This is the slide for the talk in JCConf Taiwan 2019

1. Slide 1
Structuring your Java application
黃俊喬 Woody Huang
woody0927@gmail.com
2019.10.4

2. Slide 2
Background
Our Product:
▪ Monolith
▪ More than 10 years of development
▪ More than 10 developers working on the project
Problems:
▪ Not possible to know/remember everything of the system
▪ Inefficient or even fail to find suitable reusable method
▪ Redundant code
▪ Wrong method invocation
▪ Runtime environment is not controlled by us

3. Slide 3
Outline
▪ Different approaches to structuring your application
▪ Pros and cons of each approach
▪ Common problems and solutions
▪ Example

4. Slide 4
Different approaches to structuring your application
▪ Everything in one or a few classes
▪ Packing by technical layers
▪ Packing by feature
▪ Grouping relevant features into module

5. Slide 5
Everything in one or a few classes
Concept: Putting all classes in one package
Pros:
▪ No-brainer design
Cons:
▪ Hard to maintain
▪ Hard to debug
▪ Changes are prone to error

6. Slide 6
Packing by technical layers
Concept: All classes in the package are of the same type, e.g. Service.
Collaboration among packages are usually required to complete a feature.
Pros:
▪ Easy and better structured
▪ Information is at certain level protected
▪ Changes sometimes can be localized to one layer
Cons:
▪ Low cohesion and high coupling
▪ High Ripple effects
▪ Modifiers have to be public for cross package access
▪ Internal information leaked via public modifier

7. Slide 7
Packing by feature
Concept: Classes get used together are putting together
Pros:
▪ Even better structured - high cohesion and low coupling
▪ Information is protected
▪ Minimizing ripple effects
Cons:
▪ Modifiers have to be public if we create sub-packages
▪ Internal information is leaked via public modifier

8. Slide 8
Grouping relevant features into module
Concept: Features having same domain concept are putting together
and form an multi-module project. This is used together with grouping
by feature / technical layers and inherits the pros and cons.
Pros:
▪ Most of changes can be isolated to one module

9. Slide 9
Common problems to different grouping approach
▪ Limited encapsulation
▪ public modifier is inevitable - enterprise application is unlikely to
be built without sub-packages
▪ Confusions and wrong method invocations due to leaked
information

10. Slide 10
Solutions
▪ internal package - works with interface and factory to get the
runtime instance (regulated by convention)
▪ api package - (regulated by convention and code analysis rule)
▪ Java 9 - Java Platform Module System whenever possible

11. Slide 11
Example: Overwhelmed by implementation detail
Module: Onboarding
Module:
Risk-assessment
ClientService
+ register
+ backgroundCheck
+ listProspectClients
+ listRealClients
+ …..
listRealClients

12. Slide 12
Example: API exposure - information hiding
Module: Onboarding Module: Risk
assessment
ClientSe
rvice
+ …... listRealClients
API
listRealClients

13. Slide 13
Reference
▪ Sample: https://github.com/woody0927/structuring-java-app
▪ Package by feature, not layer
▪ Package by type, -by layer, -by feature vs “Package by layered feature”

14. Slide 14
QUESTIONS & DISCUSSIONS