Annotations

SpringFrame work Annotations

• @Component : Class managed by Spring framework. It is a class-level annotation and is used to denote a class as a Component. We can use @Component across the application to mark the beans(instances or objects) as Spring’s managed components. @Component is an annotation that allows Spring to automatically detect our custom beans. In other words, without having to write any explicit code, Spring will:
• Scan our application for classes annotated with @Component.
• Instantiate them and inject any specified dependencies into them.
• Inject them wherever needed.
• @ComponentScan : It is an Interface. This annotation is an alternative to <context:component-scan> XML tag. Tells Spring that where to look for Spring Components explicitly. @ComponentScan annotation is used with @Configuration annotation.
• @ComponentScan without attribute tells that Spring to discover components in current package and within it’s sub packages.
• We can tell Spring to which packages to scan specifically by using basePackages attribute. @ComponentScan(basePackages = “BasePackageName1”), @ComponentScan(basePackages={ “BasePackageName1”, “BasePackageName2”}) with multiple packages as attributes. @ComponentScan(“packageName”), @ComponentScan({“packageName1”,”packageName2”})
• @Autowired : @Autowired annotation is performing Dependency Injection. you can turn off the default behavior by using (required=false) option with @Autowired like @Autowired(required=false). @Autowired can be applied to:
• a) field: then the dependency is stored in this field
• b) setter: then the setter is invoked, with the parameter that is determined by the same algorithm like for the field dependency injection
• c) constructor: then the constructor is invoked with the parameters determined by the same algorithm like for the field dependency injection
• @RestController : It is applied to a class to mark it as a request handler, It converts the response to Json or xml. We can expose the Web Url by annotating the class with @RestController. Spring 4.0 introduced the @RestController annotation in order to simplify the creation of RESTful web services. It’s a convenient annotation that combines @Controller and @ResponseBody, which eliminates the need to annotate every request handling method of the controller class with the @ResponseBody annotation. ex: http://localhost:8080.
• @Controller: This is class level annotation that tells spring that the class is controller class where we write all the end points.
• Diff B/W @RestController & @Controller:
• 1. The @Controller is a common annotation which is used to mark a class as Spring MVC Controller while the @RestController is a special controller used in RESTFul web services and the equivalent of @Controller(class level) + @ResponseBody(method level).
• 1. One of the key difference between @Controller and @RestController in Spring MVC is that once you mark a class as @RestController then every method is return a domain object(instance of class)(bussiness object which holds data and its behaviour. ex: student object, which holds student data and its behaviour like whether student name, student is properly studying or not) instead of a view(jsp which is a view technology)(view mean which calls the respective jsp page that is placed in (resourses/WEB-INF/resourses/WEB-INF/jsp/filename.jsp), .jsp file is a view.
• 1. You don’t need to use the @Controller and the @RestponseBody annotation. Instead you can use the @RestController to provide the same functionality, that means we don’t need to use @ResponseBody on every handler method once you annotate the class with @RestController as shown below.
• with @RestController:

  @RestController
  public class Book{
  @RequestMapping(value = "/books", method = RequestMethod.GET)
  public Book getBook(){
  return book;
  }
  }
  

• without @RestController:

  @Controller
  public class Book{
  @RequestMapping(value = "/books", method = RequestMethod.GET)
  @ResponseBody
  public Book getBook(){
  //...
  return book;
  }
  }
  

• @GetMapping : The @GetMapping annotated methods in the @Controller annotated classes handle the HTTP GET requests matched with given URI expression. EX: @GetMapping(“/path”) –> http://localhost:8080/path.
• Diff B/W @RestController & @GetMapping: RequestMapping is a generic request handler where we can use with GET, POST, PUT or any other request methods using in method attribute. GetMapping is a specific for GET request method. It is just an extension of RequestMapping to improve clarity.
• Link
• @PostMapping : @PostMapping annotation maps HTTP POST requests onto specific handler methods. It is a composed annotation that acts as a shortcut for @RequestMapping (method = RequestMethod.POST).
• @PutMapping: used for mapping HTTP put requests onto specific handler methods. It is a composed annotation that acts as shortcut for @RequestMapping(method=RequestMethod.PUT).
• @DeleteMapping: Maps HTTP Delete request onto specific handler methods. Handler Methods when a request is received the dispatcherServlet consults the Handler Method for the appropriate controller. It basically decides what to do/show when you call the url.
• @SpringBootConfiguration : @SpringBootApplication = @Configuration + @ComponentScan + @EnableAutoConfiguration. The @SpringBootApplication annotation is a combination of following three Spring annotations and provides the functionality of all three with just one line of code.
• @EnableAutoConfiguration : It is an interface. The @EnableAutoConfiguration annotation enables Spring Boot to auto-configure the application context. Therefore, it automatically creates and registers beans based on both the included jar files in the classpath and the beans defined by us.
• @RequestBody & @ResponseBody: Behind the scenes these annotations uses HTTP message convertors(JacksonHttpMessageConvertersConfiguration) to convert the body of Http request/Response to domain objects as json or xml. @RequestBody this annotation indicates that Spring should deserialize a request body into an object. @RequestBody annotation must correspond to the JSON sent from our client-side controller .Using @ResponseBody,Spring converts the return value and writes it to the HTTP response automatically. [Bean <-> Json]
• @Autowired: Provides more fine-grained control over where and how autowiring should be accomplished. this feature of spring framework enables you to inject the object dependency implicitly. Ex: controller class is dependent on service so controller class will be autowired with service class object. It internally uses setter or constructor injection. Autowiring cant be used to inject primitive and string values. Basically this annoatation tells spring framework to find dependency.
• @ConfigurationProperties: Spring Boot @ConfigurationProperties is annotation for externalized configuration. To inject property value from a property file to a class, we can add @ConfigurationProperties at a class level with stereotype annotations such as @Component or add @ConfigurationProperties to a @Bean method in a @Configuration class. @ConfigurationProperties is used to bind and validate external properties from property files such as .properties file. @ConfigurationProperties has following optional elements.
  • a) ignoreInvalidFields: Boolean to ignore invalid fields.
  • b) ignoreUnknownFields: Boolean to ignore unknown fields.
  • c) prefix: The prefix of the properties to bind to this object.
  • d) value: The prefix of the properties to bind to this object.
• @ConfigurationProperties
• @PathVariable: @PathVariable annotation can be used to handle template variables in the request URI mapping, and set them as method parameters. java @GetMapping("/api/employees/{id}") public String getEmployeesById(@PathVariable String id) { return id; }