Introduction

• The @Autowired annotation in Spring is a powerful tool that simplifies dependency injection.
• It is used for automatic dependency injection in Spring.
  
  

• Advantages of @Autowired :

  • Reduces boilerplate code for dependency injection.
  • Enhances readability and maintainability.
  • Provides flexibility with @Qualifier, @Primary and collections.

• How @Autowired Works ?

  • It uses reflection to identify a suitable bean for injection.
  • The Spring container scans for a bean matching the type of the field or parameter marked with @Autowired.

• Placement of @Autowired :

  @Autowired annotation can be used with Constructors, Setter Methods & Fields to achieve particular injection which are explained as below:
  
  
  1. Constructor Injection:
     • Injects dependencies via the class constructor.
     • Example:

       @Autowired
       public Car(Engine engine) {
           this.engine = engine;
       }

     • It is the most preferred approach as it ensures immutability and simplifies testing.
       
       
  2. Setter Injection:
     • Injects dependencies via the setter method.
     • Example:

       @Autowired
       public void setEngine(Engine engine) {
           this.engine = engine;
       }

       
       
  3. Field Injection:
     • Injects directly into the field.
     • Example:

       @Autowired
       private Engine engine;

     • We should avoid Field Injection because it makes testing harder as it requires reflection-based mocking.
       
       
• Click Here for @Autowired program used with constructor, setter methods and fields.

• Scenarios for @Autowired :

  • Single Bean: @Autowired works properly if only one bean of the type exists. Also note that @Autowired annotation resolves dependencies by type.
  • Multiple Beans: If there are multiple beans of same type, then we have to use @Qualifier or @Primary for bean resolution. If we dont resolve this then it will throws a NoUniqueBeanDefinitionException exception
  • No Bean Found: If there is no bean found, then it will throws NoSuchBeanDefinitionException unless required = false is used.