Observer: Implement with pattern (subject & observer) or inter-thread communication (wait & notify)_问答_开发者

I usually use the Observer pattern, my colleague at work though has implemented an Observer using Thread intercommunication (using wait and notify/notifyAll).

Should I implement my observers using the pattern or inter-Thread-communication using wait and notify? Are there any good reasons to avoid one approach and always use the other?

I've always gone with the first one, using the pattern, out of convention, and because it seems more expressive (involved identifiers are a good way to express and understand what is communicated and how).

EDIT:

I'm using the pattern in a Swing GUI, he's using the inter-thread solution in an Android application.

In his solution one thread generates data and then calls notify, to wake up another thread that paints the generated data and calls wait after every paint.

His argument for the wait/notify solution is that it creates less threads and even several concurrent calls to notify will cause only 1 paint event, whereas an observer-based solution would call a repaint with every call. He says it's just another valid approach, but doesn't claim he's done it for performance reasons.

My argument is that I would express the communication among objects on the OO design level rather than use a language-specific feature that makes the communic开发者_如何转开发ation almost invisible. Also, low-level thread communication is hard to master, might be hard to understand by other readers, and should rather be implemented on a higher level, i. e. using a CyclicBarrier. I don't have any sound arguments for one or the other solution, but I was wondering if there are any sound arguments for either one or the other approach (i. e. "This-and-that can happen, if you use this approach, whereas in the other one that's not possible.").

You are comparing apples and oranges. The wait/notify mechanism is used for thread synchronization, and while your colleague may have used it within an Observer/Observable implementation, it is not, in itself the pattern implementation. It simply means it is a multithreaded implementation.

There are many implementations of this pattern, and they are typically tailored to the environment in which you are working. There are event mechanisms built into most UI frameworks/toolkits. JMS for distributed environments, ...

I don't find much use for the generic Observer/Observable classes provided by the JDK, and from experience I haven't found many other developers use them either. Most will use a provided mechanism, if appropriate, or roll their own specific and ultimately more useful implementation if needed.

Since I have done most of my coding in an OSGi environment of late, I have a preference for a variation of observer/observable called the whiteboard pattern. This may or may not be feasible for you, depending on your environment.

You should avoid, or rather refrain from, inter-thread communication in 99.99% of the cases. If there is a real need for a multi threaded solution, you should use a higher level concurrency mechanism such as an ExecutorService or a good concurrency library such as jetlang: http://code.google.com/p/jetlang/.

Difficult. I would normally use Observer / Observable when not explicitly writing a multithreaded application. However, convention in this case might be for you to use his design. Perhaps see if you can abstract it out somehow so that you can replace it with the Observer pattern at a later stage if necessary?

However, I found these two articles which seem to indicate that the Observer/Observable pattern in Java is not ideal and should be avoided.

An inside view of Observer and The event generator idiom