You can only claim you know Spring if you fully understand the Proxy design pattern, and how the framework uses it to do its magic. Join this live-coding session to explore 6 ways of intercepting method calls that you can use to cast your own spells and dramatically simplify your codebase. Grab a strong coffee and prepare yourself for a whirlwind of live-coding, interwoven with deep theoretical concepts and implementation details that you have to master if you are using the Spring Framework. During this session, Victor will share one of the best parts of his Design Patterns training, applied to Spring. You'll hear about the Decorator pattern plus 2 ways to wire it with Spring, bare-hands interface proxies, concrete classes proxies, @Aspect applied to custom annotations, plus some of the most common standard off-the-shelf Spring aspects. Come and learn from a hands-on practitioner real-world best practices of using Aspects, design hints, under-the-hood implementation details, debugging tips, performance impact of aspects, all in an interactive, entertaining and extremely dynamic session. - Talk held at JPoint 2019, Moscow

1. The Magic of Spring
The Proxy Fairy
Victor Rentea - Independent Trainer VictorRentea.ro @victorrentea

2. When something is painful
but you can't avoid doing it…
postpone it

3. When something is painful
but you can't avoid doing it…
delegate it

4. When something is painful
but you can't avoid doing it…
Do It More Often!
"Bring The Pain Forward!"

5. Continuous Integration
Pair Programming
Continuous Refactoring
TDD
XP
"Bring The Pain Forward!"

6. Victor Rentea
14 years of Java
Clean Code Evangelist
VictorRentea.ro
30+ talks, 12 meetups
.NET
Lead Architect
Tech Team Lead and Consultant
Software Craftsman
XP: Pair Programming, Refactoring, TDD

7. VictorRentea.ro @victorrentea victor.rentea@gmail.com
Independent
Technical Trainer & Coach
HibernateSpring Java 8
Architecture, DDDDesign Patterns
Clean Code Unit Testing, TDD
Java Performance and much more…Scala
180+ days1300 devs6 years
VictorRentea.rovictor.rentea@gmail.com
30 companies
Posting daily on

12. ▪Log the arguments passed to m1()
▪The same for m2(), please
▪And for m3,m4,...m30
▪I love it! I want the same
for all methods in this package
copy-paste
A Success Story
?!!...Argh!!... copy-paste ...
You add a log.debug(...) in m1
You copy-paste in m2 and adjust it
Whaat?!!! ... copy-paste in 110 methods.
You feel bad. For 5 minutes.
Next task...
copy-paste
copy-paste
Time passes by

13. ▪BUG: a method
is not logged !!!%@^!
Time passes by
(2 weeks later)
I’ll put this method here...
▪Please change the log format Oh boy! 1, 2, 3, .. 109, Done!
I  my job!▪BUG: you missed a spot !!!%@^!
A Success Story

15. ▪Log the arguments passed to m1()
▪The same for m2(), please
▪And for m3,m4,...m30
▪I love it! I want the same
for all methods in this package
copy-paste
A Success Story
?!!...Argh!!... copy-paste ...
You add a log.debug(...) in m1
You copy-paste in m2 and adjust it
Whaat?!!! ... copy-paste in 110 methods.
You feel bad. For 5 minutes.
Next task...
copy-paste
copy-paste
Time passes by

16. copy-pasteNever logic
R
decompose it in smaller methods
and call them as you need
Don’t Repeat Yourself
The Capital Sin in Programming
copy-pastecopy-paste
D Y

17. What if I could magicaly
intercept method calls
and do stuff for each call ?
Aspect-Oriented Programming
Logging
Transactions
Access Control
Audit
...
“Cross-cutting concerns”:
“But I will still need to add one line in each method!

18. placeholder intermediating interactions with an object
•The client wants to work with the Real Subject
•But we trick him to intercept his calls, to do
(1) AOP
(2) Remote calls (RMI, Web Services)
Proxy
«interface»
Subject
someMethod()
RealSubject
someMethod()
Proxy
someMethod()
represents
calls
actually uses
generated

19. do more things in an object’s methods, by composition
• Implement the Subject interface,
execute code before/after/instead calling the real method
➢Can be nested: new Decor1(new Decor2(original));
➢Usually written by hand
Decorator
Proxy«interface»
Subject
someMethod()
RealSubject
someMethod()
Proxy
someMethod()
represents
calls
actually uses
Decorator
delegate
someMethod()

20. •Decorator + Proxy Patterns
•Interface Proxies
•Class Proxies
•In-depth Spring AOP
•Custom Aspects - Best Practices
Contents

22. Every time you don’t understand how Spring does something…
it's a Proxy
@Cacheable
@Transactional
@Aspect
@Async
@Secured
@Validated
Request/Thread scope
…
The Magic of Spring

23. class A {
private B b;
...
}
class BImpl
implements B {
...
}
interface B {
...
}
Interface Proxy
has a
:A
B
:BImpl
implem
call
Static
Runtime
implem
proxy
Created at run-time with
java.lang.reflect.Proxy

24. class A {
private B b;
...
}
interface B {
...
}
Class Proxy
has a
:A
B
:B
call
Static
Runtime
proxy
Concrete classes are proxyied
via bytecode generation
class B {
extends
*
*

25. •A method of a non-Spring bean?
•A final method/class?
• For a class-proxy:
•Field access?
•A private method call?
Can AOP Intercept…
NO!
NO!
NO!
Well…
Actually…
You know…
You can do any of that via:
Bytecode Enhancement
(compile-time hacking)
Or Instrumentation
(classload-time hacking)
NO!

