CODEWITHSUNDEEP
javaspringinterfacedependency-injection
Arturas M
Arturas M

Reputation: 4369

Spring Dependency injection for interfaces

Well I've been watching some tutorials about Spring dependency injection as well as MVC, but I still seem to not understand how we can instantiate classes specifically?

I mean if for instance I have a variable

@Autowired
ClassA someObject;

How can I make spring create someObject as an Instance of ClassB which would extend ClassA? like someObject = new ClassB();

I don't really understand how it works in spring, does the ContextLoaderListener do it automatically or do we have to create some kind of configuration class where we specify exactly what spring should instantiate those classes to? (In this case I haven't seen that anywhere in the tutorials) If yes, then how do we specify and how does it look like? And how do we configure it to work in web.xml, etc?

Upvotes: 12

Views: 41013

Answers (4)

San Jaisy
San Jaisy

Reputation: 17048

Best Practices

@RestController
@RequestMapping("/order")
public class OrderController {
    private final IOrderProducer _IOrderProducer;

    public OrderController(IOrderProducer iorderProducer) {
        this._IOrderProducer = iorderProducer;
    }

    @GetMapping("/OrderService")
    void get() {
        _IOrderProducer.CreateOrderProducer("This is a Producer");
    }
}

Interface

@Service
public interface IOrderProducer {
    void CreateOrderProducer(String message);
}

Implementation

public class OrderProducer implements  IOrderProducer{
    private KafkaTemplate<String, String> _template;

    public OrderProducer(KafkaTemplate<String, String> template) {
        this._template = template;
    }

    public void CreateOrderProducer(String message){
        this._template.send("Topic1", message);
    }
}

You need to include Project Lombok dependency in spring boot

Gradle implementation 'org.projectlombok:lombok'

Upvotes: 0

Stefan
Stefan

Reputation: 1000

You can do it like this:

Interface: 

package org.better.place

public interface SuperDuperInterface{
    public void saveWorld();
}

Implementation:

package org.better.place

import org.springframework.stereotype

@Component
public class SuperDuperClass implements SuperDuperInterface{
     public void saveWorld(){
          System.out.println("Done");
     }
}

Client:

package org.better.place

import org.springframework.beans.factory.annotation.Autowire;

public class SuperDuperService{
       @Autowire
       private SuperDuperInterface superDuper;


       public void doIt(){
           superDuper.saveWorld();
       }

}

Now you have your interface defined, written an implementation and marked it as a component - docs here. Now only thing left is to tell spring where to find components so they can be used for autowiring.

<beans ...>

     <context:component-scan base-package="org.better.place"/>

</beans>

Upvotes: 34

Sumit Desai
Sumit Desai

Reputation: 1760

You have to specify the type of the class that you want to create object of in your applicationContext.xml file or you can directly annotate that class with any of @Component , @Service or @Repository if you are using latest version of Spring. In web.xml, you have to specify path of xml files as a context-param to servlet, if you are using xml-based configuration.

Upvotes: 1

sorencito
sorencito

Reputation: 2625

Yes, you have to provide a context.xml file in which you specify the instances. Give it to the ApplicationContext and it will autowire all fields for you.

http://alvinalexander.com/blog/post/java/load-spring-application-context-file-java-swing-application

Upvotes: 0

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