Difference Between @Autowired and Constructor Injection (without @Autowired)

@Autowired Annotation

• Purpose: Tells Spring to automatically inject a dependency into a field, setter, or constructor.

• Usage:

MyService.java

@Component
public class MyService {
    private final MyRepository repo;

    @Autowired
    public MyService(MyRepository repo) {
        this.repo = repo;
    }
}

• Behavior:

  • Spring scans the context and finds a matching bean (MyRepository).
  • It injects that bean into the constructor automatically.
  • Useful when multiple constrctors exist (Spring needs to know which one to use).

Constructor Injection Without @Autowired

• Since Spring 4.3: If a class has only one constructor, Spring will automatically use it for dependency injection without @Autowired.
• Example:

MyService

@Component
public class MyService {
    private final MyRepository repo;

    public MyService(MyRepository repo) {  // No @Autowired needed
        this.repo = repo;
    }
}

• Behavior:
  • Spring sees there's only one constructor.
  • It assumes that's the one to use for injection.
  • Cleaner code, less boilerplate.

If MyService has more than one constructors, it'll throw compile time exception Class doesn't contain a matching constructor for autowiring:

MyService

@Component
class MyService {
    private final MyRepository repo;

    // We need set @Autowired here
    public MyService(MyRepository repo) {
        this.repo = repo;
    }

    // or here, but make sure OtherBean was existed
    public MyService(MyRepository repo, OtherBean test) {
        this.repo = repo;
    }
}

To correct above example, we need to tell Spring, which constructor to use for injecttion by using @Autowired keyword:

MyService

@Component
class MyService {
    private final MyRepository repo;

    @Autowired
    public MyService(MyRepository repo) {  // Everything fine now
        this.repo = repo;
    }

    public MyService(MyRepository repo, OtherBean test) {
        this.repo = repo;
    }
}

Comparation Table

Aspect	@Autowired Constructor	Constructor Without @Autowired
Spring version	Works in all versions	Works automatically since Spring 4.3
Multiple constructors	Needed to specify which one	Must use @Autowired to avoid ambiguity
Readability	Explicit, clear intent	Cleaner, less annotation noise
Best practice	Use when >1 constructor	Prefer no @Autowired if only one constructor

So in short:

• No @Autowired = works fine if there’s only one constructor (Spring auto-injects).
• With @Autowired = explicit, necessary when multiple constructors exist.