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Java Spring ExceptionHandler Controller syntax and good practices

I would like to handle my exceptions correctly in Spring, so I have a question about exceptionHandler syntax : Is it ok to throw specific exceptions in the controller, if they are caught by the exception handler ?
More specifically :
Here is the Exception :
public class UnknownUserException extends Exception {
private static final long serialVersionUID = 1L;
public UnknownUserException(String message) {
super(message);
}
}
Here is the ExceptionHandler with the specific method for UnknownUserException :
#ControllerAdvice
#ResponseBody
public class ControllerExceptionHandler {
#ExceptionHandler(UnknownUserException.class)
#ResponseStatus(value = HttpStatus.NOT_FOUND)
public ErrorMessage unknownUserExceptionMessage(UnknownUserException ex, WebRequest request) {
ErrorMessage message = new ErrorMessage("The user doesn't exist: " +ex.getLocalizedMessage(), ex);
return message;
}
}
Here is one example of a mapping that may raise that exception :
#GetMapping({"/user/{id}"})
public ResponseEntity<UserProfileDto> getById(#PathVariable Long id) throws UnknownUserException {
UserProfileDto user = userService.findById(id);
return ResponseEntity.ok(user);
}
The userService.findById(id) may throw the UnknownUserException.
From what I understood, the controllerAdvice "overrides" the controller in case of that specific exception thrown by a service, but then, what should I do with my controller ? should I throw the exception again (like above) or catch the specific exception and return a ResponseEntity(HttpStatus.NOT_FOUND) ?

In an ideal scenario exception should be thrown immediately when it is known in your case as you mentioned service method will throw UnknownUserException that the right thing to do. Your Controller Advice should be able to handle that exception. ContollerAdvice will handle any matching exception that is thrown during execution of the request irrespective of the origin of exception.
Refer to this link for other options on handling exception

Implementing the ResponseErrorHandler Interface

I'm trying to override the ResponseErrorHandler interface to be able to return the entire request (status code, body etc.) in case of any response other than 2xx.
I noticed that the Spring (RestTemplate) default returns an exception in case of a response other than 2xx. I do not want to return an exception, I just want to be able to return a:
new ResponseEntity(HttpStatus.STATUS_CODE)
Following some tutorials, I've found the following code:
#Component
public class LoginErrorHandler
implements ResponseErrorHandler {
#Override
public boolean hasError(ClientHttpResponse httpResponse)
throws IOException {
return (
httpResponse.getStatusCode().series() == CLIENT_ERROR
|| httpResponse.getStatusCode().series() == SERVER_ERROR);
}
#Override
public void handleError(ClientHttpResponse httpResponse)
throws IOException {
if (httpResponse.getStatusCode()
.series() == SERVER_ERROR) {
// handle SERVER_ERROR
} else if (httpResponse.getStatusCode()
.series() == CLIENT_ERROR) {
// handle CLIENT_ERROR
}
}
(Reference)
But I have not understood how I can return a ResponseEntity without changing the method return (which I can not by implementing the method).
Implementation:
HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);
RestTemplate restTemplate = new RestTemplate();
restTemplate.setErrorHandler(new LoginErrorHandler());
return restTemplate.postForEntity(url, request, String.class);

