[Index](../README.md) · [Designators →](02-designators.md)

# 1. What Spring AOP actually is

Spring AOP is a **proxy** mechanism that borrows AspectJ's **pointcut language**. Both halves
of that sentence explain a failure mode.

## It is proxies

Spring does not modify your bytecode. When a bean matches a pointcut, the container puts a
proxy in the bean registry in its place. Callers get the proxy; the proxy runs advice and then
forwards to the real object.

Everything that follows from this:

- Only **beans** can be advised. An object created with `new` has no proxy.
- Only calls **through the proxy** are intercepted. A call the object makes to itself is not.
- Only **overridable** methods can be advised: not private, not final, not static.
- Join points are **method executions**. Field access, constructor calls and exception handlers
  are not available, no matter what the pointcut language allows you to write.

## It borrows the pointcut language

`aspectjweaver` on the classpath supplies the pointcut *parser*. Spring uses it to decide which
methods match and then does its own weaving with proxies. AspectJ's own weaver is not involved.

This is why the designator list is a subset: the language can express `call()` and `cflow()`,
and a proxy cannot implement them. See [chapter 2](02-designators.md).

## The Spring Boot 4 starter rename

In Spring Boot 3 the dependency was `spring-boot-starter-aop`. In Spring Boot 4 it is
**`spring-boot-starter-aspectj`**.

```
spring-boot-starter-aop      last published 4.0.0-M2   (last GA: 3.5.16)
spring-boot-starter-aspectj  first published 4.0.0-M3
```

The contents are unchanged: `spring-boot-starter`, `spring-aop`, `aspectjweaver`. But the old
artifact is no longer in the Boot BOM, so copying a dependency block out of any pre-4 tutorial
fails resolution with a missing-version error rather than a helpful message.

Without the starter, `@Aspect` classes are ordinary beans, no pointcut is ever parsed, and
every aspect in the application silently matches nothing.

## When to use something else

If you need to advise field access, constructors, or calls between objects you do not own, you
need real AspectJ weaving (compile-time or load-time), not Spring AOP. If you need to advise
one internal call, you need to refactor — see [chapter 5](05-broken-aspect-gallery.md).