手写简单版mybatis

Mybatis是一款优秀的持久层框架，它封装了JDBC操作数据库的细节，使得开发者只需要关注SQL语句编写即可，同时还能够有效的防止SQL注入等问题。下面我来手写一个简单版的Mybatis框架。

1. 定义实体类

首先我们需要定义实体类，这里以一个User实体类为例：

public class User {
    private Integer id;
    private String name;
    private Integer age;
    // 省略getter和setter方法
}

2. 定义Mapper接口

接着我们需要定义Mapper接口，用于定义SQL语句和参数映射关系。这里同样以User为例：

public interface UserMapper {
    User selectById(Integer id);
    List<User> selectAll();
    void insert(User user);
    void update(User user);
    void deleteById(Integer id);
}

3. 编写配置文件

接下来需要编写配置文件，定义数据库连接信息、Mapper接口和SQL语句对应关系。配置文件通常包含两部分：

- 数据库连接信息：

<datasource driver="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost:3306/test" username="root" password="root"/>

- Mapper接口和SQL语句对应关系：

<mapper namespace="com.example.mapper.UserMapper">
    <select id="selectById" resultType="com.example.entity.User">
        select * from user where id=#{id}
    </select>
    <select id="selectAll" resultType="com.example.entity.User">
        select * from user
    </select>
    <insert id="insert">
        insert into user(name,age) values(#{name},#{age})
    </insert>
    <update id="update">
        update user set name=#{name},age=#{age} where id=#{id}
    </update>
    <delete id="deleteById">
        delete from user where id=#{id}
    </delete>
</mapper>

4. 编写SqlSession类

SqlSession是Mybatis的核心类，用于创建和管理数据库连接、执行SQL语句、封装结果集等操作。这里我们先定义一个简单的SqlSession类：

public class SqlSession {
    private Configuration configuration;
    private Connection connection;

    public SqlSession(Configuration configuration) {
        this.configuration = configuration;
    }

    public <T> T getMapper(Class<T> clazz) {
        return configuration.getMapper(clazz, this);
    }

    public Connection getConnection() {
        try {
            if (connection == null || connection.isClosed()) {
                connection = DriverManager.getConnection(configuration.getDataSource().getUrl(),
                        configuration.getDataSource().getUsername(),
                        configuration.getDataSource().getPassword());
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return connection;
    }
}

5. 编写Configuration类

Configuration类用于解析和管理配置文件，创建SqlSession和Mapper接口实例。这里我们主要关注两个方法getMapper和getMappedStatement：

public class Configuration {
    private DataSource dataSource;
    private Map<String, MappedStatement> mappedStatements = new HashMap<>();

    public DataSource getDataSource() {
        return dataSource;
    }

    public void setDataSource(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    public <T> T getMapper(Class<T> clazz, SqlSession sqlSession) {
        return MapperProxyFactory.getMapperProxy(sqlSession, clazz);
    }

    public MappedStatement getMappedStatement(String statementId) {
        return mappedStatements.get(statementId);
    }

    public void addMappedStatement(String statementId, MappedStatement mappedStatement) {
        mappedStatements.put(statementId, mappedStatement);
    }
}

6. 编写MappedStatement类

MappedStatement类用于封装SQL语句和参数映射信息，包括SQL语句、参数类型、返回值类型等。

public class MappedStatement {
    private String statementId;
    private String sql;
    private Class<?> parameterType;
    private Class<?> resultType;

    public MappedStatement(String statementId, String sql, Class<?> parameterType, Class<?> resultType) {
        this.statementId = statementId;
        this.sql = sql;
        this.parameterType = parameterType;
        this.resultType = resultType;
    }

    public String getStatementId() {
        return statementId;
    }

    public void setStatementId(String statementId) {
        this.statementId = statementId;
    }

    public String getSql() {
        return sql;
    }

    public void setSql(String sql) {
        this.sql = sql;
    }

    public Class<?> getParameterType() {
        return parameterType;
    }

    public void setParameterType(Class<?> parameterType) {
        this.parameterType = parameterType;
    }

    public Class<?> getResultType() {
        return resultType;
    }

    public void setResultType(Class<?> resultType) {
        this.resultType = resultType;
    }
}

7. 编写MapperProxyFactory类

MapperProxyFactory类用于创建Mapper接口的代理对象。在代理对象中我们可以根据Mapper接口的方法名获取对应的SQL语句和参数映射信息，并执行SQL语句。

public class MapperProxyFactory<T> {
    public static <T> T getMapperProxy(SqlSession sqlSession, Class<T> clazz) {
        return (T) Proxy.newProxyInstance(clazz.getClassLoader(), new Class[] { clazz },
                new MapperProxy(sqlSession, clazz));
    }
}

8. 编写MapperProxy类

MapperProxy类是Mapper接口的代理对象，用于执行SQL语句。

public class MapperProxy<T> implements InvocationHandler {
    private SqlSession sqlSession;
    private Class<T> mapperInterface;

    public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface) {
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        // 获取Mapper接口方法对应的MappedStatement
        String statementId = mapperInterface.getName() + "." + method.getName();
        MappedStatement mappedStatement = sqlSession.getConfiguration().getMappedStatement(statementId);

        // 获取SQL语句和参数映射信息
        String sql = mappedStatement.getSql();
        Class<?> parameterType = mappedStatement.getParameterType();
        Class<?> resultType = mappedStatement.getResultType();

        // 封装SQL参数
        Object param = null;
        if (args != null && args.length == 1) {
            param = args[0];
        }

        // 执行SQL语句
        Connection connection = sqlSession.getConnection();
        PreparedStatement pstmt = connection.prepareStatement(sql);
        if (param != null) {
            pstmt.setObject(1, param);
        }
        ResultSet rs = pstmt.executeQuery();

        // 封装结果集
        List<Object> resultList = new ArrayList<>();
        while (rs.next()) {
            Object result = resultType.newInstance();
            Field[] fields = resultType.getDeclaredFields();
            for (Field field : fields) {
                field.setAccessible(true);
                field.set(result, rs.getObject(field.getName()));
            }
            resultList.add(result);
        }

        // 返回结果
        if (method.getReturnType() == List.class) {
            return resultList;
        } else {
            return resultList.get(0);
        }
    }
}

到此为止，我们已经手写了一个简单版的Mybatis框架。当然，这只是一个简单的示例，实际上Mybatis还有很多复杂的功能和特性，比如动态SQL、缓存、插件等。