自定义持久层框架

下图是JDBC引起的一系列问题以及解决办法：

自定义持久层框架设计思路：

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创建两个maven工程IPersistence和IPersistence_test

--IPersistence_test引入IPersistence依赖--
​
     <groupId>com.yun</groupId>
    <artifactId>IPersistence_test</artifactId>
    <version>1.0-SNAPSHOT</version>
​
    <!--引入自定义持久层框架依赖-->
    <dependencies>
        <dependency>
            <groupId>com.yun</groupId>
            <artifactId>IPersistence</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

---

--IPersistence_test-->sqlMapConfig.xml--
<configuration>
    <!--数据库配置信息-->
    <dataSource>
        <property name="driverClass" value="com.mysql.jdbc.Driver"></property>
        <property name="jdbcUrl" value="jdbc:mysql://xxx.xxx.xx.xxx:xxxx/xxxx"></property>
        <property name="username" value="xxxx"></property>
        <property name="password" value="xxxx"></property>
    </dataSource>

    <!--存放mapper.xml的全路径-->
    <mapper resource="userMapper.xml"></mapper>

</configuration>

--IPersistence_test-->userMapper.xml--
<mapper namespace="user">

    <!--sql的唯一标识应该是由 namespace.id来组成（statementId）-->
    <select id="selectList" resultType="com.yun.pojo.User">
        select * from user
    </select>

    <!--利用反射获取到user对象的参数-->
    <select id="selectOne" resultType="com.yun.pojo.User" paramterType="com.yum.pojo.User">
        select * from user where id = #{id} and username = #{username}
    </select>

</mapper>

---

--IPersistence--
@Data
public class MappedStatement {

    //id
    private Integer id;

    //返回值类型
    private String resultType;

    //参数值类型
    private String paramterType;

    //sql语句
    private String sql;
}

--IPersistence--
@Data
public class Configuration {

    private DataSource dataSource;

    /**
     * k:statementId
     * v:封装好的MappedStatement对象
     */
    Map<String,MappedStatement> map = new HashMap<>();
}

按照设计思路编写代码

---

--解析配置文件返回流
public class Resources {

    /**
     * 根据配置文件的路径，将配置文件加载成字节输入流，存储在内存中
     * @param path
     * @return
     */
    public static InputStream getResourcesAsSteam(String path){
        InputStream resourceAsStream = Resources.class.getClassLoader().getResourceAsStream(path);
        return resourceAsStream;
    }

}

@Data
public class Configuration {

    private DataSource dataSource;

    /**
     * k:statementId
     * v:封装好的MappedStatement对象
     */
    Map<String,MappedStatement> map = new HashMap<>();

}

@Data
public class MappedStatement {

    //id
    private String id;

    //返回值类型
    private String resultType;

    //参数值类型
    private String paramterType;

    //sql语句
    private String sql;
}

--将解析的流封装到SqlSessionFactoryBuilder中
public class SqlSessionFactoryBuilder {

    public SqlSessionFactory build(InputStream is) throws Exception {
        //1，使用dom4j解析配置文件，将解析出来的内容封装到Configuration中
        XmlConfigBuilder xmlConfigBuilder = new XmlConfigBuilder();
        Configuration configuration = xmlConfigBuilder.parseConfig(is);

        //2，创建sqlSessionFactory对象,工厂类:生产sqlSession绘画对象
        DefaultSqlSessionFactory defaultSqlSessionFactory = new DefaultSqlSessionFactory(configuration);

        return defaultSqlSessionFactory;
    }
}

--将sqlMapConfig.xml和userMapper.xml流放入configuration中
public class XmlConfigBuilder {

    private Configuration configuration;

    public XmlConfigBuilder() {
        this.configuration = new Configuration();
    }

    /**
     * 该方法就是使用dom4j将配置文件解析，封装Configuration
     * @param is
     * @return
     */
    public Configuration parseConfig(InputStream is) throws Exception {

        Document document = new SAXReader().read(is);
        //获取Configuration根对象<Configuration>
        Element rootElement = document.getRootElement();
        //获取sqlMapConfig.xml里面的配置信息并且遍历
        List<Element> list = rootElement.selectNodes("//property");
        Properties properties = new Properties();
        for (Element element : list) {
            String name = element.attributeValue("name");
            String value = element.attributeValue("value");
            properties.setProperty(name,value);
        }

        //创建 c3p0 连接池
        ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();
        comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));
        comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
        comboPooledDataSource.setUser(properties.getProperty("username"));
        comboPooledDataSource.setUser(properties.getProperty("password"));

        configuration.setDataSource(comboPooledDataSource);

        //mapper.xml解析 步骤：拿到路径-->加载成字节输入流-->dom4j进行解析
        List<Element> mapperList = rootElement.selectNodes("//mapper");

        for (Element element : mapperList) {
            String mapperPath = element.attributeValue("resource");
            InputStream resourcesAsSteam = Resources.getResourcesAsSteam(mapperPath);
            XmlMApperBuilder xmlMApperBuilder = new XmlMApperBuilder(configuration);
            xmlMApperBuilder.prase(resourcesAsSteam);
        }

        return configuration;
    }
}

public class XmlMApperBuilder {

    private Configuration configuration;

