The Proxy Fairy and the Magic of Spring @JAX Mainz 2021

victorrentea@gmail.com ♦ ♦ @victorrentea ♦ VictorRentea.ro
All the code is available on the branch JAX_Mainz on https://github.com/victorrentea/spring

Victor Rentea
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34 © VictorRentea.ro
a training by
34

3
a training by
class A {
private B b;
...
}
class BImpl
implements B {
m() {} ...
}
interface B {
m(); ...
}
Interface Proxy
has a
:A
B
:BImpl
implem
Static
Runtime

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

a training by
class A {
private B b;
...
}
interface B {
m(); ...
}
Class Proxy
has a
Static
Runtime
class B {
{ }
:A
B
:B
call
proxy
m() {}
The bytecode of a subclass
is generated in memory by (eg CGLIB)
extends
Different InstanceS

a training by
38

a training by
Every time you don’t understand how Spring does something…
it's a Proxy
@Cacheable
@Transactional
@Aspect
@Async
@Secured / @PreAuthorize
@Retryable
Request/Session/Thread scope
…
The Magic of Spring

a training by
Dependency Injection is great because
1 Classes are easier to break
3 Proxies can be given to you transparently
2 Cleaner to test

a training by
:B
proxy
m() {}
Different InstanceS
Local Method Calls
don't get Proxied!
@Override
m() {}
Class Proxy Pitfalls
final
ignored:
final class
crash:
The default way
in Spring
You have to take
a reference to B
from Spring

@ Victo
r
Rentea
42
•A method of a non-Spring bean?
• Think new
•A final method/class?
• For a class-proxy:
•A static method?
•Field access?
•A local method call?
Can Spring AOP Intercept…
NO!
NO!
Spring AOP can only intercept method calls
NO!
Well…
Actually…
You know…
You can do any of that via:
Bytecode Enhancement
(compile-time hacking)
Or Instrumentation
(classload-time hacking)
NO!!!
NO!
How could it be overridden?
https://github.com/dsyer/spring-boot-aspectj#

@VictorRentea
43
proxy
methodWithTx();
myselfProxied.methodWithTx();
War Stories
@Transactional(propagation = REQUIRES_NEW)
public void methodWithTx() {
@Autowired
private CurrentClass myselfProxied;
--or–
move methodWithTx() to another @Autowired bean
--or--
CurrentClass myselfProxied =
(CurrentClass)AopContext.currentProxy();
--or--
transactionTemplate.execute(s->methodWithTx());
https://stackoverflow.com/questions/3423972/spring-transaction-method-call-by-the-method-within-the-same-class-does-not-wo

a training by
No More Magic

a training by
Spring + nativeimage?
45
•Serverless with Spring Cloud Function
•Cheaper hosting of your Spring microservices
•Good fit with Kubernetes platforms like VMware Tanzu
https://spring.io/blog/2021/03/11/announcing-spring-native-beta
Niche?
https://github.com/spring-projects-experimental/spring-native

a training by
nativeimage does NOT support
class-proxies (CGLIB)
Only JDK (interface-) proxies work,
via a Maven Plugin doing Ahead-Of-Time analysis

a training by
@Cacheable -> CacheManager
@Transactional -> TransactionTemplate
@Async -> executor.submit()
@PreAuthorize/@RolesAllowed
-> SecurityContextHolder.getContext()...
@Retryable -> RetryTemplate
Programmatic Alternatives to Annotations

@VictorRentea
48
Functional Programming vs AOP
"Execute Around" Pattern
transactionTemplate.execute(this::methodInNewTx);
(Use JFR instead)

@VictorRentea
49
The Hamburger Problem

@VictorRentea
50
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

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51
Writing Custom Aspects

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52
@annotation(my.proj.Logged)
annotation-based weaving
Custom Aspects - Best Practices
@Around("@within(my.proj.Facade)")
public Object logAround(ProceedingJoinPoint point) { … }
make them visible
@Facade
public class ExpensiveOps {
@Logged
public Boolean isPrime(int n) {…}
}

@VictorRentea
53
"execution(* com.mycomp.proj.facade.*.*(..))"
is less magic than
package names
or
class name patterns
"execution(* com.mycomp..*DAO.*(..))"
annotation-based weaving

a training by
keep them light
synchronized
files
database
http
don't intercept insane-rate calls

a training by
Annotation-based AOP can feel like changing the language
Make everyone in the team comfortable with it
(think twice before changing the proxy default behavior)
Don't fear performance
(as long as you don't apply it to 100% of methods, and you keep them light)
Consider Alternatives
When interfering with the design
The Moral?

find me on Twitter: @victorrentea
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The Proxy Fairy and the Magic of Spring @JAX Mainz 2021

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