第7章：DAO及相关实现类

• DAO：Data Access Object访问数据信息的类和接口，包括了对数据的CRUD（Create、Retrival、Update、Delete），而不包含任何业务相关的信息。有时也称作：BaseDAO
• 作用：为了实现功能的模块化，更有利于代码的维护和升级。
  
  层次结构：
  父类（抽象类）由具体类继承
  
  接口 对应某一具体表的操作规范
  
  某一具体实现 继承抽象类同时实现对应接口
  

【BaseDAO.java】

public abstract class BaseDAO {
    private Class clazz = null;

    public BaseDAO() {
        // 获取子类的类型
        this.clazz = (Class) this.getClass();
        // 获取父类的类型
        // getGenericSuperclass()用来获取当前类的父类的类型
        // ParameterizedType表示的是带泛型的类型
        ParameterizedType parameterizedType = (ParameterizedType) clazz.getGenericSuperclass();
        // 获取具体的泛型类型 getActualTypeArguments获取具体的泛型的类型
        // 这个方法会返回一个Type的数组
        Type[] types = parameterizedType.getActualTypeArguments();
        // 获取具体的泛型的类型·
        this.clazz = (Class) types[0];
    }

    // 通用的增删改操作（考虑上事物）
    public void update(Connection conn, String sql, Object... args) {
        PreparedStatement ps = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 1; i <= args.length; i++) {
                ps.setObject(i, args[i - 1]); // 注意下标
            }
            ps.execute();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(null, ps);
        }
    }
    // 通用的查询返回一条记录
    public T select(Connection conn, String sql, Object ...args) {
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1,args[i]);
            }
            rs = ps.executeQuery();
            ResultSetMetaData rsmd = rs.getMetaData();
            int columnCount = rsmd.getColumnCount();
            if(rs.next()){
                T t = (T) clazz.newInstance();
                for (int i = 0; i < columnCount; i++) {
                    Object columnValue = rs.getObject(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i + 1);
                    Field field = clazz.getDeclaredField(columnLabel);
                    field.setAccessible(true);
                    field.set(t,columnValue);
                }
                return t;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            JDBCUtils.closeResource(null,ps,rs);
        }
        return null;
    }
    // 查询返回多条记录
    public List selectMulti(Connection conn, String sql, Object ...args) {
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1,args[i]);
            }
            rs = ps.executeQuery();
            ResultSetMetaData rsmd = rs.getMetaData();
            int columnCount = rsmd.getColumnCount();
            ArrayList ts = new ArrayList<>();
            while (rs.next()){
                T t = clazz.newInstance();
                for (int i = 0; i < columnCount; i++) {
                    Object columnValue = rs.getObject(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i + 1);
                    Field field = clazz.getDeclaredField(columnLabel);
                    field.setAccessible(true);
                    field.set(t,columnValue);
                }
                ts.add(t);
            }
            return ts;
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            JDBCUtils.closeResource(null,ps,rs);
        }
        return null;
    }
    // 用于查询特殊值的通用方法
    public  E getValue(Connection conn, String sql, Object ...args) {
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1,args[i]);
            }
            rs = ps.executeQuery();
            if(rs.next()){
                return (E) rs.getObject(1);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            JDBCUtils.closeResource(null,ps,rs);
        }
        return null;
    }
}

【CustomerDAO.java】

public interface CustomerDAO {

    
    void insert(Connection conn, Customer customer);
    
    void deleteById(Connection conn,int id);
    
    void update(Connection conn,Customer customer);
    
    Customer getById(Connection conn,int id);
    
    List getAll(Connection conn);
    
    Long getCount(Connection conn);
}

【CustomerDaoImpl.java】

public class CustomerDAOImpl extends BaseDAO implements CustomerDAO {
    @Override
    public void insert(Connection conn, Customer customer) {
        String sql = "insert into customers(name,email,birth)values(?,?,?)";
        update(conn,sql,customer.getName(),customer.getEmail(),customer.getBirth());
    }

    @Override
    public void deleteById(Connection conn, int id) {
        String sql = "delete from customers where id = ?";
        update(conn,sql,id);
    }

    @Override
    public void update(Connection conn, Customer customer) {
        String sql = "update customers set name = ?,email = ?,birth = ? where id =?";
        update(conn,sql,customer.getName(),customer.getEmail(),customer.getBirth(),customer.getId());
    }

    @Override
    public Customer getById(Connection conn, int id) {
        String sql = "select id,name,email,birth from customers where id = ?";
        Customer customer = select(conn, sql, id);
        return customer;

    }

    @Override
    public List getAll(Connection conn) {
        String sql = "select id,name,email,birth from customers";
        List customers = selectMulti(conn, sql);
        return customers;
    }

    @Override
    public Long getCount(Connection conn) {
        String sql = "select count(*) from customers";
        return getValue(conn,sql);
    }
}