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

    public void prase(InputStream is) throws Exception {
        Document document = new SAXReader().read(is);
        Element rootElement = document.getRootElement();
        String namespace = rootElement.attributeValue("namespace");

        List<Element> list = rootElement.selectNodes("//select");
        for (Element element : list) {
            String id = element.attributeValue("id");
            String resultType = element.attributeValue("resultType");
            String paramterType = element.attributeValue("paramterType");
            String sqlText = element.getTextTrim();

            MappedStatement mappedStatement = new MappedStatement();
            mappedStatement.setId(id);
            mappedStatement.setResultType(resultType);
            mappedStatement.setParamterType(paramterType);
            mappedStatement.setSql(sqlText);

            //key值是由 namespace.id来组成
            String key = namespace +"."+id;
            configuration.getMap().put(key,mappedStatement);
        }
    }
}

--利用工厂模式生产sqlSession
public class DefaultSqlSessionFactory implements SqlSessionFactory{

    private Configuration configuration;

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

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

@AllArgsConstructor
public class DefaultSqlSession implements SqlSession {

    private Configuration configuration;

    @Override
    public <E> List<E> selectList(String statementId, Object... params) throws Exception {

        //将要去完成对 SimpleExecutor 里的query方法的调用
        SimpleExecutor simpleExecutor = new SimpleExecutor();
        MappedStatement mappedStatement = configuration.getMap().get(statementId);
        List<Object> list = simpleExecutor.query(configuration, mappedStatement, params);

        return (List<E>) list;
    }

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

--注册驱动，查询数据信息 并且封装返回
public class SimpleExecutor implements Executor {
    @Override                                                                                 //user
    public <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception{
        //1,注册驱动，获取连接
        Connection connection = configuration.getDataSource().getConnection();

        //2,获取sql    select * from user where id = #{id} and username = #{username}
          //转换sql    select * from user where id = ? and username = ?,转换过程中，还需要对#{}里面的值进行存储解析
        String sql = mappedStatement.getSql();

        BoundSql boundSql = getBoundSql(sql);

        //3,获取预处理对象
        PreparedStatement preparedStatement = connection.prepareStatement(boundSql.getSqlText());

        //4,设置参数
        //获取到参数的全路径
        String paramterType = mappedStatement.getParamterType();
        Class<?> paramterTypeClass =  getClassType(paramterType);

        List<ParameterMapping> parameterMappingList = boundSql.getParameterMappingList();
        for (int i = 0; i < parameterMappingList.size(); i++) {
            ParameterMapping parameterMapping = parameterMappingList.get(i);
            String content = parameterMapping.getContent();

            //反射根据content获取到实体对象中的属性值，再根据属性值获取到当前传过来的参数对象
            Field declaredField = paramterTypeClass.getDeclaredField(content);
            //暴力访问
            declaredField.setAccessible(true);
            Object o = declaredField.get(params[0]);

            preparedStatement.setObject(i+1,o);
        }

        //5,执行sql
        ResultSet resultSet = preparedStatement.executeQuery();
        //获取实体对象
        String resultType = mappedStatement.getResultType();
        Class<?> resultTypeClass = getClassType(resultType);
        //获取实体对象的具体实现
        Object instance = resultTypeClass.newInstance();

        List<Object> objects = new ArrayList<>();

        //6,封装返回结果集
        while (resultSet.next()) {
            //1,取出元数据
            ResultSetMetaData metaData = resultSet.getMetaData();
            for (int i = 1; i <= metaData.getColumnCount(); i++) {
                //获取字段名
                String columnName = metaData.getColumnName(i);
                //获取字段值
                Object value = resultSet.getObject(columnName);
                //使用反射或者内省根据数据库表和实体的对应关系，完成封装
                PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultTypeClass);
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(instance,value);
            }
            objects.add(instance);
        }

        return (List<E>) objects;
    }

    /**
     * 反射获取实体
     * @param paramterType
     * @return
     * @throws Exception
     */
    private Class<?> getClassType(String paramterType) throws Exception {
        if (!StringUtils.isNullOrEmpty(paramterType)) {
            Class<?> aClass = Class.forName(paramterType);
            return aClass;
        }
        return null;
    }

    /**
     * 完成对#{}的解析工作：1，将#{}使用?进行代替；2，解析出#{}里面的值进行存储
     * @param sql
     * @return
     */
    private BoundSql getBoundSql(String sql) {
        //标记处理类：配置标记解析器来完成对占位符的解析处理工作
        ParameterMappingTokenHandler tokenHandler = new ParameterMappingTokenHandler();
        //标记解析器，对占位符的转换
        GenericTokenParser tokenParser = new GenericTokenParser("#{", "}", tokenHandler);
        //解析出来的sql
        String parseSql = tokenParser.parse(sql);
        //#{}里面的解析出来的参数名称
        List<ParameterMapping> parameterMappings = tokenHandler.getParameterMappings();

