What is Autowiring and how it works?

When it sees @Autowired , Spring will look for a class that matches the property in the applicationContext, and inject it automatically. ... Only one constructor (at max) of any given bean class may carry this annotation, indicating the constructor to autowire when used as a Spring bean.

The @Autowired annotation is performing Dependency Injection.

If @Autowired is applied to

• a field: then the dependency is stored in this field
• a setter: then the setter is invoked, with the parameter that is determined by the same algorithm like for the field dependency injection
• a constructor: then the constructor is invoked with the parameters determined by the same algorithm like for the field dependency injection 

Constructor Injection:

@Autowired

public EngineeringTeam(EmployeeService empService) {

this.empService = empService;

}

Note:

Spring will scan for a component that implements EmployeeService interface

our example: MyEmployeeService meets the requirements

• Injecting EmployeeService in to Manager Implementation
• Spring will scan @Components

Setter Injection:

This is very simple process, you just need to replace constructor with setter method and mark the method as follows:

@Autowired.

public EngineeringTeam(EmployeeService empService) {

 this.empService = empService;

}

Replace above constructor with following setter method:

@Autowired

public void setEmployeeService(EmployeeService empService) {

 this.empService = empService;

}

Field Injection:

Configure the dependency injection with Autowired Annotation

-Applied directly to the field

-No need for setter methods

In this way, you mark data field as @Autowired

For example:

@Autowired

private EmployeeService empService;