Question

Languages such as Java and PHP support reflection, which allows objects to provide metadata about themselves. Are there any legitimate use cases where you would need to be able to do something like ask an object what methods it has outside of the realm of reverse engineering? Are any of those use cases actually implemented today?

Answer 1

There certainly are good use cases. For example, obtaining developer-provided metadata. Java APIs are increasingly using annotations to provide info about methods/fields/classes and their use. Like input validation, binding to data representations… You could use these at compile-time to generate metadata descriptors and use those, but to do it at runtime would require reflection. Even if you used the metadata descriptors, they’d end up containing things like class, method and field names that’d need to be accessed via reflection.

Another use case: dynamic languages. Take Ruby… It allows you to check up-front whether an object would respond to a method name before trying to call that method. Something like that requires reflection.

Or how about when a class or method name must be provided from outside compiled code, like when selecting an implementation of some API. That’s just gonna be a bit of text. Looking up what it resolves to comes down to reflection.