When to use @Autowired Annotation? 

The @Autowired annotation in Spring is used to automatically wire dependencies into a bean. 

It is typically used in the following scenarios:


Dependency Injection:

When you want to inject dependencies into a bean automatically, you can use the @Autowired annotation. 

Spring will scan the dependencies and inject an appropriate bean based on the type.

Dependency Resolution:

If you have multiple beans of the same type and you want to specify which one to inject, you can combine the @Autowired annotation with the @Qualifier annotation. 

This allows you to resolve the dependency by specifying the bean's name or a custom qualifier.

Constructor Injection:

The @Autowired annotation is often used on constructors to indicate that the dependencies should be injected through the constructor.

 This promotes the use of constructor-based dependency injection, which is considered a recommended practice.

Field Injection:

You can also use the @Autowired annotation on fields to enable field injection. 

Spring will automatically inject the dependency into the annotated field.

 However, this approach is less preferred compared to constructor or setter injection, as it can make testing and mocking dependencies more challenging.

Method Injection:

The @Autowired annotation can be applied to setter methods as well. 

Spring will invoke the annotated method and inject the dependency into the corresponding property or parameter.

It's worth noting that starting from Spring 4.3, the @Autowired annotation is no longer required to be explicitly used for dependency injection. 

If you have a single constructor in your bean class, Spring automatically considers it as an autowired constructor, and you can omit the @Autowired annotation.


However, using the @Autowired annotation explicitly can make the code more explicit and self-documenting, indicating the dependencies being injected.


In summary, you can use the @Autowired annotation whenever you want to enable automatic dependency injection in Spring.

 It can be applied to constructors, fields, or setter methods to indicate the dependencies that should be injected.