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Java IoT: Article

Lessons to Learn from the Hibernate Core Implementation

Hibernate Core is a good example of open source projects to learn from, don't hesitate to take a look inside it.

If our primary goal is to enforce low coupling, there's a common mistake when using interfaces that could kill the utility of using them. It's the using of the concrete classes instead of interfaces, and to explain better this problem let's take the following example:

The class A implements the Interface IA who contains the calculate() method, the consumer class C is implemented like that

public class C
{
   ....
   public void calculate()
   {
     .....
     m_a.calculate();
     ....
    }
    A m_a;

}



The Class C instead of referencing the interface IA, it references the class A, in this case we lose the low coupling benefit, and this implementation has two major drawbacks:

  • If we decide to use another implementation of IA, we must change the code of C class.
  • If some methods are added to A not existing in IA, and C use them, we also lose the contract benefit of using interfaces.

C# introduced the explicit interface implementation capability to the language to ensure that a method from the IA will be never called from a reference to concrete classes, but only from a reference to the interface. This technique is very useful to protect developers from losing the benefit of using interfaces. With JArchitect we can check this kind of mistakes using CQLinq, the idea is to search for all methods from concrete classes used directly by other methods.

from m in Methods  where m.NbMethodsCallingMe>0 && m.ParentType.IsClass
 && !m.ParentType.IsThirdParty && !m.ParentType.IsAbstract

let interfaces= m.ParentType.InterfacesImplemented

from i in interfaces where i.Methods.Where(a=>a.Name==m.Name && 
a.ParentType!=m.ParentType).Count()>0 

select new { m,m.ParentType,i }

hibernate2

For example the method getEntityPersister from SessionFactoryImpl which implements SessionFactoryImplementor interface is concerned by this problem.

Let's search for methods invoking directly SessionFactoryImpl.getEntityPersister.

from m in Methods where m.IsUsing ("org.hibernate.internal.SessionFactoryImpl.getEntityPersister(String)") select new { m, m.NbBCInstructions }

hibernate3

Methods like SessionImpl.instantiate invoke directly getEntityPersister, instead of passing by interface. What break the benefit of using interfaces. Fortunately hibernate core doesn't contains many methods having this problem.

Coupling with external jars
When external libs are used, it’s better to check if we can easily change a third party lib by another one without impacting the whole application, there are many reasons that can encourage us to change a third party lib. The other lib could:

- Have more features.
- More performent.
- More secure.

Let’s take the example of antlr lib which used to parse the hql queries, and imagine that another parser more performent than antlr was created, could we change the antlr by the new parser easily?

To answer this question let's search which methods from hibernate use it directly:

from m in Methods where m.IsUsing ("antlr-2.7.7")
select new { m, m.NbBCInstructions }

hibernate4

And which ones used it indirectly:

from m in Projects.WithNameNotIn( "antlr-2.7.7").ChildMethods()
let depth0 = m.DepthOfIsUsing("antlr-2.7.7")
where depth0 > 1 orderby depth0
select new { m, depth0 }

hibernate5

Many methods use antlr directly what makes hibernate core highly coupled with it, and changing antlr with another one is not an easy task. this fact not means that we have a problem in hibernate design, but we have to be careful when using a third party lib and well check if a third party lib must be low coupled or not with the application.

Cohesion
The single responsibility principle states that a class should have one, and only one, reason to change. Such a class is said to be cohesive. A high LCOM value generally pinpoints a poorly cohesive class. There are several LCOM metrics. The LCOM takes its values in the range [0-1]. The LCOMHS (HS stands for Henderson-Sellers) takes its values in the range [0-2]. Note that the LCOMHS metric is often considered as more efficient to detect non-cohesive types.

LCOMHS value higher than 1 should be considered alarming.

In general classes more concerned by the cohesion are the classes having many methods and fields.

Let's search for types having many methods and fields.

from t in Types where
(t.Methods.Count() > 40 || t.Fields.Count()>40) && t.IsClass
orderby t.Methods.Count() descending
select new { t, t.InstanceMethods, t.Fields,t.LCOMHS }

hibernate6

Only few types are concerned by this query, and for all them the LCOMHS is less than 1.

Using Annotations
Annotation-based development relieves Java developers from the pain of cumbersome configuration. And give us a powerful feature to free the source code from the boilerplate code. The resulting code is also less likely to contain bugs.

Let's search for all annotations defined by hibernate core.

from t in Types where t.IsAnnotationClass && !t.IsThirdParty select t

hibernate7

Many annotations are defined, what make hibernate easy to use by developers, and the headache of configuration files is avoided.

Conclusion
Hibernate Core is a good example of open source projects to learn from, don't hesitate to take a look inside it.

More Stories By Lahlali Issam

Lahlali Issam Lead Developer at JavaDepend, a tool to manage and understand complex Java code. With JavaDepend, software quality can be measured using Code Metrics, visualized using Graphs and Treemaps, and queried using CQL language, a SQL like to query the code base.

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