When you use scala object you can define the static method for others to use. Scala will create a Class which implements the singleton pattern for that class object.
If the object name is A, you can find the singleton class name with A$. Using javap to recompile the class A and A$, you will find A has bunch of static method, and A$ doesn't has any of them. If you specify the converter class package name in META-INF/service/org/apache/camel/TypeConverter, camel will load the class A and A$ at the same time. As the A$ construction method is not supposed to be invoked, camel will complain that he cannot load the converter method which you are suppose to use because of he can't create a instance of A$.
To avoid this kind of error, we need to specify the full class name of A in the TypeConverter to let camel load the converter directly.