最大数据量为1MB（1,048,576 bytes）。如果超过，会抛 KeeperExecption。

运行创建节点的代码：

    @Test
    public void testCreate() throws KeeperException, InterruptedException {
        zk.create(ZNODE_PATH, "anna2019".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    }

可以通过日志信息得到节点创建成功：

DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x101402626bb000b, packet:: clientPath:null serverPath:null finished:false header:: 1,1  replyHeader:: 1,12884901937,0  request:: '/zk_demo,#616e6e6132303139,v{s{31,s{'world,'anyone}}},0  response:: '/zk_demo

在服务端查看,/zk_demo 节点创建成功：

[zk: 127.0.0.1:2181(CONNECTED) 21] ls /
[zookeeper, zk_demo]
[zk: 127.0.0.1:2181(CONNECTED) 22] stat /zk_demo
cZxid = 0x300000031
ctime = Tue Dec 17 12:52:50 CST 2019
mZxid = 0x300000031
mtime = Tue Dec 17 12:52:50 CST 2019
pZxid = 0x300000031
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 8
numChildren = 0

3.3 获取

public byte[] getData(String path,
                      boolean watch,
                      Stat stat)
               throws KeeperException,
                      InterruptedException

Return the data and the stat of the node of the given path.
If the watch is true and the call is successful (no exception is thrown), a watch will be left on the node with the given path. The watch will be triggered by a successful operation that sets data on the node, or deletes the node.

A KeeperException with error code KeeperException.NoNode will be thrown if no node with the given path exists.

Parameters:
path - the given path
watch - whether need to watch this node
stat - the stat of the node
Returns:
the data of the node
Throws:
KeeperException - If the server signals an error with a non-zero error code
InterruptedException - If the server transaction is interrupted.

指定路径的节点不存在时就抛 KeeperException.NoNode 异常。

运行：

    @Test
    public void testGet() throws KeeperException, InterruptedException {
        byte[] data1 = zk.getData(ZNODE_PATH, false, null);
        byte[] data2 = zk.getData(ZNODE_PATH_PARENT, false, null);
        byte[] data3 = zk.getData(ZNODE_PATH_CHILDREN, false, null);
        LOGGER.info("{}的信息：{}", ZNODE_PATH, new String(data1) );
        LOGGER.info("{}的信息：{}", ZNODE_PATH_PARENT, new String(data2) );
        LOGGER.info("{}的信息：{}", ZNODE_PATH_CHILDREN, new String(data3) );
    }

结果：

13:51:00.288 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - /zk_demo的信息：anna2019
13:51:00.288 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - /app1的信息：anna2019
13:51:00.289 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - /app1/app1_1的信息：anna2020

3.4 更新

public Stat setData(String path,
                    byte[] data,
                    int version)
             throws KeeperException,
                    InterruptedException
                    
Set the data for the node of the given path if such a node exists and the given version matches the version of the node (if the given version is -1, it matches any node's versions). Return the stat of the node.
This operation, if successful, will trigger all the watches on the node of the given path left by getData calls.

A KeeperException with error code KeeperException.NoNode will be thrown if no node with the given path exists.

A KeeperException with error code KeeperException.BadVersion will be thrown if the given version does not match the node's version.

The maximum allowable size of the data array is 1 MB (1,048,576 bytes). Arrays larger than this will cause a KeeperException to be thrown.

Parameters:
path - the path of the node
data - the data to set
version - the expected matching version
Returns:
the state of the node
Throws:
InterruptedException - If the server transaction is interrupted.
KeeperException - If the server signals an error with a non-zero error code.
IllegalArgumentException - if an invalid path is specified

主要注意以下几点：

版本为-1 时，即代表适配指定路径节点的所有版本。
如果指定路径的节点不存在会抛 KeeperException.NoNode 异常，该节点没有传入的版本，会抛 KeeperException.BadVersion 异常。
byte[] data允许的最大数据量为1MB（1,048,576 bytes）。如果超过，会抛 KeeperExecption。

运行：

    @Test
    public void testSet() throws KeeperException, InterruptedException {
        Stat stat = zk.setData(ZNODE_PATH, "yuanrengu".getBytes(), -1);
        byte[] data = zk.getData(ZNODE_PATH, false, null);
        LOGGER.info(new String(data));
    }

可以看到数据已经更新：

15:46:16.472 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - yuanrengu

更新的接口提到了版本的概念，上面提到版本为-1 时，即代表适配指定路径节点的所有版本。节点每次setData时版本会加1，更新时指定的版本不存在会报KeeperException.BadVersion异常。我们做个测试：