You can use Spring's ControllerAdvice and ExceptionHandler annotations to handle exceptions through your application. Below code returns 500 http status code if any exception encountered in your request. You can add other Exception classes or your own custom class to handle specific cases and return specific status codes to client.
Edit
Handling each code will not be a good idea. Rather you can wrap them in your custom exception and provide proper message to your client service. Still you can try something like below.
#Component
#ControllerAdvice
public class MyExceptionHandler {
#ExceptionHandler(HttpClientErrorException.BadRequest.class)
#ResponseStatus(code=HttpStatus.BAD_REQUEST, reason="Bad Request", value=HttpStatus.BAD_REQUEST)
public void handleBadRequest(HttpClientErrorException.BadRequest e) {
//handle bad request exception
}
#ExceptionHandler(HttpClientErrorException.NotFound.class)
#ResponseStatus(code=HttpStatus.NOT_FOUND, reason="Not Found", value=HttpStatus.NOT_FOUND)
public void handleNotFound(HttpClientErrorException.NotFound e) {
//handle Not Found
}
#ExceptionHandler(HttpServerErrorException.InternalServerError.class)
#ResponseStatus(code=HttpStatus.INTERNAL_SERVER_ERROR, reason="Internal Server Error", value=HttpStatus.INTERNAL_SERVER_ERROR)
public void handleInternalServerError(HttpServerErrorException.InternalServerError e) {
//handle internal server error
}
//more methods for each code.
}
Then handle the codes from in your rest template as below. Here you won't be able to return body of the response to the client.
#Component
public class LoginErrorHandler
implements ResponseErrorHandler {
#Override
public boolean hasError(ClientHttpResponse httpResponse)
throws IOException {
return (httpResponse.getStatusCode() != HttpStatus.OK);
}
#Override
public void handleError(ClientHttpResponse httpResponse)
throws IOException {
if (httpResponse.getRawStatusCode() >=400 && httpResponse.getRawStatusCode()<500 ) {
throw HttpClientErrorException.create(httpResponse.getStatusCode(), httpResponse.getStatusText(), httpResponse.getHeaders(), null, null);
}else if(httpResponse.getRawStatusCode() >=500){
throw HttpServerErrorException.create(httpResponse.getStatusCode(), httpResponse.getStatusText(), httpResponse.getHeaders(), null, null);
}else {
//throw some other exceptions for other codes and catch them in controller advice.
}
}
}

You can do this:
#Component
#ControllerAdvice
public class LoginErrorHandler{
#ExceptionHandler(HttpClientErrorException.class)
#ResponseBody
public void handleError(HttpClientErrorException e, HttpServletResponse response) throws IOException {
response.sendError(e.getRawStatusCode(), e.getStatusText());
}
}
This you generify all status code that should drop an exception, and it will return in the body.

There are two extensively used and very convenient exception handlers which are provided by Spring framework for centralized exception management in Spring Boot applications.
ResponseEntityExceptionHandler : exception thrown by our endpoint methods(methods annotated with #RequestMapping)
A convenient base class for #ControllerAdvice classes that wish to provide centralized exception handling across all #RequestMapping methods through #ExceptionHandler methods.
ResponseErrorHandler : Spring provides a hook ResponseErrorHandler which can be implemented to handle the exception thrown by external services. To call any external service most likely you will be using a RestTemplate. A RestTemplate can throw three types of exception as listed below :
HttpClientErrorException : For 4xx series status codes or Client errors.
HttpServerErrorException : For 5xx series status codes or server errors
RestClientException : Any other status codes like 3xx series
To simplify things we can handle these exceptions as a catch block separately but it lead to lot of boilerplate code scattered across our service. Strategy interface used by the RestTemplate to determine whether a particular response has an error or not.
There are two steps to using ResponseErrorHandler :
Step 1: Create a custom error handler class by implementing ResponseErrorHandler and implements its methods hasError and handleError
Step 2: We need to inject the ResponseErrorHandler implementation into the RestTemplate instance as follows :
By default in RestTemplate the errorHandler points to DefaultResponseErrorHandler.
Source

How to get all exception handlers annotated by #ExceptionHanlder?

How to get all exception handlers annotated by #ExceptionHanlder and I can call them manually?
Background
I need to handle some exceptions by my own exception handlers but in some situation my handled exceptions are not thrown directly by spring, and they are wrapped in the cause by. So I need to handle these caused by exceptions in one place using my own exception handling strategy in the existing #ExceptionHandlers. How can I do that?

Try to use Java Reflection Api to find classes annotated with "ExceptionHanlder". And invoke any method or whatever you want.

