Mybatis的执行流程和sql生成原理

借鉴：https://blog.youkuaiyun.com/weixin_49267169/article/details/127348722
https://blog.youkuaiyun.com/xxxzzzqqq_/article/details/130258855
https://blog.youkuaiyun.com/weixin_32128491/article/details/125602941

一：Mybatis的执行流程

1.读取Mybatis的全局配置文件Mybatis-Config.xml :运行环境（数据库连接的参数等）

2.加载映射文件Mapper.xml

3.构建会话工厂SqlSessionFactory

4.构建一个Sqlsession对象

5.Executor执行器，它将根据SqlSession传递的参数动态地生成需要执行的SQL语句，同时负责查询缓存的维护

6.Mapped Statement对象，mapper.xml 的一个Sql语句对应Mapped statement的一个对象，mapper.xml中的id与mapped statement中的id对应起来；

7.输出结果

二：sql生成原理

@Mapper
public interface UserMapper {
 
    UserEntity getById(Long id);
}

1.实现类SqlSessionTemplate调用sqlSession接口的getMapper方法sqlSession.getMapper(RoleMapper.class)。
2.调Configuration类的getMapper(),调MapperRegistry的getMapper()方法返回MapperProxyFactory
2.MapperProxyFactory工厂创建代理对象 mapperProxyFactory.newInstance(sqlSession)返回MapperProxy代理类
3.创建的mapperProxy实现了InvocationHandler接口，invoke方法中对代理对象接口方法的调用进行了增强。通过invoke方法去执行sql操作mapperMethod.execute(sqlSession, args)。

SqlSession接口的实现类SqlSessionTemplate的getMapper()方法中调用了Configuration类中的getMapper()方法

try(InputStream in = Resources.getResourceAsStream("com/myboy/demo/sqlsession/mybatis-config.xml")){
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
    SqlSession sqlSession = sqlSessionFactory.openSession();
    # 拿到代理类对象
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    # 执行方法
    UserEntity userEntity = mapper.getById(2L);
    System.out.println(userEntity);
    sqlSession.close();
}catch (Exception e){
    e.printStackTrace();
}

Configuration 类的getMapper()方法

public class Configuration {
	.......
	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
	        return this.mapperRegistry.getMapper(type, sqlSession);
	    }
}

MapperRegistry 类的getMapper()方法：

public class MapperRegistry {
	.........
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        } else {
            try {
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception var5) {
                throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
            }
        }
    }

MapperProxyFactory类的newInstance()方法：

public class MapperProxyFactory<T> {
   ......
    public T newInstance(SqlSession sqlSession) {
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }
}

MapperProxy的invoke()方法：

public class MapperProxy<T> implements InvocationHandler, Serializable {
    private static final long serialVersionUID = -4724728412955527868L;
    ........

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            return Object.class.equals(method.getDeclaringClass()) ? method.invoke(this, args) : this.cachedInvoker(method).invoke(proxy, method, args, this.sqlSession);
        } catch (Throwable var5) {
            throw ExceptionUtil.unwrapThrowable(var5);
        }
    }