元数据

元数据

在jdbc中获取数据库的定义，例如：数据库、表、列的定义信息。就用到元数据。

 数据库元数据、参数元数据、结果集元数据

.MetaData()

1.数据库元数据

​​package com.cn.metadata;​​​​​​​​import java.sql.Connection;​​​​import java.sql.DatabaseMetaData;​​​​​​​​import org.junit.Test;​​​​​​​​import com.cn.util.JdbcUtil;​​​​​​​​public class Demo1 {​​​​  ​​​​  /**​​​​  * 1.数据库元数据​​​​  * @throws Exception​​​​   */​​​​ @Test​​​​ public void test1() throws Exception {​​​​    //获取连接​​​​    Connection conn = JdbcUtil.getConnection();​​​​   //获取数据库元数据​​​​    DatabaseMetaData metaData = conn.getMetaData();//alt+shift+L   快速获取方法返回值​​​​    ​​​​    /**​​​​    * 获取数据库元数据​​​​    */​​​​   System.out.println(metaData.getUserName());​​​​   System.out.println(metaData.getURL());​​​​    System.out.println(metaData.getDatabaseProductName());​​​​    System.out.println(metaData.getDatabaseProductVersion());​​​​   System.out.println(metaData.getDriverVersion());​​​​    ​​​​  }​​​​}​​

 2.参数元数据

​​package com.cn.metadata;​​​​​​​​import java.sql.Connection;​​​​import java.sql.ParameterMetaData;​​​​import java.sql.PreparedStatement;​​​​​​​​import org.junit.Test;​​​​​​​​import com.cn.util.JdbcUtil;​​​​​​​​public class Demo1 {​​​​ ​​​​  /**​​​​  * 2.参数元数据​​​​   * @throws Exception​​​​   */​​​​ @Test​​​​ public void test2() throws Exception {​​​​    //获取连接​​​​    Connection conn = JdbcUtil.getConnection();​​​​   ​​​​    //sql​​​​   String sql = "SELECT * FROM dept where id=? and deptName=?";​​​​    ​​​​    PreparedStatement pstmt = conn.prepareStatement(sql);​​​​   ParameterMetaData paraMetaData = pstmt.getParameterMetaData();​​​​    ​​​​    //获取参数的个数​​​​   int count = paraMetaData.getParameterCount();​​​​   System.out.println(count);//结果为2​​​​  }​​​​}​​

3.结果集元数据

​​package com.cn.metadata;​​​​​​​​import java.sql.Connection;​​​​import java.sql.PreparedStatement;​​​​import java.sql.ResultSet;​​​​import java.sql.ResultSetMetaData;​​​​​​​​import org.junit.Test;​​​​​​​​import com.cn.util.JdbcUtil;​​​​​​​​public class Demo1 {​​​​ ​​​​  /**​​​​  * 3.结果集元数据​​​​  */​​​​ @Test​​​​ public void test3() throws Exception {​​​​    //获取连接​​​​    Connection conn = JdbcUtil.getConnection();​​​​   ​​​​    //sql​​​​   String sql = "SELECT * FROM dept";​​​​    ​​​​    PreparedStatement pstmt = conn.prepareStatement(sql);​​​​   ResultSet rs = pstmt.executeQuery();​​​​    ResultSetMetaData resultMetaData = rs.getMetaData();​​​​    ​​​​    /**​​​​    * 得到结果集元数据（目标：通过结果集元数据，得到列的名称）​​​​    */​​​​   //迭代每一行结果​​​​   while(rs.next()){​​​​     //1.获取列的个数​​​​      int columnCount = resultMetaData.getColumnCount();​​​​      //2.遍历，获取每一列的值​​​​      for(int i=1; i<=columnCount; i++){​​​​       //得到每一列的列名称​​​​       String columnName = resultMetaData.getColumnName(i);//从1开始​​​​        //获取该行对应的列的值​​​​        Object columnValue = rs.getObject(columnName);​​​​        System.out.print(columnName + "=" + columnValue + ", ");​​​​      }​​​​     System.out.println();​​​​   }​​​​ }​​​​}​​

