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Java Forum / General / November 2007Looking for a pattern
Hello, I am looking for a good pattern. I have a rather large software app that makes use of a service manager for its many services... configuration, colors, data lookup, units, etc. Up until now the service manager has been a singleton and anyone who wants access to a service just asks the singleton. Now we have a new requirement... run multiple instances of the software in the same application space with different configurations. <sarcasm>shocking</sarcasm> So, now I need to think about a good design pattern to help me here. I can only come up with two awkward options: 1. Pass a service manager key to every constructor of every class that needs access to the service manager. The class can go to a singleton to ask for the instance of the service manager by key. This is awkward and I don't like it. 2. Create an interface for getting a service, and have every object in the object tree implement the interface. Pass a "parent" object reference to the "child" and implement the interfaces so they climb the tree all the way to the root node to get an instance of the service manager stored in the root node. This is better, but still awkward. Is there a better design pattern out there to do what I need? I am using .NET C#, though it shouldn't matter too much (unless .NET already has a service I can leverage). Thanks, Brian >Now we have a new requirement... run multiple instances of the >software in the same application space with different configurations. ><sarcasm>shocking</sarcasm> You need some sort of factory to create your service provider. Perhaps it can cache them, and reuse an existing provider if its set of configurations have already been used before. You create a key class that contains the various distinguishing initialisation parameters, then a hashCode that xors the various fields. Then use this key class to create index into your HashMap cache of pre-built providers.  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Oct 31, 4:32 pm, Roedy Green <see_webs...@mindprod.com.invalid> wrote: > >Now we have a new requirement... run multiple instances of the > >software in the same application space with different configurations. [quoted text clipped - 11 lines] > Roedy Green Canadian Mind Products > The Java Glossaryhttp://mindprod.com Ok, that is fine... but how does all of the classes get the key? Do you pass them in to every constructor, and keep track of the key all the way down the object graph? This is what I am trying to avoid... though I can't think of a way how. > On Oct 31, 4:32 pm, Roedy Green <see_webs...@mindprod.com.invalid> > wrote: [quoted text clipped - 18 lines] > the way down the object graph? This is what I am trying to avoid... > though I can't think of a way how. Even if you pass a key all the way down, you'd be better off passing the actual object all the way down, then you eliminate the need to do a map lookup. If you're using Java, and the partition aligns nicely with threads, you can use ThreadLocal variables... Also, look into the Dependency Injection pattern, as well as other forms of Inversion of Control. You don't have to pass it into every constructor, but just make sure that your object relationships have everything you need to get the configuration you care about. As a matter of fact, you should probably have your configuration object instantiate most of the objects (think of it as a factory), and have it connect them to each other. SignatureDaniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/> >Ok, that is fine... but how does all of the classes get the key? Do >you pass them in to every constructor, and keep track of the key all >the way down the object graph? you pass your parameters to the factory. It composes the key to see if it built a suitable config object before, if not it calls the constructor with the parms and adds it to the pool. Imagine how you might cache Font bitmaps using family, size, style as the key. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > Hello, > [quoted text clipped - 26 lines] > using .NET C#, though it shouldn't matter too much (unless .NET > already has a service I can leverage). I would prefer solution #1 over #2. Much more flexible. As a long time solution that seems obvious. For a dirty hack: if the two instances of the software are actually running in different threads, then you could register each thread to a given instance of the software and have the singleton create based on thread. Arne > Hello, > [quoted text clipped - 29 lines] > Thanks, > Brian Hix, I encountered the same problem with you but I haven't found out any elegant solution yet. My application is a rich client applet, it's required to allow more than one concurrent users run multiple applets in the same browser window!! You've known, within a Firefox/IE window, the applet class loader runs on the same JVM and make the singleton to be death!! In my case, I prefer the #1 to #2 because I can use the username as the lookup-key and it quite simple :)... Now I talk about another solution that would be feasible in your case, I don't know :-). If your application is allowed to create a new classloader (my case, require a signed applet, hix...), you can use that classloader to load all your classes in another "working-space", and the singleton might work independently. Hix, after this circumstance, I swore not to use singleton if it carries my data, it's awful!! :-) > Hello, > [quoted text clipped - 15 lines] > to ask for the instance of the service manager by key. This is > awkward and I don't like it. FWIW, that's