You can extend ResponseEntityExceptionHandler and make it a #ControllerAdvise like below.
#ControllerAdvice
public class CustomExceptionHandler extends ResponseEntityExceptionHandler {
#ExceptionHandler({YourException.class})
public ResponseEntity<Object> handleMyException(Exception ex, WebRequest request) {
... handle the way you like it
return new ResponseEntity<Object>(YourErrorObject, new HttpHeaders(), HttpStatus);
}
}

Spring provides #ControllerAdvice annotation that we can use with any class to define our global exception handler. The handler methods in Global Controller Advice is same as Controller based exception handler methods and used when controller class is not able to handle the exception.
You want to use exception handling strategy in your one place. that you can define multiple exception or make message using exception in exception controller.
like this :
#ExceptionHandler(value = { HttpClientErrorException.class, HTTPException.class, HttpMediaTypeException.class,
HttpMediaTypeNotSupportedException.class, HttpMessageNotReadableException.class })
or
#ExceptionHandler
#ResponseBody
ExceptionRepresentation handle(Exception exception) {
ExceptionRepresentation body = new ExceptionRepresentation(exception.getLocalizedMessage());
HttpStatus responseStatus = resolveAnnotatedResponseStatus(exception);
return new ResponseEntity<ExceptionRepresentation>(body, responseStatus);
}
HttpStatus resolveAnnotatedResponseStatus(Exception exception) {
ResponseStatus annotation = findMergedAnnotation(exception.getClass(), ResponseStatus.class);
if (annotation != null) {
return annotation.value();
}
return HttpStatus.INTERNAL_SERVER_ERROR;
}

Here is a work around. You can catch the the wrapping exception and then check the root cause of the exception. Here is an example of MySQLIntegrityConstraintViolationException which is wrapped by DataIntegrityViolationException in spring:
#ExceptionHandler(DataIntegrityViolationException.class)
#ResponseBody
public ResponseEntity<Object> proccessMySQLIntegrityConstraint(DataIntegrityViolationException exception) {
if (exception.getRootCause() instanceof MySQLIntegrityConstraintViolationException) {
doSomething....
} else {
doSomethingElse...
}
}

How to catch all unhandled exceptions (i.e. without existing #ExceptionHandler) in Spring MVC?

I want to let HandlerExceptionResolver resolve any Exceptions that I don't explicit catch via #ExceptionHandler annotation.
Anyways, I want to apply specific logic on those exceptions. Eg send a mail notification or log additionally. I can achieve this by adding a #ExceptionHandler(Exception.class) catch as follows:
#RestControllerAdvice
public MyExceptionHandler {
#ExceptionHandler(IOException.class)
#ResponseStatus(HttpStatus.BAD_REQUEST)
public Object io(HttpServletRequest req, Exception e) {
return ...
}
#ExceptionHandler(Exception.class)
public Object exception(HttpServletRequest req, Exception e) {
MailService.send();
Logger.logInSpecificWay();
//TODO how to continue in the "normal" spring way with HandlerExceptionResolver?
}
}
Problem: if I add #ExceptionHandler(Exception.class) like that, I can catch those unhandled exceptions.
BUT I cannot let spring continue the normal workflow with HandlerExceptionResolver to create the response ModelAndView and set a HTTP STATUS code automatically.
Eg if someone tries a POST on a GET method, spring by default would return a 405 Method not allowed. But with an #ExceptionHandler(Exception.class) I would swallow this standard handling of spring...
So how can I keep the default HandlerExceptionResolver, but still apply my custom logic?

To provide a complete solution: it works just by extending ResponseEntityExceptionHandler, as that handles all the spring-mvc errors.
And the ones not handled can then be caught using #ExceptionHandler(Exception.class).
#RestControllerAdvice
public class MyExceptionHandler extends ResponseEntityExceptionHandler {
#ExceptionHandler(Exception.class)
public ResponseEntity<Object> exception(Exception ex) {
MailService.send();
Logger.logInSpecificWay();
return ... custom exception
}
}

