C++ Factories -> Java Factories?

When I was first learning C++, I came across this article about factories, Pluggable C++ Factory, and ever since I've used that pattern for my factories in C++. Now, I've been doing Java recently and on more than one occasion I've wanted to use a factory, but I can't seem to figure out a way to extend the factory at compile time.

Any implementation of a factory I can think of in Java involves telling the actual factory class about all of the underlying classes, which is rather suboptimal.

So, how can I get all subclasses of a class to register themselves in a static dictionary at c开发者_如何学运维ompile time/program instantiation?

EDIT It seems that my question was too vague. Let me elaborate,

In Java if I were to try to replicate this pattern like so: Factory.java

abstract class Factory { 
private static Dictionary<int, Factory> dict;
public Factory(int index) {
  dict[int] = self;
}
public Foo getFoo(int index) {return dict[index].createFoo();}
protected abstract Foo makeFoo();
}

Derived.java

class Derived extends Factory {
public Derived() {super(DERIVED_INDEX);}
private static Derived tmp = new Derived();
public Foo makeFoo() {return new FooImplementation();}
}

The factory won't update with the reference to Derived (and thus won't create Derived instances) unless I manually register it myself, which defeats the purpose of having the static tmp member.

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I think the ServiceLoader class can help you here.

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Think of implementing Factory Method using Reflection.

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You might find the Reflections library useful.

Using Reflections you can query your metadata such as:

* get all subtypes of some type
* get all types/methods/fields annotated with some annotation, w/o annotation parameters matching
* get all resources matching matching a regular expression 

Using this library you can define an interface and a factory which can create any implementation of that interface (by searching for those implementations)