what I used: http://www.edmundkirwan.com/servlet/fractal/cs1/code/package47.html#doPost In that method, an access is createed each time the user clicks a button on a web page, and that access is passed to every object in the system that needs it. There is no magic solution. Yes, it feels awkward. Yes, it feels as though there, "Should," be a better solution. There isn't. Signature.ed www.EdmundKirwan.com - Home of the mathematical laws of encapsulation. > I am looking for a good pattern. I have a rather large software app > that makes use of a service manager for its many services... [quoted text clipped - 20 lines] > service manager stored in the root node. This is better, but still > awkward. Just out of curiosity, which method would you have chosen if someone had told you up front "no globals"? (After all, a singleton is just a global with a pretty wrapper.) > > I am looking for a good pattern. I have a rather large software app > > that makes use of a service manager for its many services... [quoted text clipped - 24 lines] > told you up front "no globals"? (After all, a singleton is just a global > with a pretty wrapper.) if singletons are globals, then "no globals" is a mere illusion :-( >> Just out of curiosity, which method would you have chosen if someone had >> told you up front "no globals"? (After all, a singleton is just a global >> with a pretty wrapper.) > > if singletons are globals, then "no globals" is a mere illusion :-( There's no official definition of a "global", so it's more a matter of semantics than of illusion. In the strictest sense, a singleton or other static member is not a global at all. In a relaxed sense, any static member of any top-level class is a global, and in an even more relaxed sense any static member of any class is a global. Personally, I prefer the strictest definition of "global": an identifier that is accessible globally, without any decoration at all. This correlates reasonably well to the usual use of the word "global" in programming languages. If you would consider a local static variable in a C++ function to be a "global", then I'd agree that singletons and other static things in C# would be "globals" as well. Otherwise, I'd say it's a bit of a stretch to claim that. Pete > Diego <jose.diego@gmail.com> said: > > > Just out of curiosity, which method would you have chosen if > > > someone had told you up front "no globals"? (After all, a [quoted text clipped - 5 lines] > There's no official definition of a "global", so it's more a matter > of semantics than of illusion. The whole point of a singleton is to "Ensure a class only has one instance, <i>and provide a global point of access to it.</i>" (GoF pg. 127 emphasis added.) Global access, means global IMO. And the question still stands to the OP, if you had known from the outset of the new feature, how would you have implemented it? > The whole point of a singleton is to "Ensure a class only has one > instance, <i>and provide a global point of access to it.</i>" (GoF pg. > 127 emphasis added.) Global access, means global IMO. I understand that "singleton" has a perhaps a very specific meaning in the particular book in which (it seems) this whole idea of "design patterns" was first suggested. However, I see nothing to prevent someone from implementing a singleton that is not accessible globally. And I would still call it a singleton. Furthermore, in C# any singleton itself is not accessible globally; you need to start with the namespace, and then a particular class name, at a minimum. I don't see the point in raising the question of whether globals can exist in C# anyway, but assuming the question has been raised it seems silly to insist that the singleton pattern is somehow related to the question. You have to define "global" first before you can say whether the singleton pattern exists if and only if globals do, and once you've defined "global", you don't need to look at the singleton pattern to decide whether they exist in a language or not. Pete >> The whole point of a singleton is to "Ensure a class only has one >> instance, <i>and provide a global point of access to it.</i>" (GoF pg. [quoted text clipped - 3 lines] > the particular book in which (it seems) this whole idea of "design > patterns" was first suggested. They defined it and gave it the name. It is a well known source. It would be pure obfuscation to use the same well known term about something different. > Furthermore, in C# any singleton itself is not accessible globally; you > need to start with the namespace, and then a particular class name, at a > minimum. That has nothing to do with accessibility. Arne > "Daniel T." <daniel_t@earthlink.net> said: > [quoted text clipped - 20 lines] > defined "global", you don't need to look at the singleton pattern to > decide whether they exist in a language or not. Fine, ignore the reference to globals. The question to the OP still stands. I'm trying to help the OP solve his problem. In order to do so, I would like to know how he would have solved it if he had known of the design up front instead of it being a design change. > if singletons are globals, then "no globals" is a mere illusion :-( I don't know what that means. Static variables, singletons - both are globals in some sense. The singleton pattern isn't the magic bullet that so many people think. SignatureLew >Hello, > [quoted text clipped - 29 lines] >Thanks, >Brian Sounds like you just need to implement versioning in a WCF application. ---------------- AndyW, Mercenary Software Developer Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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