Question

I’m searching for a way to make a library replaceable.
For example: I have a library which is managing communication over multicast . I’m using that library in my main application. My idea was to use an interface for that:

public interface MessageHandler {
    public void sendMessage();
    public Message receiveMessage();
}

Now it should be possible to change the library. For example a library which is managing communication over broadcast. Both are implementing the same interface.

So in my main application I want to have the opportunities:

MessageHandler mHandler = new MulticastImpl();

Or

MessageHandler mHandler = new BroadcastImpl();

My problem is: Where do I place the interface definition? When I place it into my main application I can’t see it from the library (because the library don’t know the main application).
When I place it into the both libraries I have two different MessageHandler-Interfaces (for example com.multicast.MessageHandler and com.broadcast.MessageHandler)

Any ideas?

Answer 1

This is a basic plugin system. what you should do is:

1. put the plugin API in a standalone jar.
2. include this plugin API jar in the main application.
3. plugin implementations should not include the plugin API (they will just use it for compiling).
4. provide a way for the user to specify the “implementation” class for the plugin (as a configuration String)
5. on application startup, use reflection to load the implementation class from the configuration.

Note, there are more sophisticated ways to load plugins in java (e.g. using java.util.ServiceLoader and/or nested classloaders), but this is a good start.