What is reflection and why is it useful?

Question

What is reflection, and why is it useful?

I\'m particularly interested in Java, but I assume the principles are the same in any language.

Answer 1

Not every language supports reflection, but the principles are usually the same in languages that support it.

Reflection is the ability to "reflect" on the structure of your program. Or more concrete. To look at the objects and classes you have and programmatically get back information on the methods, fields, and interfaces they implement. You can also look at things like annotations.

It's useful in a lot of situations. Everywhere you want to be able to dynamically plug in classes into your code. Lots of object relational mappers use reflection to be able to instantiate objects from databases without knowing in advance what objects they're going to use. Plug-in architectures is another place where reflection is useful. Being able to dynamically load code and determine if there are types there that implement the right interface to use as a plugin is important in those situations.