How to inject all implementations of an Interface?

How to inject all implementations of an Interface?

Suppose you have multiple implementations of an Interface and you want all the implementations injected.

For example:

Handler is an Interface and it has three implementations.

public interface Handler {
    String handle();
}

All the implementation needs to be marked for @ApplicationScoped like:

import javax.enterprise.context.ApplicationScoped;

@ApplicationScoped
public class PrintHandler implements Handler {
    public String handle() {
        return "PrintHandler";
    }
}

In the class where you want to inject all the implementations, use

@Inject
Instance<Handler> handlers;

This Instance is imported from javax.enterprise.inject.Instance;

This handlers the variable will have all the implementations of Handler interface.

javax.enterprise.inject.Instance also implements the Iterable so you can iterate to it and call the required methods.

@Inject
Instance<Handler> handlers;

@GET
@Produces(MediaType.TEXT_PLAIN)
public List<String> handle() {
    List<String> list = new ArrayList<>();
    handlers.forEach(handler -> list.add(handler.handle()));
    return list;
}