Spring AOP

In a real application, your code has two types of logic:

1. Business Logic

Payment processing
Order creation
Account transfer

2. Cross-Cutting Concerns

Logging
Security
Transactions
Monitoring

Without AOP

public void pay() {
    log.info("Start");
    securityCheck();

    // business logic

    log.info("End");
}

Problems:

Same logging/security everywhere
Code duplication
Hard to maintain
If logging logic changes → update 100 places

What AOP Does

@Service
class PaymentService {

    public void pay() {
        // Only business logic
    }
}

@Aspect
@Component
class LoggingAspect {

    @Before("execution(* com.app.service.*.*(..))")
    public void log() {
        System.out.println("Logging...");
    }
}

AOP says:

Keep your business logic clean, and move common logic (logging, security, etc.) outside

👉 Business code remains clean
👉 Common logic is applied externally

So instead of writing logging everywhere, you define it once, and Spring applies it automatically where needed

Core Concepts

Aspect

A class that contains cross-cutting logic.

@Aspect
@Component
class LoggingAspect {}

Advice

The actual logic that runs.

@Before("execution(...)")
public void log() {
    System.out.println("Logging...");
}

Join Point

A point where AOP can be applied.

👉 In Spring: only method execution

Pointcut

Defines where AOP should apply.

execution(* com.app.service.*.*(..))

Target Object

The actual business class.

@Service
class PaymentService {}

Proxy

Spring creates a proxy instead of calling the object directly.

Client → Proxy → Actual Method

Internal Flow

Spring starts
Detects @Aspect
Creates proxy for beans
Intercepts method calls
Executes advice
Calls actual method

Flow:

Client → Proxy → Advice → Target Method

Why AOP is Powerful

Clean code
No duplication
Centralized logic
Easy maintenance

Common Mistakes

Ignoring proxy behavior
Wrong pointcut expressions
Self-invocation issue

AOP = Proxy + Interception + External Logic

Summary

Separates cross-cutting concerns
Uses proxy mechanism
Works on Spring beans
Improves maintainability

Advice Types