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CGLIB for implementing runtime method interception in Java

In Java, a common requirement is to intercept method calls at runtime for various purposes such as logging, performance monitoring, security checks, etc. To achieve this, we can use a library called CGLIB.

CGLIB is a code generation library for Java that enables dynamic generation of classes and interfaces at runtime. It is widely used in frameworks such as Spring and Hibernate for enhancing or proxying objects to add additional behavior.

In this blog post, we will explore how to use CGLIB to implement runtime method interception in Java.

Table of Contents

What is CGLIB?

CGLIB stands for Code Generation Library. It is an open-source library that is used for generating dynamic proxy classes and objects at runtime. CGLIB is an alternative to the standard Java Dynamic Proxy API, which only works with interfaces. CGLIB allows us to create proxies for classes that do not implement any interfaces.

Implementing Method Interception with CGLIB

To implement method interception with CGLIB, we need to follow these steps:

  1. Create a subclass of the target class.
  2. Implement a MethodInterceptor to intercept the method calls.
  3. Use CGLIB to generate a proxy class that extends the target class and implements the MethodInterceptor.
  4. Create an instance of the proxy class and use it instead of the original target class.

Using MethodInterceptor

The MethodInterceptor interface from CGLIB allows us to intercept method calls and add custom behavior before or after the method execution. It has a single method named intercept that takes three parameters: the target object, the method being invoked, and the method arguments.

Here is an example implementation of the MethodInterceptor interface:

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

import java.lang.reflect.Method;

public class LoggingInterceptor implements MethodInterceptor {
    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        // Write custom logic before method execution
        
        // Invoke the original method
        Object result = proxy.invokeSuper(obj, args);
        
        // Write custom logic after method execution
        return result;
    }
}

In the intercept method, we can add custom logic before and after invoking the original method.

Example: Logging Interceptor

Let’s say we want to add logging functionality to a UserService class. We can use CGLIB and the LoggingInterceptor as follows:

import net.sf.cglib.proxy.Enhancer;

public class UserServiceProxy {
    public static UserService createProxy(UserService userService) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(UserService.class);
        enhancer.setCallback(new LoggingInterceptor());
        return (UserService) enhancer.create();
    }
}

public class Main {
    public static void main(String[] args) {
        UserService userService = new UserServiceImpl();
        
        UserService proxy = UserServiceProxy.createProxy(userService);
        
        proxy.createUser("John Doe");
        proxy.updateUser("John Doe");
        proxy.deleteUser(123);
    }
}

In the example above, we create a proxy of the UserService interface using CGLIB and the LoggingInterceptor. The UserServiceProxy class is responsible for creating the proxy instance.

Conclusion

CGLIB is a powerful library for implementing runtime method interception in Java. It allows us to generate dynamic proxies for classes and intercept method calls to add custom behavior. By using CGLIB, we can easily add features like logging, performance monitoring, and security checks to our applications.

In this blog post, we explored how to use CGLIB to implement runtime method interception in Java. We learned about the MethodInterceptor interface and saw an example of how to create a logging interceptor.

References

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