手写Mybatis框架

分析JDBC操作问题

问题总结

1. 数据库连接创建、释放频繁造成系统资源浪费，从而影响系统性能。
2. Sql语句在代码中硬编码，造成代码不易维护，实际应用中sql变化的可能较大，sql变动需要改变 java代码。
3. 使用preparedStatement向占有位符号传参数存在硬编码，因为sql语句的where条件不一定，可能 多也可能少，修改sql还要修改代码，系统不易维护。
4. 对结果集解析存在硬编码(查询列名)，sql变化导致解析代码变化，系统不易维护，如果能将数据 库 记录封装成pojo对象解析比较方便

解决方案

1. 使用数据库连接池初始化连接资源
2. 将数据库连接配置、sql语句抽取到xml配置文件中， 使其满足开闭原则
3. 使用反射、内省等底层技术，自动将实体与表进行属性与字段的自动映射

自定义框架设计思路

架构设计图

设计思路

使用端（提供配置）

1. 使用sqlMapperConfig.xml提供全局配置信息（数据源等）
2. 使用Mapper.xml提供sql语句文件信息
3. 提供SqlSession接口类实现select、insert、update、delete操作

框架端（JDBC封装）

1. 读取配置文件

   • 读取sqlMapperconfig.xml
   • 读取Mapper.xml

2. 解析配置文件

   • 解析文件生成Configuration、MapperStatement对象
3. 执行JDBC流程

自定义框架实现

使用端

创建配置文件

sqlMapperConfig.xml(全局配置文件，包括数据源配置，mapper路径等)

mapper.xml（namespace命名空间、SQL语句配置等）

select * from user





select * from user where id = #{id} and username = #{username}





insert into user values(#{id}, #{username}, #{password}, #{birthday})





update user SET username = #{username}, password=#{password}, birthday=#{birthday} WHERE id = #{id}





delete from user where id=#{id}

使用sqlSession操作接口（读取配置文件构建SqlSession数据库操作）

InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapperConfig.xml");

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);

sqlSession = sqlSessionFactory.openSession();

List list = sqlSession.selectList("user.findAll");

框架端

解析配置文件

Configuration（全局配置）

@Data
public class Configuration {

/**
- 数据源
*/
private DataSource dataSource;

/**
- SQL statement集合
*/
private Map mapperStatementMap = new HashMap<>();

}

MapperStatement（SQL语句配置）

@Data
public class MapperStatement {

/**
    statement-id标识（namespace+id）
*/
private String id;

/**
- 参数类型
*/
private String parameterType;

/**
- 返回类型
*/
private String resultType;

/**
- sql语句
*/
private String sql;

/**
- sql类型
*/
private SqlType sqlType;

}

XmlConfigBuilder（全局配置解析）

public class XMLConfigBuilder {

public Configuration parseConfiguration(InputStream inputStream) throws DocumentException, PropertyVetoException {

Configuration configuration = new Configuration();

//解析数据源配置

Document document = new SAXReader().read(inputStream);

Element rootElement = document.getRootElement();

List list = rootElement.selectNodes("//datasource/property");

Properties properties = new Properties();

for (Element element : list) {

String name = element.attributeValue("name");

String value = element.attributeValue("value");

properties.setProperty(name,value);

}

//构建连接池

ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();

comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));

comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));

comboPooledDataSource.setUser(properties.getProperty("username"));

comboPooledDataSource.setPassword(properties.getProperty("password"));

configuration.setDataSource(comboPooledDataSource);

//读取mapper.xml文件路径，解析mapper.xml

List mapperList = rootElement.selectNodes("//mapper");

for (Element element : mapperList) {

String resource = element.attributeValue("resource");

XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(configuration);

xmlMapperBuilder.parseMapper(Resources.getResourceAsStream(resource));

}
return configuration;
}
}

XmlMapperStatementBuilder(SQL语句解析)

public class XMLMapperBuilder {

private Configuration configuration;

public XMLMapperBuilder(Configuration configuration) {

this.configuration = configuration;

}

public void parseMapper(InputStream inputStream) throws DocumentException {

//解析SQL statement并封装到集合

Document document = new SAXReader().read(inputStream);

Element rootElement = document.getRootElement();

//解析sql语句封装MapperStatement

buildStatement(rootElement, new String[]{"select", "update", "insert", "delete"});

}

private void parseStatement(String namespace, Element element) {

String id = element.attributeValue("id");

String parameterType = element.attributeValue("parameterType");

String resultType = element.attributeValue("resultType");

String sql = element.getTextTrim();

String elementName = element.getName();

SqlType sqlType = SqlType.valueOf(elementName.toUpperCase());

MapperStatement mapperStatement = new MapperStatement();

mapperStatement.setId(id);

mapperStatement.setParameterType(parameterType);

mapperStatement.setResultType(resultType);

mapperStatement.setSql(sql);

mapperStatement.setSqlType(sqlType);

String statementId = namespace + "." + id;

configuration.getMapperStatementMap().put(statementId, mapperStatement);

}

private void buildStatement(Element rootElement, String[] nodes) {

String namespace = rootElement.attributeValue("namespace");

for (String node : nodes) {

List list = rootElement.selectNodes("//" + node);

for (Element element : list) {

parseStatement(namespace, element);

}

}

}

}

SqlSession相关接口实现

SqlSessionFactoryBuilder

public class SqlSessionFactoryBuilder {

public SqlSessionFactory build(InputStream inputStream) throws PropertyVetoException, DocumentException {

XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder();

Configuration configuration = xmlConfigBuilder.parseConfiguration(inputStream);

return new DefaultSqlSessionFactory(configuration);

}

}

DefaultSqlSessionFactory

public class DefaultSqlSessionFactory implements SqlSessionFactory{

private final Configuration configuration;

public DefaultSqlSessionFactory(Configuration configuration) {

this.configuration = configuration;

}

@Override

public SqlSession openSession() {

return new DefaultSqlSession(configuration);

}

}

DefaultSqlSession

public class DefaultSqlSession implements SqlSession{

public DefaultSqlSession(Configuration configuration) {

this.configuration = configuration;

}

private Configuration configuration;

@Override

public  List selectList(String statementId, Object... params) throws Exception {

Executor selectExecutor = new SelectExecutor();

MapperStatement mapperStatement = configuration.getMapperStatementMap().get(statementId);

return selectExecutor.query(configuration, mapperStatement, params);

}

@Override

public  T selectOne(String statementId, Object... params) throws Exception {

List