import javax.validation.constraints.Size;

import org.springframework.http.ResponseEntity;
import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

/**
 * @author javaQuery
 * @since 2018-01-31
 * @github https://github.com/javaquery/spring-boot-examples
 */
@RestController
@RequestMapping("/api")
@Validated
public class HelloWorldController {
 
 /**
  * This will send Hello {name-from-parameter} in response with HTTP status code 200.
  * @since 2018-02-01
  * @param firstname
  * @param lastname
  * @return
  */
   @GetMapping("/hello")
   public ResponseEntity<?> sayHello(
     @RequestParam @Size(min= 1, max = 5, message = "firstname length must be between 1 and 5") String firstname,
     @RequestParam String middlename,
     @RequestParam(required = false) String lastname){
    /* check lastname value */
    lastname = lastname != null ? lastname : "{lastname-is-optional}";
    return ResponseEntity.ok("Hello " + firstname + " " + middlename + " " + lastname);
   }
}

• @Validated - To perform validation on each method of controller if any.
• @RequestParam - To accept web request parameter in variable. (Note: All variable annotated with @RequestParam are compulsory/mandatory for request, until you set required = false @RequestParam(required = false) for that parameter)

@RequestParam @Size(min= 1, max = 5 , message = "firstname length must be between 1 and 5") String firstname

@RequestParam String middlename

@RequestParam(required = false) String lastname

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

import javax.servlet.http.HttpServletRequest;
import javax.validation.ConstraintViolation;
import javax.validation.ConstraintViolationException;

import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.MissingServletRequestParameterException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.context.request.WebRequest;
import org.springframework.web.servlet.mvc.method.annotation.ResponseEntityExceptionHandler;

/**
 * @author javaQuery
 * @since2018-02-01
 */
@Order(Ordered.HIGHEST_PRECEDENCE)
@ControllerAdvice()
public class RestExceptionHandler extends ResponseEntityExceptionHandler{
 
   @Override
   protected ResponseEntity<Object> handleMissingServletRequestParameter(MissingServletRequestParameterException ex, HttpHeaders headers,
     HttpStatus status, WebRequest request) {
    String error = ex.getParameterName() + " parameter is missing.";
    return new ResponseEntity<Object>(new ErrorResponse<>(Arrays.asList(error)), HttpStatus.BAD_REQUEST);
   }
 
   /**
    * Exception thrown when {@link org.springframework.validation.annotation.Validated} is used in controller.
    * @param ex
    * @param request
    * @return
    */
   @ExceptionHandler(ConstraintViolationException.class)
   protected ResponseEntity<?> handleConstraintViolationException(ConstraintViolationException ex, HttpServletRequest request) {
    try {
     List<String> messages = ex.getConstraintViolations().stream().map(ConstraintViolation::getMessage).collect(Collectors.toList());
     return new ResponseEntity<>(new ErrorResponse<>(messages), HttpStatus.BAD_REQUEST);
    } catch (Exception e) {
     return new ResponseEntity<>(new ErrorResponse<>(Arrays.asList(ex.getMessage())), HttpStatus.INTERNAL_SERVER_ERROR);
    }
   }
}

import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

/**
 * @author javaQuery
 * @since 2018-02-01
 * 
 * @param <T>
 */
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(Include.NON_NULL)
public class ErrorResponse<T> {
   private List<T> errors = new ArrayList<>(1);
 
   public ErrorResponse(List<T> errors) {
    this.errors = errors;
   }
 
   public List<T> getErrors() {
    return errors;
   }
}

-code 400
-body {"errors":["firstname parameter is missing."]}

-code 400
-body {"errors":["firstname length must be between 1 and 5"]}

-code 200
-body Hello vicky v {lastname-is-optional}

-code 200
-body Hello vicky v thakor