What could be the reasons for NullPointerException when using dependency injection in Spring if the constructor is not used for injection?

NullPointerException when using dependency injection in Spring without a constructor usually occurs because the fields annotated with @Autowired were not properly initialized. Main reasons:

• The class is not a Spring Bean (not annotated with @Component, @Service, @Repository, or not declared in configuration), so Spring does not manage its lifecycle and does not inject dependencies.
• The object is created manually via new, not through the Spring container, so autowiring does not happen.
• The @Autowired annotation is missing or used incorrectly on the field.
• Using @Autowired on private fields without proper configuration (although Spring can inject into private fields).

To avoid NullPointerException, ensure:

• The class is a Spring Bean.
• Use constructor injection — this is more reliable and recommended.
• If using field injection, ensure the object is created by Spring.

Example of correct field injection:

@Component
public class MyService {
    @Autowired
    private Dependency dependency;

    public void doSomething() {
        dependency.action();
    }
}

If MyService is created via new MyService(), then dependency will be null, leading to NullPointerException when calling doSomething().