what is the difference between decorator and proxy

[Ghajini Johnson]

Hi,
What is the difference between decorator and proxy?

Both are looking like similar..

[Hong Anderson]

Hm, what is the similarity? For me, they are quite different.

Decorator is for enhancement (as the name guide), Proxy is for ... well proxy, it can help to improve performance like lazy loading, control access, hiding remoteness.

[Ghajini Johnson]

Please look at the class diagram and then you can see the similaties.

Definitions are absolutely different. Nothing to worry in the defintions......

The aggregation in decorator is pointing to the target abstraction.. where as the proxy is having the similar aggregation to the concrete class. No much difference.. Both are adding additional functionality..

See Mikalai Zaikin's book for patterns..

[aleem khan]

java.io is built on decorator pattern. InputStream, BufferedInputStream. At runtime
an object is given additional responsibility

Ejb container uses proxy pattern where EjbObject(proxy) from server interacts with the Enterprise beans and adds additional responsibilites like security, concurrency and ejb feautures on the enterprise bean

Hope this helps

[Hong Anderson]

Ghajini Johnson wrote:Please look at the class diagram and then you can see the similaties.

Definitions are absolutely different. Nothing to worry in the defintions......

I think you've already answered your own question. That is the different you have been looking for.

[Hong Anderson]

aleem khan wrote:java.io is built on decorator pattern. InputStream, BufferedInputStream. At runtime
an object is given additional responsibility

Ejb container uses proxy pattern where EjbObject(proxy) from server interacts with the Enterprise beans and adds additional responsibilites like security, concurrency and ejb feautures on the enterprise bean

Hope this helps

From definition, adding responsibilities is the purpose of Decorator pattern.
I'm not sure about EJB container, maybe it uses both Proxy and Decorator patterns.

[Ghajini Johnson]

kentkaj

Can you be more specific....

Decorator is not for adding additional responsibilities. It is the alternative to subclassing. You can even override also..

Same capabilities are there in Proxy also, so I am asking where does that finer difference comes in.

Again, please let me know if you understand it, otherwise, thank you...

[Hong Anderson]

Ghajini Johnson wrote:kentkaj

Can you be more specific....

Decorator is not for adding additional responsibilities.

Quoted from Design Patterns [GoF].

Decorator
Intent
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing
for extending functionality.

We can use subclassing, but if we have many options to combine, we'll end up with too many classes.
When call a Decorator's operation, it will call wrapped component's operation *and* may call additional operations.

Proxy
Intent
Provide a surrogate or placeholder for another object to control access to it.

If looking at structural, implementations of Decorator and Proxy are similar, but their purposes are different.
By definition Proxy will not add additional responsibilities, it will control access, for example, remote proxy, virtual proxy (lazy loading), protection proxy (check access rights).

If a Proxy also add responsibilities, we can say that it's also a Decorator.