    @Test
    public void testSetForVersion() throws KeeperException, InterruptedException {
        String pathVersion = "/versionDemo";
        zk.create(pathVersion, "yuanrengu2019".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

        Stat stat = zk.setData(pathVersion, "yuanrengu2020".getBytes(), -1);
        LOGGER.info("============1111111  start======================");
        LOGGER.info(String.valueOf(stat));
        LOGGER.info("version:{}", stat.getVersion());
        byte[] data1 = zk.getData(pathVersion, false, null);
        LOGGER.info("data1:{}", new String(data1));
        LOGGER.info("============1111111  end======================");

        Stat stat2 = zk.setData(pathVersion, "yuanrengu2021".getBytes(), stat.getVersion());
        LOGGER.info("============222222  start======================");
        LOGGER.info(String.valueOf(stat2));
        LOGGER.info("version2:{}", stat2.getVersion());
        byte[] data2 = zk.getData(pathVersion, false, null);
        LOGGER.info("data2:{}", new String(data2));
        LOGGER.info("============222222  end======================");

        Stat stat3 = zk.setData(pathVersion, "yuanrengu2022".getBytes(), stat.getVersion());
        LOGGER.info("============3333333  start======================");
        LOGGER.info(String.valueOf(stat3));
        LOGGER.info("version3:{}", stat3.getVersion());
        byte[] data3 = zk.getData(pathVersion, false, null);
        LOGGER.info("data3:{}", new String(data3));
        LOGGER.info("============3333333  end======================");
    }

运行结果如下：

09:56:00.931 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x1014dce26220008, packet:: clientPath:null serverPath:null finished:false header:: 1,5  replyHeader:: 1,12884902005,0  request:: '/versionDemo,#7975616e72656e677532303230,-1  response:: s{12884901996,12884902005,1576720362715,1576720560918,1,0,0,0,13,0,12884901996} 
09:56:00.940 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - ============1111111  start======================
09:56:00.941 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - 12884901996,12884902005,1576720362715,1576720560918,1,0,0,0,13,0,12884901996

09:56:00.942 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - version:1
09:56:00.971 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x1014dce26220008, packet:: clientPath:null serverPath:null finished:false header:: 2,4  replyHeader:: 2,12884902005,0  request:: '/versionDemo,F  response:: #7975616e72656e677532303230,s{12884901996,12884902005,1576720362715,1576720560918,1,0,0,0,13,0,12884901996} 
09:56:00.971 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - data1:yuanrengu2020
09:56:00.971 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - ============1111111  end======================
09:56:00.988 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x1014dce26220008, packet:: clientPath:null serverPath:null finished:false header:: 3,5  replyHeader:: 3,12884902006,0  request:: '/versionDemo,#7975616e72656e677532303231,1  response:: s{12884901996,12884902006,1576720362715,1576720561002,2,0,0,0,13,0,12884901996} 
09:56:00.988 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - ============222222  start======================
09:56:00.988 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - 12884901996,12884902006,1576720362715,1576720561002,2,0,0,0,13,0,12884901996

09:56:00.990 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - version2:2
09:56:01.017 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x1014dce26220008, packet:: clientPath:null serverPath:null finished:false header:: 4,4  replyHeader:: 4,12884902006,0  request:: '/versionDemo,F  response:: #7975616e72656e677532303231,s{12884901996,12884902006,1576720362715,1576720561002,2,0,0,0,13,0,12884901996} 
09:56:01.017 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - data2:yuanrengu2021
09:56:01.017 [main] INFO com.yuanrengu.demo.ZooKeeperDemo - ============222222  end======================
09:56:01.037 [main-SendThread(106.12.111.172:2181)] DEBUG org.apache.zookeeper.ClientCnxn - Reading reply sessionid:0x1014dce26220008, packet:: clientPath:null serverPath:null finished:false header:: 5,5  replyHeader:: 5,12884902007,-103  request:: '/versionDemo,#7975616e72656e677532303232,1  response::  

org.apache.zookeeper.KeeperException$BadVersionException: KeeperErrorCode = BadVersion for /versionDemo

	at org.apache.zookeeper.KeeperException.create(KeeperException.java:122)
	at org.apache.zookeeper.KeeperException.create(KeeperException.java:54)
	at org.apache.zookeeper.ZooKeeper.setData(ZooKeeper.java:2384)

测试代码进行了 3 次 setData 操作，第一次 setData 时传入的版本为-1，成功后 version 变为 1；第二次 setData 时传入的 version 为 1，成功后 version 变为 2；第三次 setData 时传入的版本为 1，此时就抛了 KeeperException.BadVersion 异常。如果第三次 setData 传入的版本为-1，能更新成功。

3.5 删除

public void delete(String path,
                   int version)
            throws InterruptedException,
                   KeeperException
                   
Delete the node with the given path. The call will succeed if such a node exists, and the given version matches the node's version (if the given version is -1, it matches any node's versions).
A KeeperException with error code KeeperException.NoNode will be thrown if the nodes does not exist.

