Spring

@Inject vs. @Autowired

@Inject and @Autowired both annotations are used in Java and Spring do achieve Dependency Injection respectively. Please read our blog Dependency Injection for better understanding.

@Inject is part of the Java CDI (Contexts and Dependency Injection) standard introduced in Java EE 6 (JSR-299). Basically, Classes typically acquire references to other Classes through dependency injection. The dependency injection mechanism is completely type safe and CDI allows us to use Dependency Injection using @Inject  annotation. @Inject identifies a point at which a dependency on a Java class or interface can be injected and the container then provides the needed resource.

E.g

public class Login {
    @Inject 
    Credentials credentials;
}

or 

class Login {
    private Credentials credentials;
    @Inject Login (Credentials credentials) {
       this.credentials = credentials;
    }
}

After doing this, the container will inject the Credentials bean into Login bean.

Along with @Inject there are other annotations as well that help us to achieve Dependency Injection.

  • @Inject. Identifies injectable constructors, methods, and fields.
  • @Qualifier. Identifies qualifier annotations. Qualifiers are strongly-typed keys that help distinguish different uses of objects of the same type.
  • @Scope. Identifies scope annotations.
  • @Singleton. Identifies a type that the injector only instantiates once
  • @Named. A String-based qualifier.

 

@Autowired annotation comes from Spring  2.5 world. All the things that we can do with @Inject can be done in Spring using @Autowired and then it becomes Spring containers’s job to resolve dependencies for classes. Spring @Autowired also comes with one more optional attribute ‘required‘. To handle the situation in which there is no wiring beans are available we set requiredattribute to false. But what do I mean by required attribute. From the Spring doc.

“By default, the autowiring fails whenever zero candidate beans are available; the default behavior is to treat annotated methods, constructors, and fields as indicating required dependencies. This behavior can be changed by setting the required attribute to false”. E.g: @Autowired(required=false). In simple terms, “The required attribute indicates that the property is not required for autowiring purposes, the property is ignored if it cannot be autowired“.

E.g

@Autowired(required=false)
private Party person;

By Spring 3.0, Spring offers support for JSR-330 dependency injection annotations (@Inject, @Named, @Singleton).

The only difference between @Autowired and @Inject is that @Autowired has required attribute.

Note: 

By Spring 4.3 release. If a bean has single constructor, we can omit @Autowired annotation. But there are some cases like below.

1. When single constructor is present and setter is marked with @Autowired annotation, than both constructor & setter injection will be performed one after another:

@Component
public class Employee {
    private Address address;

    public Employee(Address address) {
     this.address = address; //Called first
   }

   @Autowired
   public void setAddress(Address add) {
     this.address = add; //Called second
   }
}

2. At the other hand, if there is no @Autowire at all, then object will be injected once via constructor and setter can be used in it’s common way without any injections.

 

Hope this clears you doubt!!

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