        BoundSql boundSql = new BoundSql(parseSql,parameterMappings);

        return boundSql;

    }
}

开始测试

public class IPersistenceTest {

    @Test
    public void test() throws Exception {
        //获取配置文件流
        InputStream resourcesAsSteam = Resources.getResourcesAsSteam("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourcesAsSteam);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        //调用
        User user = new User();
        user.setId(1);
        user.setUsername("张三");
        User user2 = sqlSession.selectOne("user.selectOne", user);
        System.out.println(user2);
    }
}

结果：

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我们可以自定义DAO层来处理与数据库交互

---

public interface IUserDao {

    /**
     * 查询所有
     * @return
     */
    public List<User> findAll() throws Exception;

    /**
     * 根据条件进行查询
     * @return
     */
    public User findByCondition(User user) throws Exception;
}

public class IUserDaoImpl implements IUserDao {
    @Override
    public List<User> findAll() throws Exception {
        InputStream inputStream = Resources.getResourcesAsSteam("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuild().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        User user = new User();
        user.setId(1);
        user.setUsername("Tom");
        List<User> users = sqlSession.selectList("user.selectList");
        users.forEach(item ->{
            System.out.println(item);
        });
        return users;
    }

    @Override
    public User findByCondition(User user) throws Exception {
        InputStream inputStream = Resources.getResourcesAsSteam("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuild().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        user.setId(1);
        user.setUsername("Tom");
        //调用
        User user2 = sqlSession.selectOne("user.selectOne", user);
        System.out.println(user2);
        return user2;
    }

}

---

不过又出现以下的问题：

既然咱们想出可以使用代理模式生成Dao层接口的代理实现，那咱们就开始动手测试一下，代码如下：

---

public interface SqlSession {

    /**
     * 查询所有
     * @param statemnetId
     * @param parmes
     * @param <E>
     * @return
     */
    public <E> List<E> selectList(String statemnetId,Object...parmes) throws Exception;

    /**
     * 根据条件查询
     * @param statemnetId
     * @param parmes
     * @param <T>
     * @return
     */
    public <T> T selectOne(String statemnetId,Object...parmes) throws Exception;

    /**
     * 为Dao接口生成代理实现类
     * @param mapperClass
     * @param <T>
     * @return
     */
    public <T> T getMapper(Class<?> mapperClass);
}

    @Override
    public <T> T getMapper(Class<?> mapperClass) {
        //使用JDK动态代理来为Dao接口生成代理对象，并返回
        Object instance = Proxy.newProxyInstance(DefaultSqlSession.class.getClassLoader(), new Class[]{mapperClass}, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                /*
                 * 本质上底层还是去执行jdbc代码，
                 * 其实可以根据不同的情况来调用selectList或者selectOne,
                 * 但是，invole方法是没办法获取到映射配置文件中namespace和id的值,
                 * 不过我们可以借助method对象来获取当前执行的方法名和当前方法所在文件的全限定名，
                 * 所以：namespace.id == 接口全限定名.方法名
                 */
                //准备参数1,statementId:sql语句的唯一标识(namespace.id)，parmes
                //方法名
                String methodName = method.getName();
                //接口全限定名
                String className = method.getDeclaringClass().getName();

                String statementId = className + "." + methodName;

                //准备参数2, parms : args
                //首先获取被调用方法的返回值类型，
                Type genericReturnType = method.getGenericReturnType();
                //判断是否进行了 泛型类型参数化-->意思就是当饭返回值有泛型，则返回list无泛型，则返回实体对象
                if (genericReturnType instanceof ParameterizedType) {
                    List<Object> objects = selectList(statementId, args);
                    return objects;
                }

                return selectOne(statementId,args);
            }
        });
        return (T) instance;
    }

此时，xml文件就得做出相对应的修改：

<mapper namespace="com.yun.dao.IUserDao">

    <!--sql的唯一标识：namespace.id来组成 ： statementId-->
    <select id="findAll" resultType="com.yun.pojo.User" >
        select * from user
    </select>


    <!--
        User user = new User()
        user.setId(1);
        user.setUsername("Tom")
    -->
    <select id="findByCondition" resultType="com.yun.pojo.User" paramterType="com.yun.pojo.User">
        select * from user where id = #{id} and username = #{username}
    </select>


</mapper>
-- 与IUserDao接口的全限定名和方法名绑定;

---

接下来咱们进行测试

	@Test
    public void test() throws Exception {
        InputStream inputStream = Resources.getResourcesAsSteam("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuild().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        User user = new User();
        user.setId(1);
        user.setUsername("Tom");

        //返回代理对象
        IUserDao userDao = sqlSession.getMapper(IUserDao.class);
        User user1 = userDao.findByCondition(user);
        System.out.println(user1);

    }

---

测试结果如下：

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以上就是自定义持久层框架，其实本质上就是Mybatis的雏形；