Understanding JDK Dynamic Proxy: Implementation Principles and Code Walkthrough

JDK dynamic proxy creates proxy classes at runtime based on interfaces, using java.lang.reflect.Proxy and an InvocationHandler to intercept method calls.

Typical usage involves defining an interface (e.g., HelloWorld ), its implementation, a custom InvocationHandler that logs before and after invocation, and creating the proxy via Proxy.newProxyInstance .

The core of proxy generation resides in Proxy.getProxyClass0 , which builds a unique class name, generates bytecode via ProxyGenerator.generateProxyClass , caches classes with WeakCache , and defines the class with defineClass0 .

ProxyGenerator can optionally save the generated .class file when the system property sun.misc.ProxyGenerator.saveGeneratedFiles is set to true, allowing inspection of the generated bytecode.

Generated proxy classes extend java.lang.reflect.Proxy , implement the target interface, and override Object methods ( equals , hashCode , toString ) to delegate to the InvocationHandler , which explains why JDK proxies only support interface‑based proxies.

Understanding these mechanisms clarifies how Spring AOP leverages JDK dynamic proxies and provides a foundation for more advanced proxy or AOP implementations.

Example interface and implementation:

package com.mikan.proxy;

public interface HelloWorld {
    void sayHello(String name);
}

public class HelloWorldImpl implements HelloWorld {
    @Override
    public void sayHello(String name) {
        System.out.println("Hello " + name);
    }
}

Custom InvocationHandler:

package com.mikan.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class CustomInvocationHandler implements InvocationHandler {
    private Object target;

    public CustomInvocationHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Before invocation");
        Object retVal = method.invoke(target, args);
        System.out.println("After invocation");
        return retVal;
    }
}

Proxy creation and execution:

package com.mikan.proxy;

import java.lang.reflect.Proxy;

public class ProxyTest {
    public static void main(String[] args) throws Exception {
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        CustomInvocationHandler handler = new CustomInvocationHandler(new HelloWorldImpl());
        HelloWorld proxy = (HelloWorld) Proxy.newProxyInstance(
                ProxyTest.class.getClassLoader(),
                new Class[]{HelloWorld.class},
                handler);
        proxy.sayHello("Mikan");
    }
}