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手写MyBatis

news 来源：原创 2024/9/20 23:33:11

1. 前言

本篇博客，将使用JDK动态代理、注解、反射等技术，编写一个最简单的MyBatis,可基本实现对象的增删查改

2. 注解的定义

2.1 Delete注解

/*** @ClassName Delete* @Descriiption 删除注解* @Author yanjiantao* @Date 2019/6/27 11:03**/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Delete {public String value();
}

2.2 Insert注解

/*** @ClassName Delete* @Descriiption 保存注解* @Author yanjiantao* @Date 2019/6/27 11:03**/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Insert {public String value();
}

2.3 Select注解

/*** @ClassName Delete* @Descriiption 查询注解* @Author yanjiantao* @Date 2019/6/27 11:03**/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Select {public String value();
}

2.4 Update注解

/*** @ClassName Delete* @Descriiption 更新注解* @Author yanjiantao* @Date 2019/6/27 11:03**/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Update {public String value();
}

3. jdk动态代理

3.1 方法代理类

/*** @ClassName MethodProxy* @Descriiption 方法代理* @Author yanjiantao* @Date 2019/6/27 11:11**/
public class MethodProxy implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {return DaoOperatorHandler.handle(method,args);}
}

该类实现JDK的InvocationHandler方法，并且实验invoke方法，即可实现JDK的动态代理

3.2 动态代理工厂类

/*** @ClassName MethodProxyFactory* @Descriiption 代理工厂类* @Author yanjiantao* @Date 2019/6/28 15:40**/
public class MethodProxyFactory {@SuppressWarnings("unchecked")public static <T> T getBean(Class<T> clazz) {final MethodProxy methodProxy = new MethodProxy();return (T) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),new Class[]{clazz},methodProxy);}
}

该工厂的方法主要是得到Mapper的实例，并且把Mapper交给JDK进行动态代理

4. 数据库操作

4.1 数据库操作处理类

/*** @ClassName DaoOperatorHandler* @Descriiption 数据库操作处理器* @Author yanjiantao* @Date 2019/6/27 11:39**/
public class DaoOperatorHandler {public static Object handle(Method method, Object[] parameters) throws SQLException, ClassNotFoundException {String sql = null;// 插入if (method.isAnnotationPresent(Insert.class)) {sql = checkSql(method.getAnnotation(Insert.class).value(), Insert.class.getSimpleName());insert(sql, parameters);// 更新}else if (method.isAnnotationPresent(Update.class)) {sql = checkSql(method.getAnnotation(Update.class).value(), Update.class.getSimpleName());return update(sql, parameters);// 查询}else if (method.isAnnotationPresent(Select.class)) {sql = checkSql(method.getAnnotation(Select.class).value(), Select.class.getSimpleName());Class returnType = method.getReturnType();if (List.class.isAssignableFrom(returnType)) {return  selectMany(sql, parameters);}else {return selectMany(sql, parameters).get(0);}}else if (method.isAnnotationPresent(Delete.class)) {sql = checkSql(method.getAnnotation(Delete.class).value(), Delete.class.getSimpleName());return update(sql, parameters);}System.out.println(sql);return null;}/*** 插入* @param sql sql* @param parameters    参数* @throws SQLException SQLException* @throws ClassNotFoundException   ClassNotFoundException*/private static void insert(String sql, Object[] parameters) throws SQLException, ClassNotFoundException {Connection connection = JDBCUtils.getConnection();PreparedStatement statement =  connection.prepareStatement(sql);for (int i = 0; i < parameters.length; i++) {statement.setObject(i+1, (String) parameters[i]);}statement.execute();connection.close();}/*** 插入* @param sql sql* @param parameters    参数* @throws SQLException SQLException* @throws ClassNotFoundException   ClassNotFoundException*/private static Integer update(String sql, Object[] parameters) throws SQLException, ClassNotFoundException {Connection connection = JDBCUtils.getConnection();PreparedStatement statement =  connection.prepareStatement(sql);for (int i = 0; i < parameters.length; i++) {statement.setObject(i+1, parameters[i]);}int result = statement.executeUpdate();connection.close();return result;}/*** 插入* @param sql sql* @param parameters    参数* @return List<T>* @throws SQLException SQLException* @throws ClassNotFoundException   ClassNotFoundException*/private static  <T> List<T> selectMany(String sql, Object[] parameters) throws SQLException, ClassNotFoundException {Connection connection = JDBCUtils.getConnection();PreparedStatement statement =  connection.prepareStatement(sql);for (int i = 0; parameters != null && i < parameters.length; i++) {statement.setObject(i+1, parameters[i]);}ResultSet resultSet = statement.executeQuery();List<T> result = new ResultToMapper<T>().mapToObject(resultSet,User.class);return result;}/*** 检查sql* @param sql   sql* @param type  type* @return  the sql* @throws SQLException SQLException*/private static String checkSql(String sql, String type) throws SQLException {String sqlType = sql.split(" ")[0];if (!sqlType.equalsIgnoreCase(type)) {throw new SQLException("SQL语句错误");}return sql;}}