 附上自定义的Jdbc工具类

​​package com.cn.util;​​​​​​​​import java.io.InputStream;​​​​import java.sql.Connection;​​​​import java.sql.DriverManager;​​​​import java.sql.ResultSet;​​​​import java.sql.SQLException;​​​​import java.sql.Statement;​​​​import java.util.Properties;​​​​​​​​/**​​​​ * jdbc的工具类​​​​ * @author liuzhiyong​​​​ *​​​​ */​​​​public class JdbcUtil {​​​​  private static String url = null;​​​​ private static String user = null;​​​​  private static String password = null;​​​​  private static String driverClass = null;​​​​ ​​​​  /**​​​​  * 静态代码块（只调用一次）​​​​  */​​​​ static{​​​​   try {​​​​     //读取db.properties文件​​​​     Properties properties = new Properties();​​​​     ​​​​      /**​​​​      * 使用类路径的读取方式​​​​      *  /  斜杠表示classpath的根目录​​​​       * 在java项目下，classpath的根目录从bin目录开始​​​​      * 在web项目下，classpath的根目录从WEB-INF/classes目录开始​​​​       */​​​​     InputStream in = JdbcUtil.class.getResourceAsStream("/db.properties");​​​​      properties.load(in);​​​​      //读取信息​​​​      url = properties.getProperty("url");​​​​      user = properties.getProperty("user");​​​​      password = properties.getProperty("password");​​​​      driverClass = properties.getProperty("driverClass");​​​​      Class.forName(driverClass);​​​​   } catch (Exception e) {​​​​     e.printStackTrace();​​​​      System.out.println("驱动程序注册出错！");​​​​    }​​​​ }​​​​ ​​​​  /**​​​​  * 获取连接对象的方法​​​​   */​​​​ public static Connection getConnection(){​​​​   ​​​​    try {​​​​     Connection conn = DriverManager.getConnection(url, user, password);​​​​     return conn;​​​​    } catch (SQLException e) {​​​​      // TODO Auto-generated catch block​​​​      e.printStackTrace();​​​​      throw new RuntimeException(e);​​​​    }​​​​ }​​​​​​​​ /**​​​​  * 释放资源的重载方法​​​​   */​​​​ public static void close(Connection conn, Statement stmt, ResultSet rs){​​​​    ​​​​    //关闭资源（顺序：后打开，先关闭）​​​​    if(rs != null){​​​​     try {​​​​       rs.close();​​​​     } catch (SQLException e) {​​​​        System.out.println("ResultSet关闭失败！");​​​​       throw new RuntimeException(e);​​​​      }​​​​   }if(stmt != null){​​​​      try {​​​​       stmt.close();​​​​     } catch (SQLException e) {​​​​        System.out.println("Statement关闭失败！");​​​​       throw new RuntimeException(e);​​​​      }​​​​   }​​​​   if(conn != null){​​​​     try {​​​​       conn.close();​​​​     } catch (SQLException e) {​​​​        System.out.println("Connection关闭失败！");​​​​        throw new RuntimeException(e);​​​​      }​​​​   }​​​​ }​​​​ ​​​​  /**​​​​  * 释放资源的重载方法​​​​   */​​​​ public static void close(Connection conn, Statement stmt){​​​​    ​​​​    //关闭资源（顺序：后打开，先关闭）​​​​    if(stmt != null){​​​​     try {​​​​       stmt.close();​​​​     } catch (SQLException e) {​​​​        System.out.println("Statement关闭失败！");​​​​       throw new RuntimeException(e);​​​​      }​​​​   }​​​​   if(conn != null){​​​​     try {​​​​       conn.close();​​​​     } catch (SQLException e) {​​​​        System.out.println("Connection关闭失败！");​​​​        throw new RuntimeException(e);​​​​      }​​​​   }​​​​ }​​​​ ​​​​}​​