自定义实现ORMapping框架

自定义实现ORMapping框架的核心在于反射和注解的应用，关于这部分的内容可以参考文章：
Java反射总结
Java自定义注解的实现

在本篇文章中主要实现save方法，关键在于以下几步：

• 自定义注解的实现
• 通过反射获取表名和字段名
• SQL拼接的实现
• DBUtils的开发

具体代码如下
annotation：

/**
 * @Author: huhu
 * @Date: 2019-08-10 19:21
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface HuHuBean {

    String value();
}

/**
 * @Author: huhu
 * @Date: 2019-08-10 19:22
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface HuHuField {

    String value();
}

dao：

/**
 * @Author: huhu
 * @Date: 2019-08-10 19:28
 */
public interface BaseDAO {

    /**
     * insert into xxx(col1, col2, col3...) values(?,?,?...)
     *
     * @param t
     * @param 
     * @return
     */
    <T>Serializable save(T t);

}

/**
 * @Author: huhu
 * @Date: 2019-08-10 19:35
 */
public class BaseDAOImpl implements BaseDAO {

    /**
     * 入参是T：User
     *
     * insert into t_user (name,age,birth_day) values (?,?,?);
     *
     * t: user
     * user.getId() user.getName() 通过这种方式依次将值给获取出来
     *
     * 常见面试考点：StringBuffer与StringBuilder的区别
     *
     * @param t
     * @param 
     * @return
     */
    @Override
    public <T> Serializable save(T t) {

        StringBuilder builder = new StringBuilder("insert into ");
        String table = Tools.getTable(t.getClass());
        builder.append(table).append(" (");

        Class<?> clazz = t.getClass();
        Field[] fields = clazz.getDeclaredFields();
        for (Field field : fields) {
            //判断获取的字段是否为id，id为自增主键，因此不需要拼接
            if (!field.getName().equals("id")) {
                String column = Tools.getColumn(field);
                builder.append(column).append(",");
            }
        }

        // 删除逗号
        builder.deleteCharAt(builder.toString().length()-1)
                .append(") values (");

        // 有几个?取决于需要拼接几个字段
        for (Field field : fields) {
            if (!field.getName().equals("id")) {
                builder.append("?,");
            }
        }

        // 删除逗号
        builder.deleteCharAt(builder.toString().length()-1)
                .append(")");

//        System.out.println(builder.toString());

        Connection connection = null;
        PreparedStatement pstmt = null;
        ResultSet rs =null;
        int index = 1;

        try {
            connection = DBUtils.getConnection();
            // 将拼接好的SQL传入
//            pstmt = connection.prepareStatement(builder.toString());
            // 传入拼接好的sql以及需要返回的字段名id
            pstmt = connection.prepareStatement(builder.toString(), new String[]{"id"});

            for (Field field : fields) {
                if (!field.getName().equals("id")) {
                    String getMethod = Tools.getMethod(field);
                    Method method = clazz.getDeclaredMethod(getMethod);
                    Object obj = method.invoke(t);
                    pstmt.setObject(index++, obj);
                }
            }

            int rowCount = pstmt.executeUpdate();
            System.out.println("rowCount: " + rowCount);
            if (rowCount > 0) {
                rs = pstmt.getGeneratedKeys();
                rs.next();
                // 返回id字段的最新值
                return (Serializable) rs.getObject(1);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }

        return null;
    }

}

domain：

/**
 * @Author: huhu
 * @Date: 2019-08-10 19:18
 */
@HuHuBean("t_user")
public class User {

    private Integer id;

    private String name;

    private Integer age;

    @HuHuField("birth_day")
    private Date bithday;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Date getBithday() {
        return bithday;
    }

    public void setBithday(Date bithday) {
        this.bithday = bithday;
    }

    public User() {}

    public User(String name, Integer age, Date bithday) {
        this.name = name;
        this.age = age;
        this.bithday = bithday;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", bithday=" + bithday +
                '}';
    }
}

utils：

/**
 * @Author: huhu
 * @Date: 2019-08-11 20:41
 */
public class DBUtils {

    public static Connection getConnection() {

        String url = "jdbc:mysql://localhost:3306/orm";
        String user = "root";
        String password = "root";
        Connection connection = null;

        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection(url, user, password);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }

        return connection;
    }

    public static void close(Connection connection) {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

/**
 * @Author: huhu
 * @Date: 2019-08-11 16:31
 */
public class Tools {

    /**
     * 根据注解获取表名
     *
     * @param clazz
     * @return
     */
    public static String getTable(Class<?> clazz) {

        String tableName = "";

        HuHuBean huHuBean = clazz.getAnnotation(HuHuBean.class);

        if (huHuBean != null) {
            // 获取注解中的值
            tableName = huHuBean.value();
        } else {
            // 如果为空，就直接获取类名
            tableName = clazz.getSimpleName();
        }

        return tableName;

    }

    /**
     * 根据注解获取属性名称
     *
     * @param field
     * @return
     */
    public static String getColumn(Field field) {

        String column = "";

        HuHuField huHuField = field.getAnnotation(HuHuField.class);
        if (huHuField != null) {
            // 获取注解中的值
            column = huHuField.value();
        } else {
            // 如果为空，就直接获取字段名
            column = field.getName();
        }

        return column;
    }

    /**
     * 根据字段获取对应的get方法
     *
     * @param field
     * @return
     */
    public static String getMethod(Field field) {
        String fieldName = field.getName();

        // id==>getId
        // name==>getNamet
        String name = fieldName.substring(0,1).toUpperCase() + fieldName.substring(1);
        return "get" + name;
    }
}

Test：

/**
 * @Author: huhu
 * @Date: 2019-08-11 17:52
 */
public class BaseDAOImplTest {

    @Test
    public void test01() {
        BaseDAO dao = new BaseDAOImpl();
        User user = new User("HuHu", 22, new Date());

        System.out.println(dao.save(user));

    }
}

测试结果：

数据插入成功