How do I know if a table column can have a null value or not?

In this example we’ll show how to use ResultSetMetaData.isNullable() method to know if a column can be null or not. This method returns an integer which values defined in the constants of ResultSetMetaData.columnNullable, ResultSetMetaData.columnNoNulls and ResultSetMetaData.columnNullableUnknown.

package org.kodejava.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/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(
                    "SELECT id, isbn, title, published_year, price FROM book");

            int columnIndex = 2;
            // The ResultSetMetaData is where all metadata related
            // information for a result set is stored.
            ResultSetMetaData metadata = resultSet.getMetaData();
            String columnName = metadata.getColumnName(columnIndex);
            int nullable = metadata.isNullable(columnIndex);

            // Check the nullable status of a column (`isbn`)
            if (nullable == ResultSetMetaData.columnNullable) {
                System.out.printf("`%s` can have a null value.%n", columnName);
            } else if (nullable == ResultSetMetaData.columnNoNulls) {
                System.out.printf("`%s` does not allowed to have a " +
                                  "null value.%n", columnName);
            } else if (nullable == ResultSetMetaData.columnNullableUnknown) {
                System.out.println("Nullability unknown.");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>