If Foo has a reference to Bar and you want to replace Bar's implementation (BarImpl) at runtime without effecting Foo then you are going to have to decouple them, by using some sort of interceptor pattern with dynamic proxies, or what have you. As long as Bar's interface doesn't change then everything should work.
And that's not particularly hard. Where it gets interesting is when setting up this relationship in the first place since you have to give Foo a proxy implementing the Bar interface instead of a direct reference to BarImpl. This is where dependency injection could come into play since it can do whatever pre-processing it wants on the objects before connecting them together.
And the real tricky part is synchronizing all of this so that Foo doesn't get confused if Bar gives it a different answer than it expects (because it's implementation changed mid-interrogation).
In the end, you end up with a whole lot of class loaders and extra synchronization just so you don't have to reload the application space when one or two classes change. It may be worth it for some key interfaces, but as a general practice it's probably prohibitively expensive and error prone. And for those key interfaces, it's almost always better to have both parties involved in the negotiation, ie: tell Foo it has a new Bar implementation so it can synch its logic properly.
-Paul
Dakota Jack wrote:
Paul and Craig
I am getting what I expect with the following code (see below), thanks to Craig, but not with the code which you can find at http://131.191.32.112:8080/classes.zip and I am not sure what the difference is. Essentially, I am trying to keep a WeakReference to Point classes so that when I update the Point.class I can change the classes for all the PointImpl objects out there. Not sure what is going wrong. If you could take a peek, that would be great. Thanks.
Jack
package com.crackwillow.deploy;
import java.lang.ref.Reference; import java.lang.ref.WeakReference;
public class MyReference { public static void main(String [] params) { CheckPlease cp = new CheckPlease(); cp.setBill("1.01"); WeakReference wr = new WeakReference(cp); ((CheckPlease)((Reference)wr).get()).setBill("5.03"); System.out.println(cp); } }
class CheckPlease { private String bill = "0.00";
public void setBill(String add) { if(add == null) add = "freebee"; else bill += " + " + add; }
public String getBill() { return bill; }
public String toString() { return bill; } }
"You can't wake a person who is pretending to be asleep."
~Native Proverb~
"Each man is good in His sight. It is not necessary for eagles to be crows."
~Hunkesni (Sitting Bull), Hunkpapa Sioux~
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