This document discusses improving testability and maintainability of applications through dependency injection with Spring. It begins by showing how plain Java code without separation of concerns is difficult to test. It then introduces the service locator pattern, which improves testability but has downsides like being a single point of failure. Finally, it explains how Spring implements inversion of control through its core functionality, allowing for dependency injection that is more extensible and testable than the service locator pattern alone. Key Spring annotations and configurations are also outlined.

1. More Maintainable, Testable
Applications with Dependency
Injection with Spring

2. INTRODUCTION
Maintainable and testable code is
always desirable, but not always
easy to write.
Spring and Spring Boot solve this,
let's see how!

3. Plain Java
No separation of
concerns (SoC), not
testable
01
Service Locator
Pattern
A common pattern for
improving testability
02
Conclusions
What now?
04
IoC & Spring
How do Spring and
Spring Boot make life
easier?
03

4. Plain Java
01

5. Go to code!

6. The WaffleCreator
`prepare`
method has a dependency on
`MacronutrientsProvider`
Production Code

7. Production Code
prepare(...) {
}
MacronutrientsProvider mp =
new MacronutrientsProvider(...)
//..
mp.fetch() fetch(...)
{
}
new(...) {
}
HTTP call
WaffleCreator MacronutrientsProvider

8. Testing this Code
prepare(...) {
}
MacronutrientsProvider mp =
new MacronutrientsProvider(...)
//..
mp.fetch() fetch(...)
{
}
new(...) {
}
HTTP call
WaffleCreator MacronutrientsProvider
We have no
control over the
HTTP call

9. Not testable
We can't control the
dependencies.

10. At the moment, there is
also no easy way to have
different implementations
for different use-cases.
So what can we do?

11. Service Locator
Pattern
02

12. Service Locator
get(...) {
}
put(...) {
}
MacronutrientsProvider
MacronutrientsProviderLocator
MacronutrientsProvider
fetch(...)
{
}
new(...) {
}
HTTP call

13. Service Locator
get(...) {
}
put(...) {
}
MacronutrientsProvider
MacronutrientsProviderLocator
MacronutrientsProvider
fetch(...) {
}
new(...) {
}
Stubbedcall

14. WaffleCreator
prepare(...) {
}
MacronutrientsProvider mp =
MacronutrientsProviderLocator.get(...);
//..
mp.fetch()
Config
main(...) {
}
MacronutrientsProviderLocator
.put(new MacronutrientsProvider(...));
Using the Service Locator

15. WaffleCreator
prepare(...) {
}
MacronutrientsProvider mp =
MacronutrientsProviderLocator.get(...);
//..
mp.fetch()
WaffleCreatorTest
init(...) {
}
MacronutrientsProviderLocator.put(mock(MacronutrientsProvider));
Testing the Service Locator

16. Go to code!

17. Extendable!
We can add multiple service
providers without modifying
code.

18. Testable!
We can stub or mock the
dependencies.

19. Stub vs Mock
Stub Mock
Taught how to behave Verified with expectations

20. Generic Service Locator
get(...) {
}
put(...) {
}
ServiceLocator
Some Other Services
Multiple
implementations

21. Go to code!

22. WaffleCreator
prepare(...) {
}
MacronutrientsProvider mp =
ServiceLocator.get(MacronutrientsProvider.class,“nosugar);
mp.fetch();
Config
init(...) {
}
ServiceLocator.put(“nosugar”,
new MacronutrientsProvider(...),
MacronutrientsProvider.class));
Using the Generic Service Locator

23. WaffleCreator
prepare(...) {
}
MacronutrientsProvider mp =
ServiceLocator.get(MacronutrientsProvider.class,“nosugar);
mp.fetch();
Config
init(...) {
}
ServiceLocator.put(“nosugar”,
new MacronutrientsProvider(...),
MacronutrientsProvider.class));
Using the Generic Service Locator

24. But now too much boilerplate code!
ServiceLocator.get(HttpClient.class, "with timeouts");
ServiceLocator.get(MacronutrientsProvider.class, "high sugar");
ServiceLocator.get(WaffleDatabase.class, "mysql");

25. Single Point of Failure

26. Service Locator has many dependencies
Service Locator

27. Inversion of Control

28. Inversion of Control
options
Dependency
Injection
Service
Locator
Events
Aspects
Strategy
pattern via constructor
via setter

29. Inversion of Control
IoC
Container

30. Spring Core
03

31. Beans

32. Go to code!

33. Key Annotations
@Bean
added when using
java config
@Configuration
added for classes
containing @Bean
declarations
@Component
added when using
component scan
@Repository
component that is a
repository
@Service
component that is a
business service
@Qualifier
mocks/stubs bean
@MockBean
specifies name of the
injected bean
@SpringBootTest
automatically bootstraps
IoC

34. Java Config
Beans are not polluted
with annotations
Component Scan
No need to add additional
configuration classes
XML
Not used much nowadays
Bean Registration
There are three different ways that
Spring can register dependencies.

35. Go to code!

36. CREDITS: This presentation
template was created by Slidesgo,
including icons by Flaticon, and
infographics & images by Freepik
Thanks!
Repo: github.com:pilloPl/waffles.git
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