Java 泛型與反射實現通用 BaseDAO：消除重複 CRUD 代碼

問題背景

多表查詢時，findById、findAll 等邏輯高度相似，僅實體類型不同。利用泛型 + 反射可將通用邏輯提取到父類。

領域模型

// User 實體
public class User {
    private Integer id;
    private String username;
    private String password;
    private String hobby;
    // getter/setter 省略
}

// Student 實體
public class Student {
    private Integer id;
    private String name;
    private Integer age;
    // getter/setter 省略
}

DAO 接口體系

// 基礎接口
public interface BaseDao {
    T findById(Integer id);
    List findAll();
    int insert(T entity);
    int update(T entity);
    int deleteById(Integer id);
}

// 具體接口繼承並固定泛型
public interface UserDao extends BaseDao {}
public interface StudentDao extends BaseDao {}

通用實現：BaseDaoImpl

public abstract class BaseDaoImpl implements BaseDao {

    private final String tableName;
    private final Class entityClass;
    private final QueryRunner qr = new QueryRunner(DataSourceUtils.getDataSource());

    @SuppressWarnings("unchecked")
    protected BaseDaoImpl() {
        // 1. 獲取子類泛型實際類型：UserDaoImpl extends BaseDaoImpl
        Type superclass = getClass().getGenericSuperclass();
        ParameterizedType pt = (ParameterizedType) superclass;
        this.entityClass = (Class) pt.getActualTypeArguments()[0];
        this.tableName = entityClass.getSimpleName().toLowerCase(); // 假設表名=類名小寫
    }

    @Override
    public T findById(Integer id) {
        String sql = "SELECT * FROM " + tableName + " WHERE id = ?";
        try {
            return qr.query(sql, new BeanHandler<>(entityClass), id);
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }

    @Override
    public List findAll() {
        String sql = "SELECT * FROM " + tableName;
        try {
            return qr.query(sql, new BeanListHandler<>(entityClass));
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }

    @Override
    public int insert(T entity) {
        // 利用反射獲取非空屬性構建 INSERT
        Map props = BeanUtils.describe(entity);
        props.remove("class");
        props.values().removeIf(Objects::isNull);
        
        String columns = String.join(", ", props.keySet());
        String placeholders = props.keySet().stream().map(k -> "?").collect(Collectors.joining(", "));
        String sql = "INSERT INTO " + tableName + " (" + columns + ") VALUES (" + placeholders + ")";
        
        try {
            return qr.update(sql, props.values().toArray());
        } catch (SQLException e) {
            throw new DaoException(e);
        }
    }
    // update/delete 同理...
}

具體實現類（零代碼）

public class UserDaoImpl extends BaseDaoImpl implements UserDao {}
public class StudentDaoImpl extends BaseDaoImpl implements StudentDao {}

測試驗證

@Test
public void testGenericDao() {
    UserDao userDao = new UserDaoImpl();
    User u = userDao.findById(1);
    System.out.println("User: " + u);

    StudentDao studentDao = new StudentDaoImpl();
    Student s = studentDao.findById(1);
    System.out.println("Student: " + s);
}

核心技術點解析

1. 獲取實際泛型類型：getClass().getGenericSuperclass() → ParameterizedType → getActualTypeArguments()[0]
2. BeanHandler/BeanListHandler：DbUtils 自動將 ResultSet 映射為實體，依賴無參構造器 + setter
3. 表名約定：類名小寫，或用註解 @Table("t_user") 顯式指定
4. 異常轉換：捕獲 SQLException 拋出自定義 DaoException，上層無需處理 SQL 異常

進階擴展方向

• 分頁查詢：PageHelper 或手寫 LIMIT/OFFSET
• 動態條件構建：Example 模式或 QueryDSL
• 事務管理：Spring @Transactional 或手動 Connection.setAutoCommit(false)
• 緩存整合：Redis 注解攔截 findById

此模式是 MyBatis、Hibernate、JdbcTemplate 等 ORM 框架的核心設計思想縮影。