Spring Dependency injection for interfaces

Related searches

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?

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>

Spring Dependency injection for interfaces, Take a deep dive into Dependency Injection in Spring — what it is, how dependency that the Car class has on the Engine interface, though. What is Dependency Injection: Dependency Injection is the main functionality provided by Spring IOC (Inversion of Control). The Spring-Core module is responsible for injecting dependencies through either Constructor or Setter methods. The design principle of Inversion of Control emphasizes keeping the Java classes independent of each other and the container frees them from object creation and maintenance.

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.

Inversion of Control and Dependency Injection with Spring, Spring will perform the Dependency Injection for us, and inject the dependent components into the object returned to us. It is important to remember, the Spring Context is returning to us Spring Managed beans. This means Spring will be managing the dependency injection for us.

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

Dependency Injection with the Spring Framework, This dependency spring-context is the minimum requirement to use dependency injection with Spring. Next, write code for Java classes in the package net.codejava under the src/main/java folder as below: Code of the MyClient interface: package net.codejava; public interface MyClient { void doSomething(); } Code of the ClientImpl class:

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'

Dependency Injection in Spring, Spring framework. Play framework. Guice. Weld-CDI. You should note that Dependency injection is not just for Object oriented Programming languages. For example, You can inject method references to a functional programming modules.

Dependency Injection Dependency Injection (DI) is a design pattern that removes the dependency from the programming code so that it can be easy to manage and test the application. Dependency Injection makes our programming code loosely coupled. To understand the DI better, Let's understand the Dependency Lookup (DL) first:

My question is, how do I set up dependency injection, so far I need to prepare a fake service (called a stub or fake). Whenever it comes with the new service, can I add a service reference or web context and simply edit the spring.net config file to swap into the service in the consumer?

Spring dependency injection can be configured by using annotations directly in your Java classes. Another available alternative is to specify it in the Spring XML configuration file as we will see in this tutorial. We will use a previous tutorial as the basis for the next following sections: Spring dependency injection example

Comments
  • Thanks, sounds like a really awesome answer, I think I do get it now, I'll try it now. Another question, what if we would need to pass parameters to some constructor? How would it look like or what would we use?
  • You cannot use stereotype Annotations to mark constructor based beans. Either use a default constructor with autowired dependencies or resort to a Java / XML Bases configuration (remove the @Component annotation and the context:component-scan element). See this post here for details: javalobby.org/java/forums/t18396.html
  • Is someone still reading this thread? My question is - how would you achieve the same without using @Autowired?
  • Cool, what if we have multiple implementations ? How would you autowire ? Because you are autowiring at service level right ? I tried to autowire the implementation directly and it fails. Can you explain. Thanks in advance !
  • I have the same question as @PavanSandeep. Does anyone have an answer?
  • Alright, so i let's say I have to have only ClassB annotated with Component annotation and spring will automatically instantiate a new ClassB() object for me? if Both the ClassA and ClassB would be annotated with Component annotation what would spring then do? Do I understand this correctly?
  • If both are annotated with @Component, then while auto-wiring, you will have to use Qualifier annotation which instructs spring to inject that bean which is matching with name you provide with the qualifier.
  • Also consider using xml descriptors if this is a common case for you.