CGLIB for implementing runtime event subscribing in Java

04 Oct 2023

In Java, event-driven programming is a common practice where objects can subscribe to events and be notified when those events occur. Traditionally, this is achieved using interfaces or abstract classes. However, when you need to add event-subscribing behavior to an existing class or a third-party library that you don’t have control over, you may find yourself in a situation where modifying the class hierarchy is not feasible.

This is where CGLIB (Code Generation Library) can come to the rescue. CGLIB is a powerful code generation library for Java that allows you to create dynamic proxies and enhance classes at runtime. In this blog post, we will explore how to use CGLIB to implement runtime event subscribing in Java.

What is CGLIB?

CGLIB is a widely-used code generation library that operates at the bytecode level. It allows you to create dynamic proxies, intercept method invocations, and manipulate classes at runtime. It is commonly used for tasks such as creating proxies for AOP (Aspect-Oriented Programming) and adding runtime behavior to existing classes.

Implementing Event Subscribing with CGLIB

To implement runtime event subscribing with CGLIB, follow these steps:

First, make sure you have the necessary dependencies in your project. You will need the CGLIB library, which you can include in your build configuration.
Define an event interface that specifies the events that can be subscribed to. This interface should include methods representing the events that other objects can subscribe to.

public interface EventListener {
    void onEvent1();
    void onEvent2();
}

Create a class that will act as the event publisher. This class will be responsible for keeping track of the subscribed listeners and notifying them when events occur.

public class EventPublisher {
    private List<EventListener> listeners = new ArrayList<>();

    public void subscribe(EventListener listener) {
        listeners.add(listener);
    }

    public void unsubscribe(EventListener listener) {
        listeners.remove(listener);
    }

    public void fireEvent1() {
        listeners.forEach(EventListener::onEvent1);
    }

    public void fireEvent2() {
        listeners.forEach(EventListener::onEvent2);
    }
}

Now, comes the interesting part. We will use CGLIB to create a dynamic proxy that intercepts method invocations on the event publisher class and notifies the subscribed listeners.

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

public class EventProxy implements MethodInterceptor {
    private EventPublisher publisher;

    private EventProxy(EventPublisher publisher) {
        this.publisher = publisher;
    }

    public static EventPublisher createProxy(EventPublisher publisher) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(EventPublisher.class);
        enhancer.setCallback(new EventProxy(publisher));
        return (EventPublisher) enhancer.create();
    }

    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        Object result = method.invoke(publisher, args);

        if (method.getName().startsWith("fireEvent")) {
            publisher.getListeners().forEach(listener -> {
                try {
                    Method eventMethod = listener.getClass().getMethod(method.getName());
                    eventMethod.invoke(listener);
                } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
                    e.printStackTrace();
                }
            });
        }

        return result;
    }
}

Now, let’s see how to use the dynamic proxy in your code. Create an instance of the EventPublisher class and then create a proxy using the EventProxy class:

EventPublisher publisher = new EventPublisher();
EventPublisher proxy = EventProxy.createProxy(publisher);

Now, you can subscribe to events using the proxy object and receive notifications when the events occur:

EventListener listener = new MyEventListener();
proxy.subscribe(listener);

Finally, trigger the events and observe the event notifications:

proxy.fireEvent1(); // This will trigger the onEvent1 method of all subscribed listeners
proxy.fireEvent2(); // This will trigger the onEvent2 method of all subscribed listeners

Conclusion

CGLIB is a powerful code generation library that can be used to implement runtime event subscribing in Java. By creating dynamic proxies and intercepting method invocations, we can add event-subscribing behavior to existing classes without modifying their class hierarchy. This allows for more flexibility and extensibility in event-driven programming.

Using CGLIB, you can easily enhance your Java classes with event publishing capabilities, making your code more modular and easier to maintain.