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

As I find it best to explain by example and none of the answers seem to do that...

A practical example of using reflections would be a Java Language Server written in Java or a PHP Language Server written in PHP, etc. Language Server gives your IDE abilities like autocomplete, jump to definition, context help, hinting types and more. In order to have all tag names (words that can be autocompleted) to show all the possible matches as you type the Language Server has to inspect everything about the class including doc blocks and private members. For that it needs a reflection of said class.

A different example would be a unit-test of a private method. One way to do so is to create a reflection and change the method's scope to public in the test's set-up phase. Of course one can argue private methods shouldn't be tested directly but that's not the point.