MyBatis: There is no getter for property named... with single Parameter method

A few days ago, this exception was thrown in a MyBatis Mapper within a project: There is no getter for property named 'foo' in 'class baz.bar.Foo'.

The Java interface and Mapper XML had the following content:

// Java Interface
void updateFoo(Foo foo);

// XML Mapper File
<update id="updateFoo">
  UPDATE foo SET attr1 = #{foo.attr1} WHERE id = #{foo.id}
</update>

On the first sight, this seems to be correct. Afterwards, the code got changed like this:

// Java Interface
void updateFoo(Foo foo, Bar bar)

// XML Mapper File
<update id="updateFoo">
  UPDATE foo SET attr1 = #{foo.attr1} WHERE id = #{foo.id} AND bar_id = #{bar.id}
</update>

To our surprise, this code worked. But why?

A few days later, I had a flash of thought: MyBatis is using a shortcut in the first example, because foo is the only parameter, so its attributes can be resolved directly using #{attr1}. So when we use only a single parameter, removing the parameter name seems to be mandatory instead of being just a shortcut.