JDK and CGLIB proxies

Definition

Spring adds behaviour by wrapping a bean in a proxy. If the bean implements an interface Spring can use a JDK proxy, which implements the same interface. Otherwise (or when forced) it subclasses the class with CGLIB. Spring Boot uses CGLIB by default.

Example code

Which kind of proxy you get

import org.springframework.aop.framework.ProxyFactory;
import org.springframework.aop.support.AopUtils;

interface Greeter { String greet(); }

class PoliteGreeter implements Greeter {
    public String greet() { return "Good morning"; }
}

class PlainService {
    public String run() { return "done"; }
}

class Demo {
    public static void main(String[] args) {
        ProxyFactory withInterface = new ProxyFactory(new PoliteGreeter());       // has an interface
        Object jdkProxy = withInterface.getProxy();
        System.out.println("JDK proxy:   " + AopUtils.isJdkDynamicProxy(jdkProxy));

        ProxyFactory withoutInterface = new ProxyFactory(new PlainService());     // a class with no interface
        Object cglibProxy = withoutInterface.getProxy();
        System.out.println("CGLIB proxy: " + AopUtils.isCglibProxy(cglibProxy));
        System.out.println(((Greeter) jdkProxy).greet());
    }
}

// Output:
// JDK proxy:   true
// CGLIB proxy: true
// Good morning