手写实现Mybatis —>单表查询

1. Mybatis核心概念

2. Mybatis处理流程图

• 各层包搭建树状图

• │  │  │  └─com
  │  │  │      └─mybatis
  │  │  │          ├─binding
  |  |  |              └─ MapperMethod.class
  |  |  |				 ├─ MapperProxy.class
  |  |  |				 └─ MapperRegister.class
  │  │  │          ├─executor
  |  |  |              └─ MapperMethod.class
  |  |  |				 ├─ MapperProxy.class
  |  |  |				 └─ MapperRegister.class
  │  │  │          ├─resultset
  |  |  |				 └─ DefaultResultHandler.class
  |  |  |				 └─ ResultSetHandler.class
  │  │  │          ├─session
  |  |  |				 └─ Configuration.class
  |  |  |				 ├─ DefaultSQLSession.class
  |  |  |				 ├─ SQLSession.class
  |  |  |				 ├─ SQLSessionFactory.class
  |  |  | 			 └─ sqlSessionFactoryBuilder.class
  │  │  │          └─statement
  │  │  │				 └─ StatementHandler.class
  |  |  |             
  │  │  └─resources
  │  │      └─mybatis
  │  └─test
  │      └─java
  │          └─com
  │              └─qitian
  │                  ├─mapper
  │                  └─pojo
  

3. Mybatis 实现的主要类

• Configuration

• SqlSessionFactory

• Executor

1. Configuration

configuration类的作用主要是读取mybatis-congig.xml的配置文件，以及mapper注册（将配置文件的XXXMapper.xml存储到一个Map集合中）。

Mappper注册

MapperRegister.class

@Data
public class MapperRegister {
    private  Map<String, MapperMethod> knownMappers = new HashMap<String, MapperMethod>();

}

Configuration.class

/**
 * 读取xml文件
 */
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Configuration {
    /**
     * 定义io 读取xml文件
     */
    private InputStream inputStream;
    private MapperRegister mapperRegister = new MapperRegister();

    public void loadCOnfigurations() throws Exception {
        try {
            Document document = new SAXReader().read(inputStream);
            Element root = document.getRootElement();
            //读取mappers下配置的mapper
            List<Element> mappers = root.element("mappers").elements("mapper");
            for (Element mapper : mappers) {
                //获取mapper里的属性resource
                /**
                <!--mapper映射-->
    			<mappers>
        			<mapper resource="XXXMapper.xml"/>
    			</mappers>
                */
                if (mapper.attribute("resource") != null) {
                    mapperRegister.setKnownMappers((Map<String, MapperMethod>) loadXMLConfiguration(mapper.attributeValue("resource")));
                }
                if (mapper.attribute("class") != null) {
                }
            }
        } catch (DocumentException e) {
            System.out.println("读取配置文件出错");
            e.printStackTrace();
        } finally {
            inputStream.close();
        }
    }

    /**
     * 读取XXXMapper.xml的信息
     *
     * @param reource
     * @return
     */
    private Object loadXMLConfiguration(String reource) throws Exception {
        Map<String, MapperMethod> map = new HashMap<String, MapperMethod>();
        InputStream is = null;
        try {
            //获取XXXMapper.xml文件
            is = this.getClass().getClassLoader().getResourceAsStream(reource);
            Document document = new SAXReader().read(is);
            Element root = document.getRootElement();
            if (root.getName().equalsIgnoreCase("mapper")) {
                //读取mapper中的命名空间
                String namespace = root.attribute("namespace").getText();
                //读取查询方法的关键字select
                for (Element select : (List<Element>) root.elements("select")) {
                    MapperMethod mapperMethod = new MapperMethod();
                    mapperMethod.setSql(select.getText().trim());
 //获取返回值的类型
                    mapperMethod.setType(Class.forName(select.attribute("resultType").getText()));
                    map.put(namespace + "." + select.attribute("id").getText(), mapperMethod);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            is.close();
        }
        return map;
    }
}

2. SqlSessionFactory

SqlSessionFactory利用SqlSessionFactoryBuilder.builer(configuration)方法创建对象，将configuration作为参数传入，以便获取mybatis-config.xml的信息，该类的主要作用是用于构建SqlSession对象，SqlSession对象可以通过方法openSession获得。(sqlSessionFactory.openSession(configuration)