Spring Boot Annotations List: Every Annotation You Need, with Real Examples

@Component goes on your class and component scanning finds it. @Bean goes on a method inside a @Configuration class and registers whatever the method returns, which is how you register a class you cannot edit. The demo has both: EmailNotifier is a @Component, and AppConfig.appName() is a @Bean. The beans from the demo package:

REPORT beans from package demo:
REPORT   app-demo.AppProps -> AppProps
REPORT   appConfig -> AppConfig$$SpringCGLIB$$0
REPORT   demoApplication -> DemoApplication
REPORT   emailNotifier -> EmailNotifier
REPORT   itemController -> ItemController
REPORT   itemRepository -> ItemRepository
REPORT   itemService -> ItemService
REPORT   propsConfig -> PropsConfig$$SpringCGLIB$$0
REPORT   report -> Report
REPORT   sms -> SmsNotifier
REPORT   validController -> ValidController
REPORT   viewController -> ViewController

Note appConfig is a $$SpringCGLIB$$ subclass: Spring proxies @Configuration classes so @Bean methods return the same singleton on repeated calls. Source: Components.java.

A @Controller method returns a view name. A @RestController method returns the body. Returning a String from @Controller in an app with no view technology gives a 404, which is the usual confusion:

$ curl localhost:8080/view   (@Controller returning a String)
{"timestamp":"2026-10-21T16:04:04.735Z","status":404,"error":"Not Found","path":"/view"}
HTTP 404

The method returned "items", Spring looked for a view called items, found none, and answered Not Found. Add @ResponseBody or switch to @RestController. Source: More.java.

Two beans implement Notifier. @Primary on EmailNotifier makes it the default; @Qualifier("sms") on a parameter overrides that for one injection point. Same controller, both used:

$ curl localhost:8080/api/notify?msg=hi
{"qualified":"sms:hi","primary":"email:hi"}

Without either, startup fails. This is the real failure when two Greeter beans exist and nothing says which to use:

Parameter 0 of constructor in demo.NeedsGreeter required a single bean, but 2 were found:
	- enGreeter: defined in URL [<jar>/demo/EnGreeter.class]
	- frGreeter: defined in URL [<jar>/demo/FrGreeter.class]
Consider marking one of the beans as @Primary, updating the consumer to accept multiple beans, or using @Qualifier to identify the bean that should be consumed

The error names both candidates and the three ways out. Source: More.java. More on resolution: @Autowired Explained: By-Type Resolution, @Qualifier, @Primary, ObjectProvider and List Injection.

@Value("${app.greeting}") injects one property. @ConfigurationProperties(prefix = "app") binds the whole app.* group to a record and validates it (here @NotBlank and @Size(max = 40)). Both read the same file:

$ curl localhost:8080/api/config
{"props.maxItems":5,"value":"Hello from properties","props.greeting":"Hello from properties"}

Use @Value for a one-off; use @ConfigurationProperties when properties belong together or need validation. One surprise from the bean listing above: the properties record is registered under the name app-demo.AppProps, prefix first. Source: Components.java. Deeper: @ConfigurationProperties vs @Value in Spring Boot 4: Binding, Validation and Relaxed Rules.

@PathVariable reads from the path template, @RequestParam from the query string. One handler uses both:

$ curl localhost:8080/api/items/1?style=bold
bold:book
$ curl localhost:8080/api/items/abc
{"timestamp":"2026-10-21T16:04:04.715Z","status":400,"error":"Bad Request","path":"/api/items/abc"}
HTTP 400

A non-numeric path variable is a 400, not a 500: the conversion failure is mapped for you. Source: Components.java.

@Valid @RequestBody NewItem triggers the constraints on the record (@NotBlank name, @Min(1) qty). A bad body is rejected before your method runs:

$ curl -XPOST -H "Content-Type: application/json" -d {"name":"","qty":0} localhost:8080/api/new
{"timestamp":"2026-10-21T16:04:04.682Z","status":400,"error":"Bad Request","path":"/api/new"}
HTTP 400
$ curl -XPOST -H "Content-Type: application/json" -d {"name":"cup","qty":2} localhost:8080/api/new
{"created":"cup","qty":2}
HTTP 200

Boot’s default 400 body says nothing about which field failed. If clients need that, return a ProblemDetail: Global Exception Handling with ProblemDetail (RFC 9457) in Spring Boot 4. Source: More.java.

DevOnly is annotated @Profile("dev"). With no active profile it is not created:

REPORT DevOnly (@Profile("dev")) present: false

Profiles in depth: Spring Boot Profiles Done Right: Config Import, Config Trees and Kubernetes ConfigMaps.

All three register a bean. @Service and @Controller add only meaning; @Repository also enables exception translation for data-access errors.

It combines @SpringBootConfiguration, @EnableAutoConfiguration and @ComponentScan.

@RestController is @Controller plus @ResponseBody, so return values become the response body.

@Component for your own classes, @Bean for third-party classes or when construction needs logic.

Mark one @Primary, qualify the injection point with @Qualifier, or inject a List of all of them.