Autowired in spring

  1. Dependency Injection (DI):

  2. Spring's core principle is Dependency Injection (DI), where objects define their dependencies without creating them.

  3. Autowired Annotation:

  4. @Autowired is a Spring annotation used for automatic injection of bean dependencies.

  5. Bean Declaration:

  6. Declare a class as a Spring bean using @Component, @Service, or @Repository annotations.

  7. Component Scan:

  8. Enable component scanning in the configuration to let Spring automatically discover and register beans in the application context.

  9. ApplicationContext:

  10. Create an ApplicationContext to manage the Spring beans and their dependencies.

  11. Autowired Injection:

  12. Use @Autowired on fields, setter methods, or constructors in the dependent class to indicate the need for automatic dependency injection.

  13. Constructor Injection:

  14. Prefer constructor injection by annotating the class constructor with @Autowired. Spring automatically injects dependencies when the bean is created.

  15. Setter Injection:

  16. Use setter methods annotated with @Autowired to inject dependencies. This is an alternative to constructor injection.

  17. Field Injection:

  18. Annotate class fields directly with @Autowired. This approach is concise but has some drawbacks compared to constructor injection.

  19. Qualifier Annotation:

    • When multiple beans of the same type exist, use @Qualifier along with @Autowired to specify the bean's name to be injected.

  20. Primary Annotation:

    • Mark a primary bean using @Primary when multiple beans of the same type exist, indicating it as the primary candidate for injection.

  21. Autowired on Methods:

    • Annotate methods with @Autowired to inject dependencies. This is useful when dealing with non-bean classes.

  22. Autowired with Optional:

    • Use Optional along with @Autowired to handle scenarios where the dependency might be missing.

  23. Autowired with Collections:

    • Inject a collection of beans using @Autowired on a List, Set, or Map field.

  24. Autowired in XML Configuration:

    • In XML configuration, use <context:component-scan> and <bean> elements to achieve autowiring.

  25. Autowiring Modes:

    • Specify autowiring modes using @Autowired with required attribute, indicating whether the dependency is mandatory.

  26. Autowiring with Qualifier in XML:

    • In XML configuration, use <qualifier> to specify the bean name when autowiring by type.

  27. Custom Qualifier Annotation:

    • Create a custom qualifier annotation by extending @Qualifier for more meaningful and type-safe autowiring.

  28. Autowired with Optional:

    • In Spring 4.0 and later, use @Autowired on Optional types for more flexible dependency handling.

  29. Conditional Autowiring:

    • Use @ConditionalOn... annotations to conditionally enable autowiring based on specific conditions.