CGLIB (Code Generation Library) is a powerful bytecode generation library used in Java for dynamic proxy generation and method interception. In this blog post, we will explore some advanced usage scenarios of CGLIB.

Table of Contents

• Introduction to CGLIB
• Enhancing Classes with CGLIB
• Method Interception with CGLIB
• Callback Filters in CGLIB
• Conclusion

Introduction to CGLIB

CGLIB is a popular library that allows you to enhance and modify Java classes at runtime, without the need for source code modifications. It achieves this by generating bytecode that subclasses the target class or interfaces. CGLIB provides a simple and efficient API for this purpose.

Enhancing Classes with CGLIB

CGLIB allows you to enhance classes by creating subclasses that inherit the behavior of the original class. This is particularly useful when you need to override or add additional behavior to a class without modifying its source code.

Here’s an example of how to enhance a class using CGLIB:

import net.sf.cglib.proxy.Enhancer;

public class MyClassEnhancer {
    public static void main(String[] args) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(MyClass.class);
        enhancer.setCallback(new MyInterceptor());

        MyClass enhancedObject = (MyClass) enhancer.create();
        enhancedObject.doSomething();
    }
}

In the example above, we create an instance of the Enhancer class, set the superclass to MyClass, and provide a callback object (MyInterceptor) that will handle method invocations on the enhanced class. Finally, we create an enhanced object and invoke its methods.

Method Interception with CGLIB

Method interception is one of the powerful features provided by CGLIB. It allows you to intercept method invocations on enhanced classes and execute custom logic before or after the original method.

To demonstrate method interception with CGLIB, let’s consider an example:

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

public class MyInterceptor implements MethodInterceptor {
    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        System.out.println("Before method execution");

        Object result = proxy.invokeSuper(obj, args);

        System.out.println("After method execution");

        return result;
    }
}

In the above example, the MethodInterceptor interface is implemented to define the custom logic to be executed before and after the method invocation. The intercept method intercepts the method invocation, executes the custom logic, invokes the original method using proxy.invokeSuper, and returns the result.

Callback Filters in CGLIB

CGLIB also provides callback filters, which allow you to selectively intercept certain methods based on a defined set of rules. Callback filters can be used to apply different behavior to different methods or to exclude specific methods from interception.

Here’s an example of using a callback filter in CGLIB:

import net.sf.cglib.proxy.Callback;
import net.sf.cglib.proxy.CallbackFilter;

public class MyFilter implements CallbackFilter {
    @Override
    public int accept(Method method) {
        if (method.getName().startsWith("get")) {
            // Intercept methods starting with "get"
            return 0; // Callback index for MyInterceptor1
        } else {
            return 1; // Callback index for MyInterceptor2
        }
    }
}

In the above example, the CallbackFilter interface is implemented, and the accept method is overridden. The accept method returns the index of the callback to be used based on the given method. In this case, methods starting with “get” are intercepted by MyInterceptor1, while all other methods are intercepted by MyInterceptor2.

Conclusion

In this blog post, we have explored some advanced usage scenarios of CGLIB in Java. We have seen how to dynamically enhance classes, intercept method invocations, and use callback filters for selective interception. CGLIB is a powerful tool that can be used to add advanced features and dynamic behavior to your Java applications.

#cglib #java