Using dependency injection for instantiating multiple implementations for an interface

[Swapnil Sonawane]

Hi,

I've a design like below -

Interface Writer {
 write();
}

Class FileWriter implements Writer {
 void write() {
 }
}

Class DatabaseWriter implements Writer {
 void write() {
 }
}

Class Document {

 void someMethod() {
 // get FileWriter bean and call its write method first
 // then get DatabaseWriter bean and call its write method 
 // We should be able to inject one or both of these beans effectively
 }
}

In the absence of DatabaseWriter, I'm currently using getBeanNamesForType method to inject the only available bean. But when I have multiple implementations of the interface, I'm not sure how to do this effectively. Using getBeanNamesForType and iterating over the retrieved list doesn't really look like an elegant way. I want to avoid hard-coding the bean names inside the someMethod(). I'm sure this must be a very common use-case today. What's the standard way of solving this using spring?

Thanks,
Swapnil

[Saifuddin Merchant]

You could autowire by type.

[Swapnil Sonawane]

But isn't it that I will have two matching beans for the interface type defined in the class which is going to make use of it eventually. I mean there will be two beans compatible with the interface type, which throws UnsatisfiedDependencyException according to the documentation. Please correct me if I'm wrong.

Thanks,
Swapnil

[Saifuddin Merchant]

What I mean is you'll need to use a collection class (or array) to accept more than one bean. You can auto-wire by type to get all the beans of that type in the collection type.

[Atul Kotwale]

Yes, swapnil you are right, autowire byType allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set.