Spring PetClinic Sample Application: additional examples on Layered, Hexagonal and DDD (Clean/Onion)

Origin

Derived from Spring PetClinic Sample Application

Objectives

• not to muddle with the original repo

• not intend to provide a silver bullet

• not to provide "production ready" code

• not intend to demostrates how to write tests

• minimal maintenance effort

Major changes compared to original

• removed gradle

• removed check style

• removed all tests

• removed all Active Record style codes

• removed all Hibernate Inheritance Mapping

• chnaged all fetching strategies to LAZY

• removed all cascade strategies

• removed uses of WebDataBinder, Validator, and @ModelAttribute annotation at method level

• reorganised packages

• applied PRG pattern

• added some basic error messages

• added archiecture tests

How to use

1. download the repo

2. read and understand the original structure in spring-petclinic-org or the original repo at https://github.com/spring-projects/spring-petclinic

3. compare spring-petclinic-org with the changes in spring-petclinic-layered

4. compare spring-petclinic-layered with the changes in spring-petclinic-layered-hex

5. compare spring-petclinic-layered with the changes in spring-petclinic-layered-ddd, spring-petclinic-layered-clean and spring-petclinic-layered-onion

Project Structures

Project	Separation of Concern	Dependency	Summary
spring-petclinic-org	No	Presentation → Model → Dao Presentation → Dao Model ← Dao	Original before fork
spring-petclinic-layered	Some	Presentation → Service → Dao Service → Domain Domain → Dao Domain ← Dao	Layered Architecture Example Rewritten from Orginal to usually how people approach traditional Layered Architecture. This is not the only form of traditional Layered Architecture.
spring-petclinic-layered-hex	Yes	Driving → Application ← Driven	Hexagonal Architecture™ Example Rewritten from Layered Architecture Example
spring-petclinic-layered-ddd	Yes	User Interface → [Application → Domain] ← Infrastructure	DDD Architecture™ Example Rewritten from Layered Architecture Example
spring-petclinic-layered-clean	Yes	User Interface → [Application → Domain] ← Infrastructure	Clean Architecture™ Example Rewritten from Layered Architecture Example. Please note that this is an example of "Clean Architecture™", not "clean architecture". "Clean Architecture™" is the architecture and exmaple deescribed in the book Clean Architecture™. "clean architecture" is more like using general English to describe an architecture that anyone can come up with and self-proclaimed as "an architecture that is clean".
spring-petclinic-layered-onion	Yes	User Interface → [Application → Domain] ← Infrastructure	Onion Architecture™ Example Rewritten from Layered Architecture Example

Layering

Type	Layers				Remarks 1	Remarts 2	Remarts 3
Original	Kind of Vertically sliced
Layered Architecture	Presentation	Service	Model	Data Access			All of these are a form of Layered Architecture
Hexagonal Architecture™ (Ports and Adapters)	Driving	Application		Driven		Hexagonal, DDD, Clean and Onion Architectures are bascically re-discovery of the same idea at different points in time in different context
DDD Architecture™	User Interface	Application Service	Domain	Infrastructure	These 3 are basically identical with different naming preferences
Clean Architecture™
Onion Architecture™

• In DDD, everything in a domain package, including sub packages, together is a Domain Model. An object like the Pet object alone is NOT the Domain Model but a domain object inside a Domain Model