A KeeperException with error code KeeperException.BadVersion will be thrown if the given version does not match the node's version.

A KeeperException with error code KeeperException.NotEmpty will be thrown if the node has children.

This operation, if successful, will trigger all the watches on the node of the given path left by exists API calls, and the watches on the parent node left by getChildren API calls.

Parameters:
path - the path of the node to be deleted.
version - the expected node version.
Throws:
InterruptedException - IF the server transaction is interrupted
KeeperException - If the server signals an error with a non-zero return code.
IllegalArgumentException - if an invalid path is specified

节点可能含有子节点，删除节点的操作有几点需要特别注意：

版本为-1 时，即代表适配指定路径节点的所有版本。
如果指定路径的节点不存在会抛 KeeperException.NoNode 异常，该节点没有传入的版本，会抛 KeeperException.BadVersion 异常。
如果节点含有子节点，删除父节点(parent node)时会抛KeeperException.NotEmpty异常。

在 ZooKeeper 中，只允许删子节点。如果一个节点存在一个或多个子节点，该节点就无法被直接删除，必须先删除所有子节点。

/app1 有子节点，我们做下删除操作：

    /**
     *  删除含有子节点的父节点
     * @throws KeeperException
     * @throws InterruptedException
     */
    @Test
    public void testDeleteHasChildrenZnode() throws KeeperException, InterruptedException {
        // 指定要删除的版本，-1表示删除所有版本
        zk.delete(ZNODE_PATH_PARENT, -1);
    }

可以看到日志：

org.apache.zookeeper.KeeperException$NotEmptyException: KeeperErrorCode = Directory not empty for /app1

	at org.apache.zookeeper.KeeperException.create(KeeperException.java:132)
	at org.apache.zookeeper.KeeperException.create(KeeperException.java:54)
	at org.apache.zookeeper.ZooKeeper.delete(ZooKeeper.java:1793)
	at com.yuanrengu.demo.ZooKeeperDemo.testDeleteHasChildrenZnode(ZooKeeperDemo.java:89)

4 总结

上面我们实现了节点的增、删、改、查的测试，后面的篇章会有更多好玩的用法，如实现分布式锁、配置中心等。

基于上面的分析，总结几个注意的点：

节点有 7种形式：

PERSISTENT：持久节点（也有叫永久节点的），不会随着会话的结束而自动删除。
PERSISTENT_SEQUENTIAL：带单调递增序号的持久节点，不会随着会话的结束而自动删除。
EPHEMERAL：临时节点，会随着会话的结束而自动删除。
EPHEMERAL_SEQUENTIAL：带单调递增序号的临时节点，会随着会话的结束而自动删除。
CONTAINER：容器节点，用于 Leader、Lock 等特殊用途，当容器节点不存在任何子节点时，容器将成为服务器在将来某个时候删除的候选节点。
PERSISTENT_WITH_TTL：带 TTL（time-to-live，存活时间）的持久节点，节点在 TTL 时间之内没有得到更新并且没有子节点，就会被自动删除。
PERSISTENT_SEQUENTIAL_WITH_TTL：带 TTL（time-to-live，存活时间）和单调递增序号的持久节点，节点在 TTL 时间之内没有得到更新并且没有子节点，就会被自动删除。

临时节点不能有子节点。如果给临时节点创建子节点会抛 KeeperException 异常。
临时节点的生命周期与客户端会话绑定。一旦客户端会话失效（客户端与 Zookeeper 连接断开不一定会话失效），那么这个客户端创建的所有临时节点都会被移除。
byte[] data允许的最大数据量为1MB（1,048,576 bytes）。

【ZooKeeper 系列】2. 用 Java 实现 ZooKeeper API 的调用

1 简介

2 测试环境搭建

3 API 测试

3.1 创建会话

3.1.1 ZooKeeper(String connectString, int sessionTimeout, Watcher watcher)

3.1.2 ZooKeeper(String connectString, int sessionTimeout, Watcher watcher, boolean canBeReadOnly)

3.2 新增

3.3 获取

3.4 更新

3.5 删除

4 总结

相关帖子

Java 笔记系列——08- 分布式协调（Zookeeper）

Zookeeper 基础（一）

ZooKeeper 基础

zookeeper 群起脚本

CentOS7 安装 Zookeeper-3.4.5

ZooKeeper 典型应用场景一览

js 代码片段模拟 window.prompt 函数

欢迎来到这里！