配置文件加载

public interface SqlSession {

    /**
     * 查询多个
     * @param statementId sql的id
     * @param params 参数列表
     * @param <E> 返回类型
     * @return List<E>
     */
    <E> List<E> selectList(String statementId, Object... params) throws Exception;

    /**
     * 按条件查询单个
     * @param statementId sql的id
     * @param params 参数列表
     * @param <T> 返回类型
     * @return T
     */
    <T> T selectOne(String statementId, Object... params) throws Exception;

    /**
     * 更新内容
     * @param statementId sql的id
     * @param params 参数列表
     * @return 受影响的行数
     */
    int update(String statementId, Object params) throws Exception;

    /**
     * 保存对象
     * @param statementId sql的id
     * @param params 参数列表
     * @return 受影响的行数
     */
    int save(String statementId, Object params) throws Exception;

    /**
     * 删除对象
     * @param statementId sql的id
     * @param params 参数列表
     * @return 受影响的行数
     */
    int delete(String statementId, Object params) throws Exception;

    /**
     * 生成实现类
     * @param mapperClass 待实现接口
     * @param <T> 类型
     * @return 动态代理类
     */
    <T> T getMapper(Class<T> mapperClass);

    /**
     * 关闭连接
     */
    void close();
}

public interface SqlSessionFactory {

    /**
     * 生产一个SqlSession
     * @return SqlSession
     */
    SqlSession openSession();
}

public interface Executor {

    /**
     * 查找
     * @param configuration 核心配置类
     * @param mappedStatement 已封装的sql
     * @param params 参数
     * @param <E> 返回值类型
     * @return 结果
     */
    <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception;

    /**
     * 更新
     * @param configuration 核心配置类
     * @param mappedStatement 已封装的sql
     * @param params 参数
     * @return 受影响的行数
     */
    int update(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception;

    /**
     * 关闭连接
     */
    void close();
}

public class SqlSessionFactoryBuilder {

    /**
     * 构建SqlSessionFactory
     * @param in 主配置文件流
     * @return DefaultSqlSessionFactory
     */
    public SqlSessionFactory build(InputStream in) throws Exception {
        //使用dom4j解析配置文件，封装到Configuration
        XMLConfigBuilder builder = new XMLConfigBuilder();
        Configuration configuration = builder.parseConfig(in);
        //创建SqlSessionFactory对象：生产SqlSession
        return new DefaultSqlSessionFactory(configuration);
    }
}

public class SimpleExecutor implements Executor {
    private PreparedStatement statement;
    private Connection connection;
    @Override
    @SuppressWarnings({"unchecked", "Duplicates"})
    public <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception {
        //注册驱动，获取连接
        connection = configuration.getDataSource().getConnection();
        //获取sql
        String sql = mappedStatement.getSql();
        //转换sql
        BoundSql boundSql = getBoundSql(sql);
        //获取预处理对象
        statement = connection.prepareStatement(boundSql.getSqlText());
        //设置参数
        String paramType = mappedStatement.getParamType();
        Class<?> type = getClassType(paramType);
        List<ParameterMapping> mappingList = boundSql.getParameterMappingList();
        for (int i = 0; i < mappingList.size(); i++) {
            ParameterMapping parameterMapping = mappingList.get(i);
            String content = parameterMapping.getContent();
            Field field = type.getDeclaredField(content);
            //开启暴力访问
            field.setAccessible(true);
            E o = (E)field.get(params[0]);
            //i从0开始，所以加一
            statement.setObject(i+1, o);
        }
        //执行sql
        ResultSet resultSet = statement.executeQuery();
        String resultType = mappedStatement.getResultType();
        Class<?> resultTypeClass = getClassType(resultType);
        //封装结果集
        List<Object> objects = new ArrayList<>();
        while (resultSet.next()) {
            Object o = resultTypeClass.getDeclaredConstructor().newInstance();
            ResultSetMetaData metaData = resultSet.getMetaData();
            for (int i = 0; i < metaData.getColumnCount(); i++) {
                //获取字段名
                String columnName = metaData.getColumnName(i+1);
                Object value = resultSet.getObject(columnName);
                //根据表和实体关系，完成封装，内省
                PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultTypeClass);
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(o, value);
            }
            objects.add(o);
        }
        return (List<E>) objects;
    }

