How can dependency injection be applied using Spring Bean?

Spring Boot supports Dependency Injection (DI) mainly in three ways:

1. Field Injection:
   The most common and simple way. Using @Autowired on a field. Not recommended for production code due to testing and encapsulation issues.

   // Field Injection
   @Service
   public class MyService {
   
       @Autowired
       private MyDependency dependency;
   
       public void doSomething() {
           dependency.execute();
       }
   }
   

2. Setter Injection:
   Using @Autowired on a setter method. Allows optional dependencies (if setter can be omitted) and easier testing.

   // Setter Injection
   @Service
   public class MyService {
   
       private MyDependency dependency;
   
       @Autowired
       public void setDependency(MyDependency dependency) {
           this.dependency = dependency;
       }
   
       public void doSomething() {
           dependency.execute();
       }
   }
   

3. Constructor Injection:
   The most recommended way. Using @Autowired on the constructor. Ensures immutability of dependencies after object creation and clearly indicates required dependencies. In Spring Boot 2.x+, @Autowired on a single constructor can be omitted.

   // Constructor Injection
   @Service
   public class MyService {
   
       private final MyDependency dependency;
   
       // @Autowired is optional for a single constructor in Spring Boot 2.x+
       public MyService(MyDependency dependency) {
           this.dependency = dependency;
       }
   
       public void doSomething() {
           dependency.execute();
       }
   }
   

Spring container scans classes with annotations like @Component, @Service, @Repository, @Controller and creates Beans from them. When a Bean with @Autowired on a field, setter, or constructor detects a need for another dependency, the container searches for a suitable Bean by type and/or name and injects it. If multiple Beans match, @Qualifier can be used for clarification.