Tuesday, March 5, 2013

Centralize validation and exception handling with @ControllerAdvice



1 Introduction

The @ControllerAdvice annotation introduced by Spring 3.2 allows us to handle several functionalities in a way that can be shared by all controllers (through its handler methods, annotated with @RequestMapping). This annotation is mainly used to define the following methods:
  • @ExceptionHandler: Handles exceptions thrown by handler methods.
  • @InitBinder: Initializes the WebDataBinder, which will be used to populate objects passed as arguments to the handler methods. Usually, it is used to register property editors or validators.
  • @ModelAttribute: Binds a parameter or return value to an attribute, which will then be exposed to a web view.

Source code can be found at github.


2 Adding validation and exception handling

The following is a description of  the controller's handler methods before implementing the @ControllerAdvice.

Add person controller:

@RequestMapping(value="/persons", method=RequestMethod.POST)
@ResponseStatus(HttpStatus.CREATED)
public void addPerson(@Valid @RequestBody Person person, HttpServletRequest request, HttpServletResponse response) {
personRepository.addPerson(person);
logger.info("Person added: "+person.getId());
response.setHeader("Location", request.getRequestURL().append("/").append(person.getId()).toString());
}

@InitBinder
public void initBinder(WebDataBinder binder) {
binder.setValidator(new PersonValidator());
}
      
@ExceptionHandler({MethodArgumentNotValidException.class})
public ResponseEntity<String> handleValidationException(MethodArgumentNotValidException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.BAD_REQUEST);
}


Besides the handler method, this controller has the following methods:
  • initBinder: Registers a validator to prevent that a person with invalid data is introduced. To make the validator validate the person object passed as a parameter, it is necessary to add the @Valid annotation to the argument. Spring 3 fully supports JSR-303 bean validation API, but it does not implement it. The reference implementation which is used in this example is Hibernate Validator 4.x.
  • handleValidationException: Handles the MethodArgumentNotValidException that can be thrown by the handler method. This exception is thrown by Spring MVC when an argument annotated with @Valid, fails its validation.


Get person controller:

@RequestMapping(value="/persons/{personId}", method=RequestMethod.GET)
public @ResponseBody Person getPerson(@PathVariable("personId") long id) {
return personRepository.getPerson(id);
}
     
@ExceptionHandler({PersonNotFoundException.class})
public ResponseEntity<String> handlePersonNotFound(PersonNotFoundException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.NOT_FOUND);
}
      

This controller adds an exception handler for handling when a request asks to retrieve a person that does not exist.





Update person controller:

@RequestMapping(value="/persons", method=RequestMethod.PUT)
@ResponseStatus(HttpStatus.CREATED)
public void updatePerson(@Valid @RequestBody Person person, HttpServletRequest request, HttpServletResponse response) {
personRepository.updatePerson(person);
logger.info("Person updated: "+person.getId());
response.setHeader("Location", request.getRequestURL().append("/").append(person.getId()).toString());
}
      
@InitBinder
public void initBinder(WebDataBinder binder) {
binder.setValidator(new PersonValidator());
}
      
@ExceptionHandler({PersonNotFoundException.class})
public ResponseEntity<String> handlePersonNotFound(PersonNotFoundException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.NOT_FOUND);
}
      
@ExceptionHandler({Exception.class})
public ResponseEntity<String> handleValidationException(MethodArgumentNotValidException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.BAD_REQUEST);
}
 
We are repeating code, since @ExceptionHandler is not global.


3 Centralizing code

@ControllerAdvice annotation is itself annotated with @Component, so the class that we are implementing will be autodetected through classpath scanning.

@ControllerAdvice
public class CentralControllerHandler {
@InitBinder
public void initBinder(WebDataBinder binder) {
binder.setValidator(new PersonValidator());
}
      
@ExceptionHandler({PersonNotFoundException.class})
public ResponseEntity<String> handlePersonNotFound(PersonNotFoundException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.NOT_FOUND);
}
      
@ExceptionHandler({MethodArgumentNotValidException.class})
public ResponseEntity<String> handleValidationException(MethodArgumentNotValidException pe) {
return new ResponseEntity<String>(pe.getMessage(), HttpStatus.BAD_REQUEST);
}
}

Now we can delete these methods from the controllers, taking rid of code duplication, since this class will handle exception handling and validation for all handler methods annotated with @RequestMapping.

 

4 Testing

The methods described below, test the retrieval of persons:

@Test
public void getExistingPerson() {
String uri = "http://localhost:8081/rest-controlleradvice/spring/persons/{personId}";
Person person = restTemplate.getForObject(uri, Person.class, 1l);
assertNotNull(person);
assertEquals("Xavi", person.getName());
}
      
@Test
public void getNonExistingPerson() {
String uri = "http://localhost:8081/rest-controlleradvice/spring/persons/{personId}";
try {
restTemplate.getForObject(uri, Person.class, 5l);
throw new AssertionError("Should have returned an 404 error code");
} catch (HttpClientErrorException e) {
assertEquals(HttpStatus.NOT_FOUND, e.getStatusCode());
}
}


The rest of tests can be found with the source code linked above.