实习已经半年多了，从刚进入公司开始接触到框架，其中ORM框架Hibernate、mybatis。到目前为止，都会用，但是具体的原理不懂，所以开始系统的学习一下。先从mybatis开始。
参照 《深入浅出Mybatis技术原理与实战——杨开振》，代码：https://github.com/yangyang5214/mybatis
传统的jdbc编程
Java程序都是通过JDBC（Java Data Base Connectivity连接数据库的，这样就可以通过Sql对数据库编码。 传统的jdbc编程主要分为下面几步：
1、连接数据库，注册驱动和数据库信息
2、操作Connection,打开Statement对象3、通过Statement执行sql，返回结果到ResultSet对象。
4、使用ResultSet读取数据，然后通过代码转换为pojo对象。
5、关闭数据库相关资源。下面以一个根据id查询用户的例子看一下具体的实现：1、基础数据，表的创建CREATE TABLE USER ( id INT PRIMARY KEY AUTO_INCREMENT, username VARCHAR (10), age LONG, pwd VARCHAR (10));INSERT INTO USER(username,age,pwd) VALUES ('爬爬',20,111), ('殃殃','10',222), ('祸祸','30',333)2、具体实现public class JdbcTest { public static void main(String[] args) { JdbcTest jdbcTest = new JdbcTest(); UserDto userDto = jdbcTest.getUserById(1); System.out.println(userDto); } /** * 注册驱动和数据库相关信息 * @return */ public Connection getConnection(){ Connection connection = null; try { Class.forName("com.mysql.jdbc.Driver"); String url = "jdbc:mysql://localhost:3306/mybatis"; String username = "root"; String pwd = "admin"; connection = DriverManager.getConnection(url,username,pwd); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (SQLException e) { e.printStackTrace(); } return connection; } /** * 通过Statement执行sql,返回结果到resultSet对象 * @param userId * @return */ public UserDto getUserById(long userId){ Connection connection = getConnection(); PreparedStatement ps = null; ResultSet rs = null; String sql = "SELECT * FROM user where id = ?"; try { ps = connection.prepareStatement(sql); ps.setLong(1,userId); rs = ps.executeQuery(); while (rs.next()){ UserDto userDto = new UserDto(); userDto.setId(rs.getLong("id")); userDto.setAge(rs.getLong("age")); userDto.setUsername(rs.getString("username")); userDto.setPwd(rs.getString("pwd")); return userDto; } } catch (SQLException e) { e.printStackTrace(); }finally { colse(rs,ps,connection); } return null; } /** * 关闭数据库相关资源 * @param rs * @param ps * @param connection */ public void colse(ResultSet rs,PreparedStatement ps,Connection connection){ try { if (rs != null && !rs.isClosed()){ try { rs.close(); } catch (SQLException e) { e.printStackTrace(); } } if (ps != null && !ps.isClosed()){ try { ps.close(); } catch (SQLException e) { e.printStackTrace(); } } if (connection != null && !connection.isClosed()){ try { connection.close(); } catch (SQLException e) { e.printStackTrace(); } } } catch (SQLException e) { e.printStackTrace(); } }}
3、UserDto类public class UserDto { private long id; private String username; private String pwd; private long age; public long getId() { return id; } public void setId(long id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getPwd() { return pwd; } public void setPwd(String pwd) { this.pwd = pwd; } public long getAge() { return age; } public void setAge(long age) { this.age = age; } @Override public String toString() { return "UserDto{" + "id=" + id + ", username='" + username + '\'' + ", pwd='" + pwd + '\'' + ", age=" + age + '}'; }}
4、结果
