Java中ResultSetMetaData元数据的具体使用

1. 前言

ResultSetMetaData 叫元数据，是数据库列对象，以列为单位封装为对象。

元数据，指的是其包含列名，列值，列类型，列长度等等有用信息。

2. 常用方法介绍

ResultSetMetaData 常用方法：

1). metaData.getColumnName(i) 获取该列的原始名字
2). metaData.getColumnLabel(i) 获取该列的别名
3). metaData.getColumnClassName(i) 获取该列的(在java中的)数据类型
4). metaData.getColumnType(i) 获取该列的(在数据库中的)数据类型对应的序号
5). metaData.getColumnTypeName(i) 获取该列的(在数据库中的)数据类型
6). metaData.getScale(i) 获取该列中小数点右边的位数
7). metaData.getColumnDisplaySize(i) 获取该列的长度
8). metaData.isAutoIncrement(i) 判断该列的值是否自动递增
9). metaData.isNullable(i) 判断该列的值是否为null
10). metaData.getTableName(i) 获取表名

3. 代码演示

先准备好一张表，如下图：

代码：

public class Test {

    private static final String URL = "jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC&useUnicode=true&characterEncoding=utf8";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "root";

    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.cj.jdbc.Driver");
        Connection connection = DriverManager.getConnection(URL, USERNAME, PASSWORD);
        // sql 语句 是使用了 别名的
        PreparedStatement preparedStatement = connection.prepareStatement("select id as ID, username as USERNAME, birthday as BIRTHDAY, sex as SEX, address as ADDRESS, money as MONEY  from user ");
        ResultSet resultSet = preparedStatement.executeQuery();
        // 获取元数据对象
        ResultSetMetaData metaData = resultSet.getMetaData();
        // 获取一共有多少列
        int columnCount = metaData.getColumnCount();
        // 将数据封装为Map
        List<Map<String, Object>> list = new ArrayList<>();
        while (resultSet.next()) {
            Map<String, Object> columnMap = new HashMap<>();
            // 注：列名的索引 起始是 1 不是 0
            for (int i = 1; i <= columnCount; i++) {
                System.out.println("getColumnName(i): " + metaData.getColumnName(i));
                System.out.println("getColumnLabel(i): " + metaData.getColumnLabel(i));
                System.out.println("getColumnClassName(i): " + metaData.getColumnClassName(i));
                System.out.println("getColumnType(i): " + metaData.getColumnType(i));
                System.out.println("getColumnTypeName(i): " + metaData.getColumnTypeName(i));
                System.out.println("getScale(i): " + metaData.getScale(i));
                System.out.println("isNullable(i): " + metaData.isNullable(i));
                System.out.println("isAutoIncrement(i): " + metaData.isAutoIncrement(i));
                System.out.println("getTableName(i): " + metaData.getTableName(i));
                System.out.println();

                String key = metaData.getColumnName(i);
                Object value = resultSet.getObject(key);
                columnMap.put(key, value);
            }
            list.add(columnMap);
        }
        System.out.println();
        System.out.println(list);
        resultSet.close();
        preparedStatement.close();
        connection.close();
    }
}

4. 结果图(一部分)

getColumnName(i): id
getColumnLabel(i): ID
getColumnClassName(i): java.lang.Integer
getColumnType(i): 4
getColumnTypeName(i): INT
getScale(i): 0
isNullable(i): 0
isAutoIncrement(i): true   // id 自增 因此为true
getTableName(i): user

getColumnName(i): username
getColumnLabel(i): USERNAME
getColumnClassName(i): java.lang.String
getColumnType(i): 12
getColumnTypeName(i): VARCHAR
getScale(i): 0
isNullable(i): 0
isAutoIncrement(i): false
getTableName(i): user

getColumnName(i): birthday
getColumnLabel(i): BIRTHDAY
getColumnClassName(i): java.time.LocalDateTime
getColumnType(i): 93
getColumnTypeName(i): DATETIME
getScale(i): 0
isNullable(i): 1
isAutoIncrement(i): false
getTableName(i): user

getColumnName(i): sex
getColumnLabel(i): SEX
getColumnClassName(i): java.lang.String
getColumnType(i): 1
getColumnTypeName(i): CHAR
getScale(i): 0
isNullable(i): 1
isAutoIncrement(i): false
getTableName(i): user

getColumnName(i): address
getColumnLabel(i): ADDRESS
getColumnClassName(i): java.lang.String
getColumnType(i): 12
getColumnTypeName(i): VARCHAR
getScale(i): 0
isNullable(i): 1
isAutoIncrement(i): false
getTableName(i): user

getColumnName(i): money
getColumnLabel(i): MONEY
getColumnClassName(i): java.math.BigDecimal
getColumnType(i): 3
getColumnTypeName(i): DECIMAL
getScale(i): 3     // 在数据库中该列值是 decimal 且是3位小数 因此得出 3
isNullable(i): 1
isAutoIncrement(i): false
getTableName(i): user

list：结果：
[
{birthday=2021-02-27T17:47:08, address=北京, money=10.580, sex=男, id=41, username=老王},
{birthday=2021-03-02T15:09:37, address=北京, money=10.580, sex=女, id=42, username=小二王},
{birthday=2021-03-04T11:34:34, address=北京, money=10.580, sex=女, id=43, username=小二王},
{birthday=2021-03-04T12:04:06, address=北京, money=10.580, sex=男, id=45, username=大王},
{birthday=2021-03-07T17:37:26, address=北京, money=10.580, sex=男, id=46, username=老王},
{birthday=2021-03-08T11:44, address=北京, money=10.580, sex=女, id=48, username=小马},
{birthday=null, address=null, money=null, sex=男, id=50, username=kkooop}
]