Java我如何知道表列是否可以具有null值？
在此示例中，我们将展示如何使用ResultSetMetaData.isNullable()method来确定列是否可以null。此方法返回的值在的常数来定义的整数ResultSetMetaData.columnNullable，ResultSetMetaData.columnNoNulls和ResultSetMetaData.columnNullableUnknown。
package org.nhooo.example.jdbc;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
public class IsNullableExample {
private static final String URL = "jdbc:mysql://localhost/nhooo";
private static final String USERNAME = "root";
private static final String PASSWORD = "";
public static void main(String[] args) {
try (Connection connection =
DriverManager.getConnection(URL, USERNAME, PASSWORD)){
Statement statement = connection.createStatement();
ResultSet resultSet = statement.executeQuery(
"SELECT isbn, title, published_year, price FROM books");
//ResultSetMetaData是所有元数据相关的地方
//存储结果集的信息。
ResultSetMetaData metadata = resultSet.getMetaData();
int nullable = metadata.isNullable(1);
//检查列的可空状态（`isbn`）
if (nullable == ResultSetMetaData.columnNullable) {
System.out.println("`isbn` can have a null value.");
} else if (nullable == ResultSetMetaData.columnNoNulls) {
System.out.println("`isbn` does not allowed to have a " +
"null value.");
} else if (nullable == ResultSetMetaData.columnNullableUnknown) {
System.out.println("Nullability unknown.");
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}Maven依赖
<!-- https://search.maven.org/remotecontent?filepath=mysql/mysql-connector-java/8.0.17/mysql-connector-java-8.0.17.jar -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.17</version>
</dependency>