Java Enterprise中的服务提供者接口

Question

I would like to code a very modular factory through the use of a Service Provider Interface, so without having to modify every time the Factory itself.

To do that I would like to receive the list of all the Concrete Classes that implement an interface and decide at runtime which one to use through a method call in each until I have a suitable Implementer.

Example: I have an interface Poligon
and a Concrete Class Triangle implements Poligon . The discriminant method would be:

boolean isSuitableFor(int i) {
    if (i == 3) 
      return true; 
    else 
      return false;
}

In Java SE I can do it using ServiceLoader: in an Enterprise environment how can I find all the implementing class and at runtime decide to use its implementation if suitable? Thx!

Answer 1

There are a couple of ways I can think of to solve this using CDI.

First, if the isSuitableFor method is as simple as you list, you can use a Qualifier with a @Nonbinding value for the int listed, and simply look up the implementation with that qualifier value. Let's say that the qualifier is @Vertices and the value for Triangle is 3 , so then the qualifier becomes:

@Qualifier
@Target({ TYPE, METHOD, PARAMETER, FIELD })
@Retention(RUNTIME)
@Documented
public @interface Vertices {
    @Nonbinding int value();
}

And Triangle becomes:

@Vertices(3)
public class Triangle implements Polygon{
}

And you would get a reference to it via:

@Inject
@Vertices(3)
private Polygon polygon;

Or you can create an AnnotationLiteral for vertices and manually look it up via instance.

Another way, is to iterate through all Instances, like this:

@Inject
@Any
private Instance<Polygon> polygons;

for(Polygon p : polygons) {
    if(p.isSuitableFor(x)) {
        // do something.
    }
}

This assuming that all polygons are registered beans. This will be slow for large numbers of polygons.

The third way is to use an event, and instead of qualifying the class, qualify the observer method. Eg

@Inject
private Event<SomePayload> somePayload;


// later on
somePayload.select(new VerticesLiteral(3)).fire(somePayload);

// and the observer method in Triangle
public void handle(@Observes @Vertices(3) SomePayload somePayload) { ... }

// and in square
public void handle(@Observes @Vertices(4) SomePayload somePayload) { ... }