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DAO的全称是Data Access Object，旨在将对数据库的操作进行封装，使开发人员可以通过调用简单的方法与数据库进行交互，而无需直接处理连接或事务管理。

DAO的组成部分

示例

假设数据表包含ID、NAME、AGE三个字段，常见的VO类如下：

public class Customer {
    private int id;
    private String name;
    private int age;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "id:" + id + " name:" + name + " age:" + age;
    }
}

数据库连接类DBUtil的实现如下：

public class DBUtil {
    private static String driver;
    private static String url;
    private static String username;
    private static String password;
    static {
        Properties prop = new Properties();
        try {
            Reader in = new FileReader("src/config.properties");
            prop.load(in);
        } catch (Exception e) {
            e.printStackTrace();
        }
        driver = prop.getProperty("driver");
        url = prop.getProperty("url");
        username = prop.getProperty("username");
        password = prop.getProperty("password");
    }
    public static Connection open() {
        try {
            Class.forName(driver);
            return DriverManager.getConnection(url, username, password);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
    public static void close(Connection conn) {
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

DAO接口的定义如下：

import java.util.List;
public interface CustomerDAO {
    void add(Customer c);
    void update(Customer c);
    void delete(int id);
    Customer getCustomerById(int id);
    List
   
     query();
}
   

DAO实现类的示例代码如下：

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class CustomerDaoImp implements CustomerDAO {
    public void add(Customer c) {
        Connection conn = DBUtil.open();
        String sql = "insert into person(name, age) values (?, ?)";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, c.getName());
            pstmt.setInt(2, c.getAge());
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn);
        }
    }
    public void update(Customer c) {
        Connection conn = DBUtil.open();
        String sql = "update person set name=?, age=? where id =?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, c.getName());
            pstmt.setInt(2, c.getAge());
            pstmt.setInt(3, c.getId());
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn);
        }
    }
    public void delete(int id) {
        Connection conn = DBUtil.open();
        String sql = "delete from person where id =?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn);
        }
    }
    public Customer getCustomerById(int id) {
        Connection conn = DBUtil.open();
        String sql = "select id, name, age from person where id =?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            ResultSet rs = pstmt.executeQuery();
            if (rs.next()) {
                Customer c = new Customer();
                c.setId(rs.getInt(1));
                c.setName(rs.getString(2));
                c.setAge(rs.getInt(3));
                return c;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn);
        }
        return null;
    }
    public List
   
     query() {
        Connection conn = DBUtil.open();
        String sql = "select id, name, age from person";
        List
    
      list = new ArrayList<>();
        try {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery(sql);
            while (rs.next()) {
                int id = rs.getInt(1);
                String name = rs.getString(2);
                int age = rs.getInt(3);
                Customer c = new Customer();
                c.setId(id);
                c.setName(name);
                c.setAge(age);
                list.add(c);
            }
            return list;
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn);
        }
        return null;
    }
}
    
   

这种结构使得代码更易于管理和扩展，同时遵循了面向接口编程的原则，提高了代码的可维护性和复用性。

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