update java document for subscription
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Bomin Zhang
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@ -295,25 +295,28 @@ $ taos
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这时，因为电流超过了10A，您应该可以看到示例程序将它输出到了屏幕上。
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您可以继续插入一些数据观察示例程序的输出。
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### jdbc使用数据订阅功能
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### Java 使用数据订阅功能
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（1）使用订阅功能前的数据准备
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订阅功能也提供了 Java 开发接口，相关说明请见 [Java Connector](https://www.taosdata.com/cn/documentation20/connector/)。需要注意的是，目前 Java 接口没有提供异步订阅模式，但用户程序可以通过创建 `TimerTask` 等方式达到同样的效果。
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```shell
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下面以一个示例程序介绍其具体使用方法。它所完成的功能与前面介绍的 C 语言示例基本相同，也是订阅数据库中所有电流超过 10A 的记录。
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#### 准备数据
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```sql
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# 创建 power 库
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taos> create database power;
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# 切换库
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taos> use power;
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# 创建超级表
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taos> create table meters(ts timestamp, current float, voltage int, phase int) tags(location binary(64), groupI
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d int);
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taos> create table meters(ts timestamp, current float, voltage int, phase int) tags(location binary(64), groupId int);
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# 创建表
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taos> create table d1001 using meters tags ("Beijing.Chaoyang", 2);
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taos> create table d1002 using meters tags ("Beijing.Haidian", 2);
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# 插入测试数据
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taos> insert into d1001 values("2020-08-15 12:00:00.000", 12, 220, 1),("2020-08-15 12:10:00.000", 12.3, 220, 2),("2020-08-15 12:20:00.000", 12.2, 220, 1);
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taos> insert into d1002 values("2020-08-15 12:00:00.000", 9.9, 220, 1),("2020-08-15 12:10:00.000", 10.3, 220, 1),("2020-08-15 12:20:00.000", 11.2, 220, 1);
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# 从超级表meters查询current大于10的数据
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# 从超级表 meters 查询电流大于 10A 的记录
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taos> select * from meters where current > 10;
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ts | current | voltage | phase | location | groupid |
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===========================================================================================================
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@ -325,7 +328,7 @@ taos> select * from meters where current > 10;
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Query OK, 5 row(s) in set (0.004896s)
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```
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（2）使用jdbc提供的订阅功能
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#### 示例程序
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```java
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public class SubscribeDemo {
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@ -337,42 +340,36 @@ public class SubscribeDemo {
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TSDBSubscribe subscribe = null;
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try {
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// 加载驱动
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Class.forName("com.taosdata.jdbc.TSDBDriver");
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// 获取Connectin
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Properties properties = new Properties();
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properties.setProperty(TSDBDriver.PROPERTY_KEY_CHARSET, "UTF-8");
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properties.setProperty(TSDBDriver.PROPERTY_KEY_TIME_ZONE, "UTC-8");
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String jdbcUrl = "jdbc:TAOS://127.0.0.1:6030/power?user=root&password=taosdata";
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connection = DriverManager.getConnection(jdbcUrl, properties);
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// 创建Subscribe，topic为主题名称，sql为查询语句，restar为true代表每次订阅消费历史数据
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subscribe = ((TSDBConnection) connection).subscribe(topic, sql, true);
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subscribe = ((TSDBConnection) connection).subscribe(topic, sql, true); // 创建订阅
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int count = 0;
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while (true) {
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// 消费数据
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TSDBResultSet resultSet = subscribe.consume();
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// 打印结果集
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if (resultSet != null) {
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while (count < 10) {
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TimeUnit.SECONDS.sleep(1); // 等待1秒，避免频繁调用 consume，给服务端造成压力
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TSDBResultSet resultSet = subscribe.consume(); // 消费数据
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if (resultSet == null) {
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continue;
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}
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ResultSetMetaData metaData = resultSet.getMetaData();
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while (resultSet.next()) {
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int columnCount = metaData.getColumnCount();
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for (int i = 1; i <= columnCount; i++) {
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System.out.print(metaData.getColumnLabel(i) + ": " + resultSet.getString(i) + "\t");
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}
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System.out.println("\n====================");
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System.out.println();
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count++;
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}
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}
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if (count > 10)
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break;
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TimeUnit.SECONDS.sleep(1);
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}
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} catch (Exception e) {
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e.printStackTrace();
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} finally {
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try {
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if (null != subscribe)
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subscribe.close(true);
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subscribe.close(true); // 关闭订阅
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if (connection != null)
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connection.close();
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} catch (SQLException throwables) {
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@ -383,38 +380,30 @@ public class SubscribeDemo {
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}
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```
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（3）订阅功能演示
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运行demo，首先，subscribe会将满足情况的历史数据消费
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运行示例程序，首先，它会消费符合查询条件的所有历史数据：
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```shell
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# java -jar subscribe.jar
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ts: 1597464000000 current: 12.0 voltage: 220 phase: 1 location: Beijing.Chaoyang groupid : 2
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====================
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ts: 1597464600000 current: 12.3 voltage: 220 phase: 2 location: Beijing.Chaoyang groupid : 2
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====================
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ts: 1597465200000 current: 12.2 voltage: 220 phase: 1 location: Beijing.Chaoyang groupid : 2
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====================
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ts: 1597464600000 current: 10.3 voltage: 220 phase: 1 location: Beijing.Haidian groupid : 2
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====================
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ts: 1597465200000 current: 11.2 voltage: 220 phase: 1 location: Beijing.Haidian groupid : 2
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====================
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```
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接着，使用taos客户端向表中新增数据
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接着，使用 taos 客户端向表中新增一条数据：
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```shell
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```sql
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# taos
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taos> use power；
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taos> insert into d1001 values("2020-08-15 12:40:00.000", 12.4, 220, 1);
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```
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查看数据消费情况
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因为这条数据的电流大于10A，示例程序会将其消费：
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```shell
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ts: 1597466400000 current: 12.4 voltage: 220 phase: 1 location: Beijing.Chaoyang groupid: 2
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====================
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```
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