Ravaan Techky

REST Example along with Request validation and HATEOAS framework

Overview:

For Spring request validation, we need to add spring-boot-validation-starter dependency. For HATEOAS (Hyperlink As The Engine Of Application State), we need to add spring-boot-hateoas-starter dependency

	<dependency> 
		<groupId>org.springframework.boot</groupId> 
		<artifactId>spring-boot-starter-validation</artifactId> 
	</dependency>

	<dependency> 
		<groupId>org.springframework.boot</groupId> 
		<artifactId>spring-boot-starter-hateoas</artifactId> 
	</dependency>

Important Points:

• Given sample example is work on Spring Boot version - 2.4.2

Validation:

• To enabled validation we need add @Validated annotation in request method. ```markdown /**
  • Save user. *
  • @param userObject the user object
  • @return the response entity */ @PostMapping(path = “/user”) public ResponseEntity
• We need to provide field level validation in Pojo class which created/bind from request.

  public class User {	
    /** The first name. */
    @Size(min = 2, message = "Minimum allowed size is 2 character")
    private String firstName;
  
    /** The enroll date. */
    @Past
    private Date enrollDate;
    ...
  

• In above example, If requested User json does not pass validation then it throw HTTP 400 (Bad Request) code. To provide field level error message, we need to modify handleMethodArgumentNotValid method from ResponseEntityExceptionHandler class

@ControllerAdvice
@RestController
public class CustomResponseEntityExceptionHandler extends ResponseEntityExceptionHandler {

	@Override
	protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException exception,
			HttpHeaders headers, HttpStatus status, WebRequest request) {
		StringBuilder fieldErrorBuilder = new StringBuilder();
		exception.getBindingResult().getFieldErrors().forEach(fieldError -> {
			fieldErrorBuilder.append("Field " + fieldError.getField() + " value " + fieldError.getRejectedValue() + " is rejected because of " + fieldError.getDefaultMessage());
		});
		ExceptionResponse exceptionResponse = new ExceptionResponse("Validation Fail. " + fieldErrorBuilder.toString(), exception.getBindingResult().toString());
		return new ResponseEntity(exceptionResponse, HttpStatus.BAD_REQUEST);
	}
}

HATEOAS (Hyperlink As The Engine Of Application State):

• HATEOAS framework is available in Spring Boot startup kit.
• With the help HATEOAS, we can able to send other request endpoints along with response which is useful from consumer prospective. Example, - If consumer requested user endpoints then in response along with user object it also bing some useful URI like - list-user, save-user, delete-user, etc.

 	/**
	 * Gets the all user.
	 *
	 * @return the all user
	 */
	@GetMapping(path = "/user")
	public CollectionModel<User> getAllUser() {
		List<User> userList = this.userService.getAllUser();
		CollectionModel<User> resource = CollectionModel.of(userList);
		WebMvcLinkBuilder userLink = WebMvcLinkBuilder.linkTo(WebMvcLinkBuilder.methodOn(this.getClass()).findUser(123));
		resource.add(userLink.withRel("user-link"));
		WebMvcLinkBuilder deleteUserLink = WebMvcLinkBuilder.linkTo(WebMvcLinkBuilder.methodOn(this.getClass()).deleteUser(123));
		resource.add(deleteUserLink.withRel("delete-user-link"));
		User userObject = new User(-1, "firstName", "lastName", "address");
		WebMvcLinkBuilder saveUserLink = WebMvcLinkBuilder.linkTo(WebMvcLinkBuilder.methodOn(this.getClass()).saveUser(userObject));
		resource.add(saveUserLink.withRel("save-user-link"));
		return resource;
	}


	/**
	 * Find user.
	 *
	 * @param userId the user id
	 * @return the entity model
	 */
	@GetMapping(path = "/user/{userId}")
	public EntityModel<User> findUser(@PathVariable(name = "userId") Integer userId) {
		User resultUser = this.userService.findUser(userId);
		if(resultUser == null) {
			throw new ResourceNotFoundException("Requested user not found. [User ID : " + userId + "]");
		}
		EntityModel<User> resource = EntityModel.of(resultUser);
		WebMvcLinkBuilder deleteUserLink = WebMvcLinkBuilder.linkTo(WebMvcLinkBuilder.methodOn(this.getClass()).deleteUser(123));
		resource.add(deleteUserLink.withRel("delete-user-link"));
		WebMvcLinkBuilder allUserLink = WebMvcLinkBuilder.linkTo(WebMvcLinkBuilder.methodOn(this.getClass()).getAllUser());
		resource.add(allUserLink.withRel("all-user"));
		return resource;
	}

• Please note, above code is with reference to new HATEOAS update.
• In Old HATEOAS framework, -

 	/**
	 * Find user.
	 *
	 * @param userId the user id
	 * @return the entity model
	 */
	@GetMapping(path = "/user/{userId}")
	public Resource<User> findUser(@PathVariable(name = "userId") Integer userId) {
		User resultUser = this.userService.findUser(userId);
		if(resultUser == null) {
			throw new ResourceNotFoundException("Requested user not found. [User ID : " + userId + "]");
		}
		 Resource<User> userResource = new Resource<>(userResultObject);
		 ControllerLinkBinder linkTo = ControllerLinkBinder.linkTo(ControllerLinkBinder.methodOn(this.getClass()).getAllUsers());
		 resource.add(allUserLink.withRel("all-user"))
		 return resource;
	}

Content Negotiation (Request & Response format setting):

• We can able to change accepted format from request header field.
• we have 2 format support with Jackson (Dependency - jackson-dataformat-xml and jackson-dataformat-json (by default included in spring framework)

	<dependency>
		<groupId>com.fasterxml.jackson.dataformat</groupId>
		<artifactId>jackson-dataformat-xml</artifactId>
	</dependency>

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spring-rest-validation-example is maintained by ravaan-techky.

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