26. •Autowiring yourself ?
•@Autowired ApplicationContext + getBean(T)
•@Autowired ObjectFactory<> + getObject()
•@Lookup method
•AopContext.currentProxy()
Call Yourself via a Proxy
To intercept a local call,
AopContext.currentProxy()

27. proxy
methodWithTx();
CurrentClass myselfProxied =
(CurrentClass)AopContext.currentProxy();
myselfProxied.methodWithTx();
AopContext.currentProxy()
@Transactional(propagation = REQUIRES_NEW)
public void methodWithTx() {

28. No More Magic

29. - an AOP Alternative -
Functional Programming
"Execute Around" Pattern
transactionTemplate.execute(this::methodInNewTx);

30. I found my code!
The Hamburger Problem
Proxies
What the heck is the rest?!
in front of any class with 1+ methods to intercept
Are you sure you want to know?
stacktrace

31. How can Aspects
(invisible logic)
communicate?
What is shared?
(within an invocation)
ThreadLocal
(invisible data)

33. ThreadLocal Data -- what about @Async calls?
Propagate Thread Data
to worker threads
Hunt me
afterwards

34. •Decorator + Proxy Patterns
•Interface Proxies
•Class Proxies
•In-depth Spring AOP
•Custom Aspects - Best Practices
Contents

35. @annotation(my.proj.Logged)
annotation-based weaving
Designing Custom Aspects - Best Practices
@Around("execution(* *(..)) && @within(my.proj.Facade))")
public Object logAround(ProceedingJoinPoint point) { … }
make them visible
@Facade
public class ExpensiveOps {
@Logged
public Boolean isPrime(int n) {…}
}

36. Designing Custom Aspects - Best Practices
"execution(* com.mycomp.proj.facade.*.*(..))"
instead of
package name
or
class name
"execution(* com.mycomp..*DAO.*(..))"
annotation-based weaving

37. keep them light
Designing Custom Aspects - Best Practices
synchronized
files
database
http
don't intercept insane-rate calls

38. Designing Custom Aspects - Best Practices
I did that!
don't intercept insane-rate calls
alternatives:
Execute-AroundFP
BeanPostProcessor
CPP

39. BPP
after init
@PostConstruct
BPP
before init
@Autowired
Designing Custom Aspects - Best Practices
BeanPostProcessor
new
*Technically, each step above hides a bit more work, and more ways to configure
Can return a different instanceproxy
WHY?

40. Average Error (99.9%) Unit
Cache Method 357.7 ± 11.7
us/op
Decorator 355.1 ± 6.0
Interface Proxy 398.7 ± 28.0
Class Proxy 386.9 ± 17.5
Class Proxy / BPP 373.9 ± 36.0
@Cacheable 8109.8 ± 1097.5
Performance
Details? Run it yourself: https://github.com/victorrentea/proxy-fairy-performance

41. Leftover Tricks
Designing Custom Aspects - Best Practices
- Double proxying
- @Aspect @Order
- Sharing the PersistenceContext
- Proxies in ORMs
Hunt me
afterwards

42. Every time you don’t understand how Spring does something…
it's a Proxy
The Magic of Spring
mixins

43. Spring Data
mixins
CustomerRepository
Customer findByName(String name);
@Query("SELECT c FROM Cust... ")
List<Customer> getActiveCustomers();
JpaRepository<E,ID>
List<E> findAll();
E findOne(ID);
void save(E);
void delete(E); ...
CustomerRepositoryCustom
List<Customer> search(CustomerSearch);
CustomerRepositoryImpl
List<Customer> search(CustomerSearch) {
...<implem>...
}
YourBaseRepository<E,ID>
abc customMethod(xyz);
YourBase
RepositoryImpl
<implem>
naming
rules

44. ✓Proxy + Decorator Patterns; wiring
✓Interface & Class Proxy - under the hood
✓Spring AOP - tricks & limitations
✓Custom Aspects - Best Practices
Key Points

46. And have a nice Spring!
Thank You!
I'm available
a proof of seniority
I use both hemispheres
Tough meetings?
Abused estimates?
Purpose of code:--Uncle Bob
1. Maintainable
2. Does its job!
Functional Party
Activist
Stay into
The Light
Trainings, Talks, Goodies Daily Posts
Clean Code
needs strength
and determination
Let's chat!
Thanks for review,
Vladimir Sitnikov
Hunt me
down