CGLIB on JVM

Simply put

CGLIB is an open-source Java library that provides powerful and high-performance code generation capabilities, often used for dynamic proxies and AOP (Aspect-Oriented Programming). CGLIB allows developers to generate subclasses of specified classes at runtime and enhance them, such as adding interceptors and method interception logic.

The underlying implementation of CGLIB relies on the ASM framework, a Java bytecode manipulation framework that can transform bytecode and generate new classes. Unlike traditional Java reflection mechanisms, CGLIB uses ASM to directly manipulate bytecode, so it usually has performance advantages, especially when handling a large number of concurrent requests.

The dynamic proxy features of CGLIB allow developers to extend the behavior of classes without modifying the source code. It supports proxying classes that do not implement any interfaces, which is different from Java’s dynamic proxies (which require implementing an interface). However, CGLIB also has its limitations, such as not being able to proxy methods declared as final, as these methods are not allowed to be overridden in the Java language specification.

CGLIB is used in various scenarios, such as the Spring AOP framework, which uses CGLIB to dynamically proxy and enhance method execution. In addition, Hibernate also uses CGLIB to proxy single-table mapping associations (such as many-to-one and one-to-one).

When using CGLIB, you can create proxy instances through the Enhancer class, and set the proxy class and one or more Callback objects. These Callback objects will be called back when the method is called. You can specify different interception strategies for different methods through CallbackFilter.

In general, CGLIB is a feature-rich library that allows developers to easily proxy and enhance classes through bytecode generation, thereby implementing advanced features such as AOP and lazy loading without modifying the source code or building new classes.

Understanding Dynamic Proxy

Dynamic proxy is a technique in Java that allows the creation of proxy objects at runtime. These proxy objects behave as stand-ins for the real objects and can intercept method calls, enabling additional logic to be applied before or after the method execution. In the context of Spring, dynamic proxy technology plays a crucial role in injecting dependencies into the target objects.

JDK Dynamic Proxy and CGLIB

Spring employs two primary dynamic proxy technologies: JDK dynamic proxy and CGLIB. JDK dynamic proxy generates proxy objects based on the target object’s interfaces, while CGLIB is used when the target object does not implement any interfaces. Spring intelligently selects the appropriate dynamic proxy technology based on the nature of the target object.

How Dependency Injection Works with Dynamic Proxy

When you define a bean in a Spring application context and specify its dependencies, Spring’s IoC (Inversion of Control) container creates and manages these objects. Through dynamic proxy, Spring can inject the required dependencies into the target objects before returning them to the client code. This approach helps in achieving loose coupling between the components and allows for easier unit testing and maintenance.

Understanding Proxy Mechanism through an Example

Let’s consider an example where we have a service class that depends on a data access object (DAO) interface. When we configure this service class as a bean in the Spring application context and specify its dependency on the DAO interface, Spring uses dynamic proxy to create a proxy that implements the DAO interface. This proxy is then injected into the service class, effectively decoupling the two components.

Conclusion

In conclusion, Spring’s dependency injection mechanism harnesses the power of dynamic proxy technology to achieve loose coupling and effective management of dependencies in enterprise applications. Understanding the underlying dynamic proxy concepts helps developers utilize this feature to its fullest potential, ultimately leading to more maintainable and testable code.

See

https://www.baeldung.com/cglib
https://docs.spring.io/spring-framework/reference/core/aop/proxying.html