Wrestling with Spring Proxies

Today, I was wrestling with Spring 2 proxies for my pet project.
I wanted to switch back from AspectJ Transaction support to Spring AOP support and although it seemed easy, I stumbled on the following exception :

Caused by: org.springframework.aop.framework.AopConfigException: Couldn't generate CGLIB subclass of class [class hrutil.service.LoginApplicationService]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given
Caused by: java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given

The following code generated problems:

public class LoginApplicationService {
private UserRepository userRepository;
public LoginApplicationService(UserRepository userRepository) {
this.userRepository = userRepository;
}
...}

and

<bean id="loginApplicationService" class="hrutil.service.LoginApplicationService">
<constructor-arg ref="userRepository"/>
</bean>

It left me paralyzed for a moment, since I did define constructor arguments. Googling my problem finally brought the solution.

Spring can use two different techniques for creating proxies at runtime: CGLIB or JDK dynamic proxies. If the target class implements one or more interfaces, then Spring will create a JDK dynamic proxy that implements every interface. If the target class implements no interfaces, Spring will use CGLIB to create a new class on the fly that is a subclass ("extends") the target class. This leads to one important difference: a JDK dynamic proxy cannot be casted to the original target class because it's simply a dynamic proxy that happens to implement the same interface(s) as the target.

Another important difference (and the one that caused the exception) : CGLIB proxies cannot be used with constructor-arguments.

Thus, the problem can be solved in two ways:

1. Create interfaces for the services you want to proxy (never a bad habit and easily produced with your IDE's Extract Interface Refactoring).
2. Do not use Spring constructor arguments when CGLIB is selected.