Understanding AOP: AspectJ Compile‑Time Weaving vs Spring AOP Dynamic Proxies

AOP (Aspect‑Oriented Programming) supplements OOP by handling cross‑cutting concerns such as transactions, logging, and caching, and is implemented via proxies that can be static (compile‑time) or dynamic (runtime).

Using AspectJ for compile‑time enhancement

AspectJ creates static proxies during compilation, weaving aspects directly into the bytecode.

public class Hello {
    public void sayHello() {
        System.out.println("hello");
    }
    public static void main(String[] args) {
        Hello h = new Hello();
        h.sayHello();
    }
}

Aspect definition:

public aspect TxAspect {
    void around(): call(void Hello.sayHello()) {
        System.out.println("开始事务 ...");
        proceed();
        System.out.println("事务结束 ...");
    }
}

Compile with ajc -d . Hello.java TxAspect.aj and run Hello, producing:

开始事务 ...
hello
事务结束 ...

The compiled Hello.class now contains additional methods generated by AspectJ, illustrating static weaving.

Using Spring AOP

Spring AOP employs dynamic proxies: JDK proxies for classes implementing interfaces and CGLIB proxies for concrete classes.

Interface and implementation example:

public interface Person {
    String sayHello(String name);
}

@Component
public class Chinese implements Person {
    @Timer
    @Override
    public String sayHello(String name) {
        System.out.println("-- sayHello() --");
        return name + " hello, AOP";
    }
    public void eat(String food) {
        System.out.println("我正在吃：" + food);
    }
}

Aspect definition:

@Aspect
@Component
public class AdviceTest {
    @Pointcut("@annotation(com.listenzhangbin.aop.Timer)")
    public void pointcut() {}

    @Before("pointcut()")
    public void before() {
        System.out.println("before");
    }
}

Spring Boot test shows JDK proxy output:

before
-- sayHello() --
class com.sun.proxy.$Proxy53

When the target class does not implement an interface, Spring falls back to CGLIB:

@Component
public class Chinese {
    @Timer
    public String sayHello(String name) {
        System.out.println("-- sayHello() --");
        return name + " hello, AOP";
    }
    public void eat(String food) {
        System.out.println("我正在吃：" + food);
    }
}

Running the same test now yields a CGLIB‑enhanced class:

before
-- sayHello() --
class com.listenzhangbin.aop.Chinese$EnhancerBySpringCGLIB$$56b89168

Summary

AspectJ weaves aspects at compile time, offering better performance but requiring a special compiler, whereas Spring AOP creates proxies at runtime, needing no extra compilation step; the choice depends on performance requirements and build constraints.