How to validate Spring MVC @PathVariable values?

You can use like this: use org.springframework.validation.annotation.Validated to valid RequestParam or PathVariable.

 *
 * Variant of JSR-303's {@link javax.validation.Valid}, supporting the
 * specification of validation groups. Designed for convenient use with
 * Spring's JSR-303 support but not JSR-303 specific.
 *

step.1 init ValidationConfig

@Configuration
public class ValidationConfig {
    @Bean
    public MethodValidationPostProcessor methodValidationPostProcessor() {
        MethodValidationPostProcessor processor = new MethodValidationPostProcessor();
        return processor;
    }
}

step.2 Add @Validated to your controller handler class, Like:

@RequestMapping(value = "poo/foo")
@Validated
public class FooController {
...
}

step.3 Add validators to your handler method:

   @RequestMapping(value = "{id}", method = RequestMethod.DELETE)
   public ResponseEntity<Foo> delete(
           @PathVariable("id") @Size(min = 1) @CustomerValidator int id) throws RestException {
        // do something
        return new ResponseEntity(HttpStatus.OK);
    }

final step. Add exception resolver to your context:

@Component
public class BindExceptionResolver implements HandlerExceptionResolver {

    @Override
    public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
        if (ex.getClass().equals(BindException.class)) {
            BindException exception = (BindException) ex;

            List<FieldError> fieldErrors = exception.getFieldErrors();
            return new ModelAndView(new MappingJackson2JsonView(), buildErrorModel(request, response, fieldErrors));
        }
    }
}

Instead of using @PathVariable, you can take advantage of Spring MVC ability to map path variables into a bean:

@RestController
@RequestMapping("/user")
public class UserController {

    @GetMapping("/{id}")
    public void get(@Valid GetDto dto) {
        // dto.getId() is the path variable
    }

}

And the bean contains the actual validation rules:

@Data
public class GetDto {
     @Min(1) @Max(99)
     private long id;
}

Make sure that your path variables ({id}) correspond to the bean fields (id);

The solution is simple:

@GetMapping(value = {"/", "/{hash:[a-fA-F0-9]{40}}"})
public String request(@PathVariable(value = "hash", required = false) String historyHash)
{
    // Accepted requests: either "/" or "/{40 character long hash}"
}

And yes, PathVariables are ment to be validated, like any user input.

@PathVariable is not meant to be validated in order to send back a readable message to the user. As principle a pathVariable should never be invalid. If a pathVariable is invalid the reason can be:

1. a bug generated a bad url (an href in jsp for example). No @Valid is needed and no message is needed, just fix the code;
2. "the user" is manipulating the url. Again, no @Valid is needed, no meaningful message to the user should be given.

In both cases just leave an exception bubble up until it is catched by the usual Spring ExceptionHandlers in order to generate a nice error page or a meaningful json response indicating the error. In order to get this result you can do some validation using custom editors.

Create a CustomerNumber class, possibly as immutable (implementing a CharSequence is not needed but allows you to use it basically as if it were a String)

public class CustomerNumber implements CharSequence {

    private String customerNumber;

    public CustomerNumber(String customerNumber) {
        this.customerNumber = customerNumber;
    }

    @Override
    public String toString() {
        return customerNumber == null ? null : customerNumber.toString();
    }

    @Override
    public int length() {
        return customerNumber.length();
    }

    @Override
    public char charAt(int index) {
        return customerNumber.charAt(index);
    }

    @Override
    public CharSequence subSequence(int start, int end) {
        return customerNumber.subSequence(start, end);
    }

    @Override
    public boolean equals(Object obj) {
        return customerNumber.equals(obj);
    }

    @Override
    public int hashCode() {
        return customerNumber.hashCode();
    }
}

Create an editor implementing your validation logic (in this case no whitespaces and fixed length, just as an example)

public class CustomerNumberEditor extends PropertyEditorSupport {

    @Override
    public void setAsText(String text) throws IllegalArgumentException {

        if (StringUtils.hasText(text) && !StringUtils.containsWhitespace(text) && text.length() == YOUR_LENGTH) {
            setValue(new CustomerNumber(text));
        } else {
            throw new IllegalArgumentException();
            // you could also subclass and throw IllegalArgumentException
            // in order to manage a more detailed error message
        }
    }

    @Override
    public String getAsText() {
        return ((CustomerNumber) this.getValue()).toString();
    }
}

Register the editor in the Controller

@InitBinder
public void initBinder(WebDataBinder binder) {

    binder.registerCustomEditor(CustomerNumber.class, new CustomerNumberEditor());
    // ... other editors
}

Change the signature of your controller method accepting CustomerNumber instead of String (whatever your ResponseObject is ...)

@RequestMapping(value = "/number/{customerNumber}")
@ResponseBody
public ResponseObject searchByNumber(@PathVariable("customerNumber") CustomerNumber customerNumber) {
    ...
}

You can create the answer you want by using the fields in the ConstraintViolationException with the following method;

    @ExceptionHandler(ConstraintViolationException.class)
    protected ResponseEntity<Object> handlePathVariableError(final ConstraintViolationException exception) {
        log.error(exception.getMessage(), exception);

        final List<SisSubError> subErrors = new ArrayList<>();
        exception.getConstraintViolations().forEach(constraintViolation -> subErrors.add(generateSubError(constraintViolation)));

        final SisError error = generateErrorWithSubErrors(VALIDATION_ERROR, HttpStatus.BAD_REQUEST, subErrors);
        return new ResponseEntity<>(error, HttpStatus.BAD_REQUEST);
    }

You need to added an @Validated annotation to Controller class and any validation annotation before path variable field

Path variable may not be linked with any bean in your system. What do you want to annotate with JSR-303 annotations? To validate path variable you should use this approach Problem validating @PathVariable url on spring 3 mvc

Actually there is a very simple solution to this. Add or override the same controller method with its request mapping not having the placeholder for the path variable and throw ResponseStatusException from it. Code given below

 @RequestMapping(value = "/number")
 @ResponseBody
 public ResponseObject searchByNumber() {
 throw new ResponseStatusException(HttpStatus.BAD_REQUEST,"customer number missing")
 }