Why does Hibernate require no argument constructor?

Question

Hibernate, and code in general that creates objects via reflection use Class<T>.newInstance() to create a new instance of your classes. This method requires a public no-arg constructor to be able to instantiate the object. For most use cases, providing a no-arg constructor is not a problem.

There are hacks based on serialization that can work around not having a no-arg constructor, since serialization uses jvm magic to create objects without invoking the constructor. But this is not available across all VMs. For example, XStream can create instances of objects that don't have a public no-arg constructor, but only by running in a so-called "enhanced" mode which is available only on certain VMs. (See the link for details.) Hibernate's designers surely chose to maintain compatibility with all VMs and so avoids such tricks, and uses the officially supported reflection method Class<T>.newInstance() requiring a no-arg constructor.

Erm, sorry everyone, but Hibernate does not require that your classes must have a parameterless constructor. The JPA 2.0 specification requires it, and this is very lame on behalf of JPA. Other frameworks like JAXB also require it, which is also very lame on behalf of those frameworks.

(Actually, JAXB supposedly allows entity factories, but it insists on instantiating these factories by itself, requiring them to have a --guess what-- parameterless constructor, which in my book is exactly as good as not allowing factories; how lame is that!)

But Hibernate does not require such a thing.

Hibernate supports an interception mechanism, (see "Interceptor" in the documentation,) which allows you to instantiate your objects with whatever constructor parameters they need.

Basically, what you do is that when you setup hibernate you pass it an object implementing the org.hibernate.Interceptor interface, and hibernate will then be invoking the instantiate() method of that interface whenever it needs a new instance of an object of yours, so your implementation of that method can new your objects in whatever way you like.

I have done it in a project and it works like a charm. In this project I do things via JPA whenever possible, and I only use Hibernate features like the interceptor when I have no other option.

Hibernate seems to be somewhat insecure about it, as during startup it issues an info message for each of my entity classes, telling me INFO: HHH000182: No default (no-argument) constructor for class and class must be instantiated by Interceptor, but then later on I do instantiate them by interceptor, and it is happy with that.

To answer the "why" part of the question for tools other than Hibernate, the answer is "for absolutely no good reason", and this is proven by the existence of the hibernate interceptor. There are many tools out there that could have been supporting some similar mechanism for client object instantiation, but they don't, so they create the objects by themselves, so they have to require parameterless constructors. I am tempted to believe that this is happening because the creators of these tools think of themselves as ninja systems programmers who create frameworks full of magic to be used by ignorant application programmers, who (so they think) would never in their wildest dreams have a need for such advanced constructs as the... Factory Pattern. (Okay, I am tempted to think so. I don't actually think so. I am joking.)

Hibernate instantiates your objects. So it needs to be able to instantiate them. If there isn't a no-arg constructor, Hibernate won't know how to instantiate it, i.e. what argument to pass.

The hibernate documentation says:

4.1.1. Implement a no-argument constructor

All persistent classes must have a default constructor (which can be non-public) so that Hibernate can instantiate them using Constructor.newInstance(). It is recommended that you have a default constructor with at least package visibility for runtime proxy generation in Hibernate.

The hibernate is an ORM framework which supports field or property access strategy. However, it does not support constructor-based mapping - maybe what you would like ? - because of some issues like

1º What happens whether your class contains a lot of constructors

public class Person {

    private String name;
    private Integer age;

    public Person(String name, Integer age) { ... }
    public Person(String name) { ... }
    public Person(Integer age) { ... }

}

As you can see, you deal with a issue of inconsistency because Hibernate cannot suppose which constructor should be called. For instance, suppose you need to retrieve a stored Person object

Person person = (Person) session.get(Person.class, <IDENTIFIER>);

Which constructor should Hibernate call to retrieve a Person object ? Can you see ?

2º And finally, by using reflection, Hibernate can instantiate a class through its no-arg constructor. So when you call

Person person = (Person) session.get(Person.class, <IDENTIFIER>);

Hibernate will instantiate your Person object as follows

Person.class.newInstance();

Which according to API documentation

The class is instantiated as if by a new expression with an empty argument list

Moral of the story

Person.class.newInstance();

is similar To

new Person();

Nothing else

Hibernate needs to create instances as result of your queries (via reflection), Hibernate relies on the no-arg constructor of entities for that, so you need to provide a no-arg constructor. What is not clear?

Actually, you can instantiate classes which have no 0-args constructor; you can get a list of a class' constructors, pick one and invoke it with bogus parameters.

While this is possible, and I guess it would work and wouldn't be problematic, you'll have to agree that is pretty weird.

Constructing objects the way Hibernate does (I believe it invokes the 0-arg constructor and then it probably modifies the instance's fields directly via Reflection. Perhaps it knows how to call setters) goes a little bit against how is an object supposed to be constructed in Java- invoke the constructor with the appropriate parameters so that the new object is the object you want. I believe that instantiating an object and then mutating it is somewhat "anti-Java" (or I would say, anti pure theoretical Java)- and definitely, if you do this via direct field manipulation, it goes encapsulation and all that fancy encapsulation stuff.

I think that the proper way to do this would be to define in the Hibernate mapping how an object should be instantiated from the info in the database row using the proper constructor... but this would be more complex- meaning both Hibernate would be even more complex, the mapping would be more complex... and all to be more "pure"; and I don't think this would have an advantage over the current approach (other than feeling good about doing things "the proper way").

Having said that, and seeing that the Hibernate approach is not very "clean", the obligation to have a 0-arg constructor is not strictly necessary, but I can understand somewhat the requirement, although I believe they did it on purely "proper way" grounds, when they strayed from the "proper way" (albeit for reasonable reasons) much before that.