07. Activators

###Adding instances with an activator

In some situations injecting objects to a container cannot be accomplished using simple mechanisms described in the earlier sections and we need an activator. To explain the concept of activators let's take a look at the following example.

container.add(DBConfig.class);
container.addActivator(DB.class, DBActivator.class);

public class DBActivator implements Callable {
    private final DBConfig config;

    public DirectoryActivator(DBConfig config) {
        this.config = config;
    }

    public DB call() throws Exception {
        if (isInMemoryDB(config)) {
            return new InMemoryDB();
        }
        return new RealDB(config));
    }
    ...
}

Activator is an implementation of a Callable. Inside a call method we define how our object should be created. In the code above we use an activator to instantiate a correct DB implementation based on the database config object. In case of any exceptions inside the call method, Yadic will raise a ContainerException.

Note: the activator will be called only once, even in a multithreaded environment.