MyBatis是如何让我们通过接口就能调用到SQL的

大致可分为如下几个步骤

1. 动态注册bean

1.1 根据配置mapperScan, 扫描对应的包, 将对应的类解析成BeanDefinition

1.2 通过替换BeanDefinition中的BeanClass为MapperFactoryBean, (原来的BeanClass是Mapper接口) 实现了在spring生成对应的对象时, 返回的对象不是本身类型的对象，而是MapperFactoryBean重写FactoryBean接口的getObject()方法返回的代理对象。该方法getObject()已经对mapper接口进行了代理, 即后续进行自动注入时, 也是返回getObject()生成的代理对象

2. 生成对应的代理对象

2.1 在getObject()方法中, 会获取到接口的全限定名称, 然后进一步对代理方法进行封装, 调用链如下

   MapperFactoryBean: 
   	public T getObject() throws Exception {
   		return getSqlSession().getMapper(this.mapperInterface);
   	}
   
   DefaultSqlSession: 
       public <T> T getMapper(Class<T> type) {
       	//configuration是mybatis的重要配置类, 在初始化的时候, 就会将mapper接口添加到configuration中
       	return configuration.getMapper(type, this);
       }
   
   Configuration: 
   	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
   		return mapperRegistry.getMapper(type, sqlSession);
   	}
   
   MapperRegistry: 
   	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
   	    //获取mapper代理工厂
   		final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
   		if (mapperProxyFactory == null) {
   			throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
   		}
   		try {
   			return mapperProxyFactory.newInstance(sqlSession);
   		} catch (Exception e) {
   			throw new BindingException("Error getting mapper instance. Cause: " + e, e);
   		}
   	}
   
   MapperProxyFactory: 
   	protected T newInstance(MapperProxy<T> mapperProxy) {
   		return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
   	}
   
   	public T newInstance(SqlSession sqlSession) {
   		//返回一个新代理对象
   		final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
   		return newInstance(mapperProxy);
   	}

2.2 在org.apache.ibatis.binding.MapperProxy#cachedInvoker中, new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration())), 并且将代理方法调用器缓存起来. MapperMethod该对象即是最终调用方法的对象.

3. 执行对应的方法

3.1 方法调用, 代理里最常见的方法invoke

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
    	//过滤掉object的方法
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else {
        return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
  }

3.2. org.apache.ibatis.binding.MapperProxy.PlainMethodInvoker重写的invoke方法中,判断该方法是否被调用过(是否存在于缓存), 若没有, 则创建一个PlainMethodInvoker方法调用器, 传入MapperMethod(MapperMethod是真正执行方法的对象), 并将新创建的PlainMethodInvoker存入缓存中(methodCache), 并调用该PlainMethodInvoker的invoke方法,

3.3. xml中的id和select等标签封装成了SqlCommand, 调用mapperMethod的execute, 执行对应的增删改查.

4. 结果集封装, 进行一些数据库数据对应java对象的转换

通过mybatis的封装和代理, 将mapper.xml转换成了接口的实例对象

如有谬误, 欢迎斧正

简化版如下: https://blog.csdn.net/sinat_25991865/article/details/89891581

public interface UserMapper {
	List<SysUser> selectAll();
}

public class MyMapperProxy<T> implements InvocationHandler {
	private Class<T> mapperInterface;
	private SqlSession sqlSession;
	
	public MyMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) {
			this.mapperInterface = mapperInterface;
			this.sqlSession = sqlSession;
	}

	@Override
	public Object invoke(Object proxy , Method method , Object[] args)
			throws Throwable {
		//针对不同的 sql 类型，需要调用sqlSession不同的方法
		//接口方法中的参数也有很多情况 ，这里只考虑没有有参数的情况
		List<T> list= sqlSession.selectList(
				mapperInterface.getCanonicalName() + ”.” + method.getName());
		//返回数据也有很多情况，这里不做处理直接返回
		return list;
	}
}

方法调用

//获取sqlSession
SqlSession sqlSession = getSqlSession();
//获取 UserMapper 接口
MyMapperProxy userMapperProxy = new MyMapperProxy(
		UserMapper.class , sqlSession) ;
UserMapper userMapper = (UserMapper) Proxy.newProxyinstance (
		Thread.currentThread().getContextClassLoader(),
		new Class[ ] {UserMapper.class},
		userMapperProxy) ;
//调 用 selectAll 方 法
List<SysUser> user= userMapper.selectAll();