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Java Forum / General / August 2007How to find all classes in package XXX implementing interface YYY
I would like to use reflection in someway to load all classes in a given package that implement an interface. Basically, this is for a factory that loads all classes in the same package as it, by default, with the ability to override. I assume this is possible but the documentation isn't jumping out at me on how that's done. Ideas anyone? Christian Bongiorno http://christian.bongiorno.org > I would like to use reflection Reflection is usually the wrong tool. Think twice before you use it. E.g. if you do this so a superclass knows all its subclasses then you probably have a design error and you should rethink your design. But you have other problems, too. > in someway to load all classes in a > given package that implement an interface. Since a package is open-ended you can't find all classes in a given package. Please spare us the discussion of this fact. If you don't believe it, read the Java spec. Without having all potential candidates you can't find all classes in a given package that implement a particular interface. > I assume this is possible but the documentation isn't jumping out at > me on how that's done. It is not possible. > Ideas anyone? Modify your requirement. For example, all classes in a jar or a directory to which you have the pathname. Or all classes from a (text) list of class names. Sideswipe wrote: >> in someway to load all classes in a >> given package that implement an interface. > Since a package is open-ended you can't find all classes in a given > package. Please spare us the discussion of this fact. If you don't > believe it, read the Java spec. Without having all potential > candidates you can't find all classes in a given package that > implement a particular interface. >> I assume this is possible but the documentation isn't jumping out at >> me on how that's done. > It is not possible. To expound slightly more on Hunter's point, though as he says if you google around the Usenet archives you will find quite a bit on this FAQ, new classes can appear in a package any time. You'd have to run your search for infinite time. It's like making a dictionary of every word in the English language. The language changes while you're compiling the list. You can never finish, Sisyphus.  SignatureLew > Sideswipe wrote: > >> in someway to load all classes in a [quoted text clipped - 18 lines] > -- > Lew Often people think of this as a solution to the service provider pattern. Java has a specification for providing service providers. I suggest you look at it. I didn't realize this was such a FAQ. More search did turn up some ideas. I also fully understand the English dictionary analogy. But, even though that's true it doesn't stop people from printing dictionaries. Which is what I was looking for. Generally I don't bother responding to a rude poster, but this time I will. I wasn't looking for the end-all be all list of classes implementing an interface in a package. Only the ones at startup and currently in the class loader. I wasn't asking you to _judge_ my question and ponder what stupid thing I might be cooking up either. This post from sun shows a possibility: http://forum.java.sun.com/thread.jspa?threadID=341935&start=15 I actually did want to limit the scope to only classes in the same jar. Try to actually be a positive member of this community and show how even a limited case could be done and what the caveats are. ----------------- Perhaps a an interesting extension to ClassLoader would be to add a listener method for every time a new class is loaded. ... >I wasn't looking ... ... >I wasn't asking ... Did you consider contracting a help-desk or consultant?* Be sure to mention those conditions when signing up, so they might charge the appropriate rate. This, OTOH, is a discussion forum. >...Try to actually be a positive member of this community Try not to spit the dummy when someone pursues the technical matter, or questions the entire strategy. >...and show >how even a limited case could be done and what the caveats are. * This is not a help desk, in case you might have any illusions. SignatureAndrew Thompson http://www.athompson.info/andrew/ On Tue, 28 Aug 2007 17:52:27 -0000, Sideswipe <christian.bongiorno@gmail.com> wrote, quoted or indirectly quoted someone who said : >I would like to use reflection in someway to load all classes in a >given package that implement an interface. see http://mindprod.com/project/interfacefinder.html SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > see http://mindprod.com/project/interfacefinder.html While I understand that you do not give any free support to takers of such a project, but I've got a bugreport about the text there: " 2.) Split it at the semicolons with Regex.split. Semicolons? That's only on MsWindows-platform, so rather than splitting at semicola, one should *always* split paths at File.pathSeparator instead. 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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