Understanding Java Proxy Mechanisms and Spring AOP: Static, Dynamic, and AspectJ

This article introduces Java's proxy mechanism and its application in Spring AOP, covering the proxy design pattern, bytecode loading, static proxies (AspectJ and JDK), dynamic proxies (JDK and CGLIB), and how Spring chooses between them.

1. Proxy Pattern

Explains the classic proxy pattern using a celebrity‑agent analogy, highlighting its advantages such as hiding the delegate implementation and decoupling client from the real subject.

2. Bytecode and Proxy

Describes how Java compiles .java files to .class bytecode and how bytecode can be generated at runtime to create classes dynamically, illustrated with a Javassist example.

public class JavassistDemo {
    public static void main(String[] args) {
        makeNewClass();
    }
    // ... (rest of the Javassist code)
}

3. Static Proxy

3.1 AspectJ Static Proxy

Shows how to add AspectJ dependencies, define a pointcut and advice, and compile with the AspectJ compiler to produce a statically woven proxy.

<dependency>
org.aspectj
aspectjrt
1.8.9
</dependency>

3.2 JDK Static Proxy

Implements a proxy class that implements the same interface as the real subject and adds pre‑ and post‑processing logic.

public class Agent implements ShowService {
    private Star star;
    private void getMoney(){ System.out.println("get money"); }
    private void writeReceipt(){ System.out.println("write receipt"); }
    @Override
    public void sing(String songName){
        getMoney();
        star.sing(songName);
        writeReceipt();
    }
    // ...
}

4. Dynamic Proxy

4.1 JDK Dynamic Proxy

Uses java.lang.reflect.Proxy and InvocationHandler to create a proxy at runtime.

ShowService proxy = (ShowService) Proxy.newProxyInstance(
    star.getClass().getClassLoader(),
    star.getClass().getInterfaces(),
    new InvocationHandlerImpl(star));
proxy.sing("Mockingbird");

4.2 CGLIB Dynamic Proxy

Creates a subclass of the target class using Enhancer and MethodInterceptor, allowing proxying of classes without interfaces.

Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(star.getClass());
enhancer.setCallback(new MethodInterceptorImpl());
ShowService proxy = (ShowService) enhancer.create();
proxy.sing("Mockingbird");

5. Spring AOP

Explains that Spring AOP is built on dynamic proxies (JDK or CGLIB) and shows both XML‑based and annotation‑based configurations, including the use of @AspectJ annotations without requiring the AspectJ compiler.

<aop:aspectj-autoproxy proxy-target-class="true"/>

6. Summary

The article provides a practical, code‑driven overview of Java proxy techniques and their integration into Spring AOP, helping readers understand when to use static versus dynamic proxies and how Spring decides between JDK and CGLIB implementations.