JPA，XML-是否普遍接受无参数的私有构造函数？

Question

There are many situations where having a private no-arg constructor makes sense, for instance:

• Instances that are designed to be immutable
• Value Objects that are built with a builder, using a fluent API

When I'm willing to use these objects in a persistence unit or / and as transferable objects in a REST interface, I add some JPA or / and XML annotations. In this process, I want my immutable objects to remain immutable objects and I want my Value Objects to remain Value Objects. After all, why should I change the nature of these objects just because I want to use them as transferable objects or as persistent data?

Example:

@XmlType
public class Endpoint {
    @XmlElement
    private String hostname = "localhost";

    @XmlElement
    private int port = 8080;

    @XmlElement
    private String path = "/";

    private Endpoint() {}

    public String hostname() {
        return hostname;
    }

    // etc...

    @XmlTransient
    static public class Builder {
        private Endpoint endpoint = new Endpoint();

        public Builder hostname(String hostname) {
            endpoint.hostname = hostname;
            return this;
        }

        // etc...

        public Endpoint build() {
            Endpoint newInstance = endpoint;
            endpoint = null;
            return newInstance;
        }
    }
}

Of course, my favorite IDE starts reporting some warnings: there should be a public no-arg constructor,... I could change my no-arg constructor and make it public, and therefore allow my clients to use it. But it breaks the initial idea to always use the builder to construct instances of the object. And anyway, there are situations where using the default constructor wouldn't make any sense.

As far as I can remember, all the JAXB or JPA implementations I've played with have been able to instantiate my objects. I've tried JAXB RI, MOXy, Hybernate, EclipseLink; all these libraries seems to accept my constructs without any problem.

Hence my question: Is the requirement to have a public no-arg constructor (and not protected or private) somehow obsolete (Java 8+) with recent libraries? Or am I taking risks and I'm just lucky?

Answer 1

JPA spec 2.1 "The Entity Class"

The entity class must have a no-arg constructor. The entity class may have other constructors as well. The no-arg constructor must be public or protected .

Some JPA providers (eg DataNucleus) don't require such a constructor at all (since they have features that add it during bytecode enhancement), but to be totally compliant (and not leaving things to being "lucky") the user should provide one.