该类主要是根据被代理类是否包含相关注解，根据注解的类型，进行增删查改的操作，最后，再将增删查改后的处理结果，使用反射映射到实体类上

5 实体类

5.1用户实体类

/*** @ClassName User* @Descriiption 用户实体类* @Author yanjiantao* @Date 2019/6/28 15:24**/
@Data
public class User {private Integer id;private String username;private String password;
}

6 工具类

6.1 JDBCUtils

/*** @ClassName JDBCUtils* @Descriiption jdbc连接工具类* @Author yanjiantao* @Date 2019/6/28 16:24**/
public class JDBCUtils {public static Connection getConnection() throws ClassNotFoundException, SQLException {Class.forName("com.mysql.cj.jdbc.Driver");String username = "root";String password = "root123456";return DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mybatis?useUnicode=true&characterEncoding=utf8&characterSetResults=utf8&serverTimezone=GMT%2B8", username, password);}}

6.2 ResultToMapper

/*** @ClassName ResultToMapper* @Descriiption mysql查询结果转换为实体bean* @Author yanjiantao* @Date 2019/6/28 17:35**/
public class ResultToMapper<T> {public List<T> mapToObject(ResultSet resultSet, Class<?> clazz) {if (resultSet == null) {return null;}List<T> result = null;try {while (resultSet.next()) {T bean = (T) clazz.newInstance();ResultSetMetaData metaData = resultSet.getMetaData();for (int i = 0; i < metaData.getColumnCount(); i++) {String columnName = metaData.getColumnName(i + 1);Object columnValue = resultSet.getObject(i + 1);Field field = clazz.getDeclaredField(columnName);if (field != null && columnValue != null) {field.setAccessible(true);field.set(bean,columnValue);}}if (result == null) {result = new ArrayList<>();}result.add(bean);}} catch (SQLException | InstantiationException | IllegalAccessException | NoSuchFieldException e) {e.printStackTrace();}if (result == null) {return Collections.emptyList();}return result;}}

该类主要是将mysql查询的结果，通过反射，映射到实体类上

7 Mapper

public interface UserMapper {@Insert("insert into user (username,password) values (?,?)")public void addUser(String name, String password);@Select("select * from user")public List<User> findUsers();@Select("select * from user where id = ?")public User getUser(Integer id);@Update("update user set username = ? , password=? where id=?")public Integer updateUser(String name, String password, Integer id);@Delete("delete from user where id=?")public Integer deleteUser(Integer id);
}

8 测试类


@Slf4j
public class UserMapperTest {@Testpublic void addUser() {UserMapper userMapper = MethodProxyFactory.getBean(UserMapper.class);userMapper.addUser("boolean-","123456");log.info("---------->");}@Testpublic void findUsers() {UserMapper userMapper = MethodProxyFactory.getBean(UserMapper.class);List<User> list = userMapper.findUsers();log.info("---------->list={}", list);}@Testpublic void getUser() {UserMapper userMapper = MethodProxyFactory.getBean(UserMapper.class);User user = userMapper.getUser(2);log.info("---------->user={}", user);}@Testpublic void updateUser() {UserMapper userMapper = MethodProxyFactory.getBean(UserMapper.class);Integer result = userMapper.updateUser("鄢剑涛update", "yjt123", 1);log.info("count={}", result);}@Testpublic void deleteUser() {UserMapper userMapper = MethodProxyFactory.getBean(UserMapper.class);Integer count = userMapper.deleteUser(1);log.info("count={}", count);}
}