Most of the enterprise
Java applications share some similarities in their design. Mostly the
packaging of these applications are driven by the framework used by them
like Spring, EJBs or Hibernate etc. Alternatively you can group you
packages by features. Like any other item regarding modeling this is not
free from any issues. Lets discuss some trade-off and how get around
them. In this post we will discuss the pros and cons of both approaches
against common usage scenarios.
Package By Layer (PBL)
This is a the first
thing that developers do when the create an enterprise application in to
split it to number of layers like DAO, SERVICE, VIEW etc.. This gives
nice separation of code when we use different frameworks at different
layers. For example if I were to use Hibernate, Spring and JSF, then I
will have all my Hibernate dependent code in the DAO layer and JSF
related code in the VIEW Layer. This is sort of good in case I need to
migrate to a new framework only at the view layer or DAO Layer.
Package By Feature (PBF)
packages to reflect the feature set. It places all items related to a
single feature (and only that feature) into a single directory/package.
This results in packages with high cohesion and high modularity, and
with minimal coupling between packages. Items that work closely together
are placed next to each other. They aren't spread out all over the
This also increases coherence as a large percentage of a the dependencies of a class are located close to that class.
Comparing the approaches
Let me compare both the approaches in below dimensions.
1. Adding a new Feature.
In case of PBL code has to be added to VIEW, SERVICE and DAO Layers and
it can be tedious. PBF solves this problem by grouping all the code
related to same feature in to a single directory.
2. Changing a framework used.
As discussed already PBL makes it easier to change a framework as all
the related code are kept at same place. Here we know exactly the scope
of the change and its impact. In case of PBF we need to dig into all the
feature set to see the framework related classes.
If you choose to migrate the framework module by module, then it could be argued that PBF is better than PBL.
3. Code Navigation.
As developers needs to
work on the features most of the time using PBF is easier for code
navigation. When you know exactly know what has to be done its not much
4. Keeping the Common Code.
Every application will
have some components which will be reused across the features ie, the
features are not always exclusive. In such case if we package all the
features separately the interactions between them can be quite messy. As
a general principle we need to reduce such interactions and increase
the cohesion inside the package. We can get around the situation be
adding such entities to a common package. This approach is used in many
projects including Hibernate.
Most application will have 4 types of classes. ie,
1. Domain Objects
2. Business Services
3. Data Retrieval Logic
4. Data Representation Logic
If we use PBF it gives
us a good structural representation but does not give any functional
representation. We need our architecture to resemble the problem domain.
So its better to use Package By Feature style. We could internally
classify the classes by using naming conventions like MyFeatureDAO
. This way we could communicate the intend of the classes.
I have found some discussions on this topic and hope that might also help you choose.
1. Java Practices Article
2. Lessons to Learn from the Hibernate Core Implementation
3. How Changing Java Package Names Transformed my System Architecture
4. Presentation By Kostis Kapelonis
5. The Principles of OOD By Uncle Bob
6. Article says its Better to Use Package By Layer
7. Question on SO Is package by feature approach good?