手写简单版mybatis

Mybatis 是一款优秀的持久层框架，它可以对 JDBC 进行封装，使得开发者可以通过 XML 文件或注解来配置 SQL 语句，从而避免了手写 JDBC 代码的繁琐和重复。下面，我将简单介绍如何手写一个简单版的 Mybatis。

1. 创建配置文件

在 resources 目录下创建一个 mybatis-config.xml 文件，用于配置 Mybatis 的各种参数。其中包括数据库连接信息、mapper 文件路径、缓存配置、插件等。以下是一个简单的配置文件示例：

<configuration>
    <dataSource type="POOLED">
        <property name="driver" value="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
    </dataSource>

    <mappers>
        <mapper resource="com/example/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

2. 创建数据库连接池

在 Mybatis 中，我们可以通过数据源来获取数据库连接。下面是一个简单的数据源示例：

public class PooledDataSource {
    private static final String driver = "com.mysql.jdbc.Driver";
    private static final String url = "jdbc:mysql://localhost:3306/mybatis";
    private static final String username = "root";
    private static final String password = "root";
    private static final int initialSize = 5;
    private static final int maxActive = 10;
    private static final int maxIdle = 8;
    private static final int minIdle = 5;
    private static final long maxWait = 60000;

    private static DataSource dataSource;

    static {
        try {
            GenericObjectPoolConfig<Connection> poolConfig = new GenericObjectPoolConfig<>();
            poolConfig.setMinIdle(minIdle);
            poolConfig.setMaxIdle(maxIdle);
            poolConfig.setMaxTotal(maxActive);
            poolConfig.setMaxWaitMillis(maxWait);

            ConnectionFactory connectionFactory = new DriverManagerConnectionFactory(url, username, password);
            PoolableConnectionFactory poolableConnectionFactory = new PoolableConnectionFactory(connectionFactory, null);
            poolableConnectionFactory.setValidationQuery("SELECT 1");

            dataSource = new PoolingDataSource(poolableConnectionFactory.getPool());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static DataSource getDataSource() {
        return dataSource;
    }
}

3. 创建 SqlSession

SqlSession 是 Mybatis 中用于执行 SQL 语句的核心接口，它提供了各种方法来执行 SQL 语句、获取 Mapper 等。下面是一个简单的 SqlSession 实现：

public class DefaultSqlSession implements SqlSession {
    private final Configuration configuration;
    private final Executor executor;

    public DefaultSqlSession(Configuration configuration, Executor executor) {
        this.configuration = configuration;
        this.executor = executor;
    }

    @Override
    public <T> T selectOne(String statement, Object parameter) {
        return executor.query(statement, parameter);
    }

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

4. 创建 Executor

Executor 是 Mybatis 中用于执行 SQL 语句的实现类，它提供了各种方法来执行 SQL 语句、获取结果等。下面是一个简单的 Executor 实现：

public class SimpleExecutor implements Executor {
    private final DataSource dataSource;

    public SimpleExecutor(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    @Override
    public <T> T query(String statement, Object parameter) {
        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        try {
            connection = dataSource.getConnection();
            preparedStatement = connection.prepareStatement(statement);
            preparedStatement.setString(1, parameter.toString());
            resultSet = preparedStatement.executeQuery();
            if (resultSet.next()) {
                return (T) resultSet.getString(1);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                resultSet.close();
                preparedStatement.close();
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return null;
    }
}

5. 创建 Mapper

Mapper 是 Mybatis 中用于定义 SQL 语句的接口，它包含了各种方法来执行 SQL 语句、获取结果等。下面是一个简单的 Mapper 接口示例：

public interface UserMapper {
    @Select("SELECT * FROM user WHERE id = #{id}")
    User selectById(Integer id);
}

6. 创建 Configuration

Configuration 是 Mybatis 中用于配置各种参数的类，它负责解析配置文件、创建 SqlSession 和 Mapper 等。下面是一个简单的 Configuration 实现：

public class Configuration {
    private final Properties properties;
    private final Map<Class<?>, MapperProxyFactory<?>> mapperProxyFactoryMap = new HashMap<>();

    public Configuration(Properties properties) {
        this.properties = properties;
    }

    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) mapperProxyFactoryMap.get(type);
        if (mapperProxyFactory == null) {
            mapperProxyFactory = new MapperProxyFactory<>(type);
            mapperProxyFactoryMap.put(type, mapperProxyFactory);
        }
        return mapperProxyFactory.newInstance(sqlSession);
    }
}

7. 创建 MapperProxyFactory 和 MapperProxy

MapperProxyFactory 是 Mybatis 中用于创建 MapperProxy 的工厂类，它负责创建 MapperProxy 并维护 Mapper 接口和对应的 MapperProxy。MapperProxy 是 Mybatis 中用于动态代理 Mapper 接口的类，它负责解析 Mapper 接口中定义的 SQL 语句，并将其转化为对应的 SQL 语句执行器。下面是一个简单的 MapperProxyFactory 和 MapperProxy 实现：

public class MapperProxyFactory<T> {
    private final Class<T> mapperInterface;

    public MapperProxyFactory(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }

    public T newInstance(SqlSession sqlSession) {
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, new MapperProxy(sqlSession, mapperInterface));
    }
}

public class MapperProxy<T> implements InvocationHandler {
    private final SqlSession sqlSession;
    private final 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 {
        Select select = method.getAnnotation(Select.class);
        if (select != null) {
            String sql = select.value()[0];
            return sqlSession.selectOne(sql, args[0]);
        }
        throw new RuntimeException("No such method: " + method.getName());
    }
}

至此，我们就手写了一个简单版的 Mybatis，它包含了配置文件、数据库连接池、SqlSession、Executor、Mapper 等核心组件。虽然这个实现比较简单，但它足以帮助我们理解 Mybatis 的核心原理。