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JRuby (and ActiveHibernate) at JavaPolis

It's nice to see that JRuby is getting the attention it deserves at JavaPolis:

• Introduction to JRuby at the university,
• JRuby on Rails at the conference,
• Birds-of-a-Feather (BOF) session on JRuby,
• A quicky on ActiveHibernate.

And, while on the topic of ActiveHibernate, I have added support for lazy loading of entities and support for Hibernate components (aka aggregation in Rails):

class Address
  attr_accessor :street
  attr_accessor :city
end

class Person
  include Hibernate
  primary_key_accessor :id, :long
  h_component :address,[[:street,:string],[:city,:string]]
end

I'll use the time that's left before JavaPolis to polish (i.e., clean up and extend the tests) the functionality that is in there today.

Anyway, I'm looking forward to meeting the JRuby team in Antwerp.

.NET Web development without ASP.NET

A couple of weeks ago, Scott Guthrie and Scott Hanselman showed the first bits of Microsoft's upcoming MVC framework for web development at the ALT.NET conference. It is amazing to see how much of ASP.NET they actually ditched to come up with a real MVC framework. Their work seems very nice (fully plugable, dependency injection, ...), and it actually looks remarkably well like MonoRail or ProMesh.NET.

My current project switched from ASP.NET to ProMesh some time ago (which was not an easy decision given ASP.NET's omnipresence), but this evolution proves it was the right thing to do.

ActiveHibernate update

Just added basic many-to-one support in the mapping DSL. I still hope to have a JRuby on Rails application running with ActiveHibernate before Javapolis.

Update:I'll be doing a Javapolis Quicky on ActiveHibernate. In the meanwhile one-to-many (has-many in ActiveRecord speak) has been added to the mapping DSL example.

To DDD or not to DDD

Imagine the application you're currently working on. Imagine your users were happy with a primitive text based UI in a console, and imagine persistence was not an issue because you have an obscene amount of RAM (a billion terrabyte) and your server never crashes (so no DBMS needed).

Would there be any complexity left to deal with in your software? If the answer is no, you're lucky (in a way). If the answer to this highly hypothetical question is still yes, then domain driven design (DDD) might be the thing for you.

Subclasses with ActiveHibernate

Finally I resumed my work on ActiveHibernate. It now uses a XmlMarkup Builder and the visitor pattern to sort out the relationships between different persistent classes and to generate the mapping XML internally. This might sound like overkill, but even to get inheritance up and running it seemed like the only way to keep things maintainable. Anyway, the Ruby code

class Payment
  include Hibernate
  hattr_accessor :amount,:double
  ...
end

class CreditCardPayment < Payment
  join_with_table do
    hattr_accessor :creditCardType,:string
    ...
  end
end

class ChequePayment < Payment
  hattr_accessor :chequeNumber,:string
  ..
end

generates the mapping

<class name="Payment" table="PAYMENT">
  <id name="id" type="long" column="PAYMENT_ID">
      <generator class="native"/>
  </id>
  <discriminator column="PAYMENT_TYPE" type="string"/>
  <property name="amount" type="double"/>
  ...
  <subclass name="CreditCardPayment" >
      <join table="CREDIT_CARD_PAYMENT">
          <key column="PAYMENT_ID"/>
          <property name="creditCardType" type="string" />
          ...
      </join>
  </subclass>
  <subclass name="ChequePayment" >
          <property name="chequeNumber" type="string" />
          ...
  </subclass>
</class>

The <join/> construct in the CreditCardPayment mapping allows to combine the table-per-class-hierarchy and the table-per-subclass mapping strategies in one inheritance hierarchy. In the Ruby mapping DSL it can be nicely expressed with a class method (table name and key are optional arguments) and a block. The fact that a Ruby class definition is just ordinary Ruby code (with self referring to a Class object) opens a whole new world to the simple Java developer that I am.

In Subversion soon.

Deep Cloning an Object Graph in .NET

The other day I was looking at the really nice NHibernate mapping examples by Davy Brion. His entity base class looks like this:

public abstract class Entity
{
   private long _id = -1;

   public virtual long Id
   {
      get { return _id; }
   }

   public virtual T Clone()
   {
      MemoryStream memoryStream = new MemoryStream();
      BinaryFormatter binaryFormatter = new BinaryFormatter();
      binaryFormatter.Serialize(memoryStream, this);
      memoryStream.Position = 0;
      return (T)binaryFormatter.Deserialize(memoryStream);
   }
}

It is used like so:

public class Order : Entity<Order>
{
   //...
}

The Order class derives from a template class that uses the Order class itself as parameter. I had never seen this before, and, for example, it gives all your entities a Clone() method that returns the proper type.

What's even more interesting is the way the Clone() method has been implemented by just serializing to a memory stream, immediately followed by a deserialization. It gives you a deep clone of your entity, together with all child entities it references. This technique is robust, simple, clean and generic (requiring no extra development effort).

I can imagine some people will object to this because of performance, but that only makes me think of all the crimes that are committed daily in the name of performance and (premature) optimization.