    @Override
    @SuppressWarnings("Duplicates")
    public int update(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception {
        connection = configuration.getDataSource().getConnection();
        String sql = mappedStatement.getSql();
        BoundSql boundSql = getBoundSql(sql);
        statement = connection.prepareStatement(boundSql.getSqlText());
        //设置参数
        String paramType = mappedStatement.getParamType();
        Class<?> type = getClassType(paramType);
        List<ParameterMapping> mappingList = boundSql.getParameterMappingList();
        for (int i = 0; i < mappingList.size(); i++) {
            ParameterMapping parameterMapping = mappingList.get(i);
            String content = parameterMapping.getContent();
            Field field = type.getDeclaredField(content);
            //开启暴力访问
            field.setAccessible(true);
            Object o = field.get(params[0]);
            //i从0开始，所以加一
            statement.setObject(i+1, o);
        }
        //执行sql
        return statement.executeUpdate();
    }

    @Override
    public void close() {
        try {
            statement.close();
            connection.close();
        } catch (SQLException ignored) {}
    }

    private Class<?> getClassType(String paramType) throws ClassNotFoundException {
        if (paramType != null) {
            return Class.forName(paramType);
        }
        throw new NullPointerException("paramType is null");
    }

    /**
     * 把 #{} 转换为 ?
     * 解析出#{}的值
     * @param sql 待处理的sql
     * @return 封装后的BoundSql
     */
    private BoundSql getBoundSql(String sql) {
        //标记处理类
        ParameterMappingTokenHandler tokenHandler = new ParameterMappingTokenHandler();
        GenericTokenParser genericTokenParser = new GenericTokenParser("#{", "}", tokenHandler);
        //解析出来的sql
        String parseSql = genericTokenParser.parse(sql);
        //解析出来的参数名
        List<ParameterMapping> mappings = tokenHandler.getParameterMappings();
        return new BoundSql(parseSql, mappings);
    }
}

public class DefaultSqlSession implements SqlSession {

    private final Configuration configuration;

    private Executor executor;

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

    @Override
    public <E> List<E> selectList(String statementId, Object... params) throws Exception {
        //调动simpleExecutor的query方法
        executor = new SimpleExecutor();
        MappedStatement mappedStatement = configuration.getMappedStatementMap().get(statementId);
        return executor.query(configuration, mappedStatement, params);
    }

    @Override
    public <T> T selectOne(String statementId, Object... params) throws Exception {
        List<T> t = selectList(statementId, params);
        if (t.size() == 1) {
            return t.get(0);
        } else if (t.size() > 1){
            throw new RuntimeException("返回结果过多");
        }
        throw new RuntimeException("返回结果为空");
    }

    @Override
    public int update(String statementId, Object params) throws Exception {
        executor = new SimpleExecutor();
        MappedStatement mappedStatement = configuration.getMappedStatementMap().get(statementId);
        return executor.update(configuration, mappedStatement, params);
    }

    @Override
    public int save(String statementId, Object params) throws Exception {
        executor = new SimpleExecutor();
        MappedStatement mappedStatement = configuration.getMappedStatementMap().get(statementId);
        return executor.update(configuration, mappedStatement, params);
    }

    @Override
    public int delete(String statementId, Object params) throws Exception {
        executor = new SimpleExecutor();
        MappedStatement mappedStatement = configuration.getMappedStatementMap().get(statementId);
        return executor.update(configuration, mappedStatement, params);
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T getMapper(Class<T> mapperClass) {
        //jdk动态代理
        Object o = Proxy.newProxyInstance(DefaultSqlSession.class.getClassLoader(), new Class[]{mapperClass}, (proxy, method, args) -> {
            String methodName = method.getName();
            String className = method.getDeclaringClass().getName();
            String statementId = className + "." + methodName;
            //获取返回值类型
            Type genericReturnType = method.getGenericReturnType();
            if (genericReturnType instanceof ParameterizedType) {
                return selectList(statementId, args);
            }
            return selectOne(statementId, args);
        });
        return (T) o;
    }

    @Override
    public void close() {
        try {
            executor.close();
        }catch(Exception ignore){}
    }
}

public class DefaultSqlSessionFactory implements SqlSessionFactory {

    private final Configuration configuration;

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

    @Override
    public SqlSession openSession() {
        return new DefaultSqlSession(configuration);
    }
}