@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Flush {
}

看其注释：
The maker annotation that invoke a flush statements via Mapper interface.
（1）maker annotation
（2）invoke a flush statements
（3）via Mapper interface
重点在于第二点，先看invoke的用法：
public Object invoke(Object obj,Object... args)throws IllegalAccessException,IllegalArgumentException,InvocationTargetException
这里的obj对象就是Mapper interface实现类，