Class Loading: How to do it with Singletons?

[Apothem]

Simply put, I want to dynamically load singletons.

i.e. Say I have:

public class Singleton {
  private static Singleton instance;
  private Singleton() {}
  public static Singleton getInstance() {
    if(instance == null) { instance = new Singleton(); }
    return instance;
  }
}

From what I have learned, you would do something like this if it wasn't a singleton:

MainClass.class.getClassLoader().loadClass("myClass").newInstance();

But I don't this will be allowed. Would I need another layer of indirection? For example, do I need to do:

public class SingletonLoader {
  public Singleton getInstance() { return Singleton.getInstance(); }
}

and from there invoke the getInstance method? Or is there a more direct solution?

[Fou-Lu]

Singleton is designed for the instance of that class. I think that the call against a .newInstance off the class loader would chain to the constructor, so a private or protected constructor will throw an exception.
To load, its a simple call to Singleton.getInstance();. There is really no way to do this dynamically since you cannot stop an class with a public constructor from instantiating a new record. Any class to be a singleton has to be an instance of a Singleton first. You can call it against a dynamically loaded class though, that's just a simple try/catch and attempt to invoke the Method of getInstance on the class or cast to a Singleton type.

So if I got what you are looking for, your dynamically loaded classes implement singleton. You can load them as so:

PHP Code:

try
{
    Method getInstance = class.getClassLoader().loadClass("MyClass").getMethod("getInstance");
    Object instance = getInstance.invoke();
}
catch (Exception ex)
{
    System.out.println(ex.getMessage());
} 


Methinks that will work.

[Apothem]

Well, after digging around I found out I was supposed to do something like:

          Class<?> c = Class.forName("MySingleton");
          Method m = c.getDeclaredMethod("getInstance", new Class[0]);
          Object obj = m.invoke(null, new Object[0]);

But I'm getting the following exception:

java.lang.NoSuchMethodException: MySingleton.getInstance()
        at java.lang.Class.getDeclaredMethod(Class.java:1954)
        at myProg.main(myProg.java:21)

Really strange because that class does in fact have a getInstance() method... as such:

  /**
   * Gets single instance of the site
   * @return the site
   */
  public static MySingleton getInstance() {
    if(instance == null) {
      instance = new MySingleton();
    }
    
    return instance;
  }

Note: I replaced the real singleton class name with "MySingleton".

Edit: Woops my bad, it seems that I was editing the wrong "MySingleton" file. It works. Thanks.

[renegadeandy]

I am interested in why you want to do this at all?

[Apothem]

The project I am currently working on makes use of the ability to dynamically load classes. Why? I want to allow other developers to develop their own subsystem based on a set of interfaces I already have, and allow users to benefit from it.

I can technically make things very simple for myself and complicated for others, but I do not believe that is the way to go. And though it may sound hypocritical to say this, I would like to uphold certain invariant so that there is design. These invariant might cause the developer to do a LITTLE more work, but it is for the "greater good" so that recompiling the entire source is not required.

[renegadeandy]

Ahhhh right I see - so the main goal is to make setting up easy so you dont need to recompile, hence why you need to deal with the classloader, gotcha, nice work!

[albert.chiu]

You can use spring framework to do this.