How to retrieve data from a database using Java?

In Java, you can retrieve data from a database using JDBC (Java Database Connectivity) technology. Here is a simple example code:

import java.sql.*;
public class DatabaseApp {
    public static void main(String[] args) {
        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        try {
            // 连接数据库
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydatabase",
             "username", "password");
            // 创建Statement对象
            stmt = conn.createStatement();
            // 执行查询语句
            String sql = "SELECT * FROM mytable";
            rs = stmt.executeQuery(sql);
            // 处理结果集
            while (rs.next()) {
                int id = rs.getInt("id");
                String name = rs.getString("name");
                int age = rs.getInt("age");
                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接
            try {
                if (rs != null) {
                    rs.close();
                }
                if (stmt != null) {
                    stmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

The code above begins by establishing a connection to the database using the `DriverManager.getConnection` method, then creates a `Statement` object for executing SQL statements. To execute a query, the `executeQuery` method can be used, and the results can be retrieved through a `ResultSet` object. Finally, each row of data is extracted and processed within a `while` loop. It is important to remember to call the `close` method to close the database connection, Statement object, and ResultSet object after using them to release resources and avoid memory leaks.