为什么 JAXB 中的 XmlAdapter 是抽象的 class？

Question

I always use interface when there is no common implementation between the subclasses. However, the built-in XmlAdapter in JAXB is an abstract class, and ALL the methods there are abstract, why it's not made interface instead of abstract class? There should be a reason.

Answer 1

XmlAdapter defines two abstract methods, and also a default no arg protected constructor.

Unmarshaller s and Marshaller s creates the instances of XmlAdapter using the default constructor (unless an instance is provided with setAdapter ).

My initial thought was that the reason was ensuring there is always a default no-arg constructor for Marshaller and Unmarshaller , but as you stated, if a subclass of XmlAdapter declared an non no-arg constructor, that would make the default constructor of XmlAdapter not visible. In fact, an XmlAdapter implementation that added a constructor with args, and did'nt provide the no-arg constructor, would cause the Marshall/Unmarshall operation to fail unless instances where provided with setAdapter methods.

So, there is not practical reason to use and abstract class instead of an interface with the current implementation .

My guess is this was a design decision as using and abstract class is best suited from an evolution perspective. If for example, at some point some kind of common initialization where required in the constructor, it could be implemented on the now empty constructor. Or if a method where required in the XmlAdapter to marshall a list of objects, it could be implemented with something like this:

public List<ValueType> marshalList(List<BoundType> list) throws Exception {
    List<ValueType> result = new ArrayList<>();
    for (BoundType b: list) {
        result.add(marshal(b));
    }
    return result;
}

And JAXB internal classes could use this method without the need of modifying any of the adapter implementations.