What is Metaobject protocol ?

In traditional programming languages like C, C++, Java, programming language designer works at different level of abstraction than programming language users. Language designers writes programmers that decides how the elements of programming languages works and how the features in the languages such as method dispatch, inheritance are implemented. And the programming language users work on writing the primary application classes without having the flexibility of changing the way language itself works.

However, in some languages like Common Lisp Object System (CLOS), Groovy and Ruby, the programming language provide interface to alter and enhance the core features of programming language. This allows changing the way language throws certain exception or implementing multiple inheritance in language that does not allow it by default.

Metaobject protocol is a interface to programming language using which programmer can add and modify the features to programming language. Here, programming language users play with the Meta objects. These objects decides how features like inheritance, method invocation, scope resolution behaves. These objects are called as Meta Objects simply because they holds information about object like methods, fields, super class etc.These objects also controls run time behavior of program and how the primary objects in the actual system works. We can say that Metaobject protocol makes features of compiler/interpreter available to the programmers.

Metaobject protocol is an important feature for meta programming (However, some programming languages like smalltalk allows to do elegant metaprogramming without MOP). It also allows programmers to adjust language according to their needs.
For example, Common Lisp Object System (CLOS) adds object oriented and dynamic language features to LISP using Metaobject protocols. The implementation of language is done by implementing classes as objects of a metaclass and allowing programmer to change behavior of base class and also allow defining new metaclasses.

Groovy, a dynamic scripting language for JVM, implements Metaobject protocol. Metaobject protocol in Groovy decides how the core language functionality like method dispatch, scope resolution for attributes works. It gives more flexible control of language in programmer's hand. And Ruby has many features that allows programmer to do amazing things like changing the behavior of class on run time, defining methods dynamically etc. The AOP implementation for Java called AspectJ is also implemented with Metaobject protocol.

In short, Metaobject protocol gives some amazing power and flexibility to the programmer which can be used with little caution to create amazing programs !