Implementing an interface from a framework vs simple java interface

Question

This concept is unclear with me.

I have worked on several frameworks for an instance Spring. To implement a feature we always implement some interfaces provided by the framework.

For an instance if I have to create a custom scope in Spring, my class implements a org.springframework.beans.factory.config.Scope interface. Which has some predefined low level functionality which helps in defining a custom scope for a bean.

Whereas in Java I read an interface is just a declaration which classes can implement & define their own functionality. The methods of an interface have no predefined functionality.

interface Car
{
topSpeed();
acclerate();
deaccelrate();
}

The methods here don't have any functionality. They are just declared.

Can anyone explain this discrepancy in the concept? How does the framework put some predefined functionality with interface methods?

Answer 1

There is no difference between how a framework handles interfaces and how your code does. There is a concept called Interception. Frameworks often times have classes that intercept calls on interfaces and run their own code before or after your implementation runs.

A simple roll-your-own way to do interception in a framework that you are writing would be to use the decorator pattern. Many DI frameworks provide a means of doing interception and use more sophisticated ways of accomplishing it such as through the use of reflection.