Well, I was facing the same problem some time back and have tried several ways like extending ResponseEntityExceptionHandler but all them were solving some problems but creating other ones.
Then I have decided to go with a custom solution which was also allowing me to send additional information and I have written below code
#RestControllerAdvice
public class MyExceptionHandler {
private final Logger log = LoggerFactory.getLogger(this.getClass());
#ExceptionHandler(NumberFormatException.class)
public ResponseEntity<Object> handleNumberFormatException(NumberFormatException ex) {
return new ResponseEntity<>(getBody(BAD_REQUEST, ex, "Please enter a valid value"), new HttpHeaders(), BAD_REQUEST);
}
#ExceptionHandler(IllegalArgumentException.class)
public ResponseEntity<Object> handleIllegalArgumentException(IllegalArgumentException ex) {
return new ResponseEntity<>(getBody(BAD_REQUEST, ex, ex.getMessage()), new HttpHeaders(), BAD_REQUEST);
}
#ExceptionHandler(AccessDeniedException.class)
public ResponseEntity<Object> handleAccessDeniedException(AccessDeniedException ex) {
return new ResponseEntity<>(getBody(FORBIDDEN, ex, ex.getMessage()), new HttpHeaders(), FORBIDDEN);
}
#ExceptionHandler(Exception.class)
public ResponseEntity<Object> exception(Exception ex) {
return new ResponseEntity<>(getBody(INTERNAL_SERVER_ERROR, ex, "Something Went Wrong"), new HttpHeaders(), INTERNAL_SERVER_ERROR);
}
public Map<String, Object> getBody(HttpStatus status, Exception ex, String message) {
log.error(message, ex);
Map<String, Object> body = new LinkedHashMap<>();
body.put("message", message);
body.put("timestamp", new Date());
body.put("status", status.value());
body.put("error", status.getReasonPhrase());
body.put("exception", ex.toString());
Throwable cause = ex.getCause();
if (cause != null) {
body.put("exceptionCause", ex.getCause().toString());
}
return body;
}
}

Create classes for exception handling in this way
#RestControllerAdvice
public class MyExceptionHandler extends BaseExceptionHandler {
}
public class BaseExceptionHandler extends ResponseEntityExceptionHandler {
}
Here ResponseEntityExceptionHandler is provided by spring and override the several exception handler methods provided by it related to the requestMethodNotSupported,missingPathVariable,noHandlerFound,typeMismatch,asyncRequestTimeouts ....... with your own exception messages or error response objects and status codes
and have a method with #ExceptionHandler(Exception.class) in MyExceptionHandler where the thrown exception comes finally if it doesn't have a matching handler.

I had the same issue and solved it creating a implementation of the interface HandlerExceptionResolver and removing the generic #ExceptionHandler(Exception.class) from the generic handler method.
.
It works this way:
Spring will try to handle the exception calling MyExceptionHandler first, but it will fail to find a handler because the annotation was removed from the generic handler. Next it will try other implementations of the interface HandlerExceptionResolver. It will enter this generic implementation that just delegates to the original generic error handler.
After that, I need to convert the ResponseEntity response to ModelAndView using MappingJackson2JsonView because this interface expects a ModelAndView as return type.
#Component
class GenericErrorHandler(
private val errorHandler: MyExceptionHandler,
private val objectMapper: ObjectMapper
) : HandlerExceptionResolver {
override fun resolveException(request: HttpServletRequest, response: HttpServletResponse, handler: Any, ex: Exception): ModelAndView? {
// handle exception
val responseEntity = errorHandler.handleUnexpectedException(ex)
// prepare JSON view
val jsonView = MappingJackson2JsonView(objectMapper)
jsonView.setExtractValueFromSingleKeyModel(true) // prevents creating the body key in the response json
// prepare ModelAndView
val mv = ModelAndView(jsonView, mapOf("body" to responseEntity.body))
mv.status = responseEntity.statusCode
mv.view = jsonView
return mv
}
}

Spring MVC #ExceptionHandler method in controller never being invoked

I have a Spring MVC controller with some simple REST service requests. I would like to add some error handling when specific exceptions are thrown from my services, but I cannot get a handler method annotated with #ExceptionHandler to actually ever be called. Here is one service I am deliberately throwing an exception to try and get my handler method to take over. The handler method is never invoked and Spring just returns a 500 error to the calling client. Do you have any ideas on what I'm doing wrong?
#ExceptionHandler(IOException.class)
public ModelAndView handleIOException(IOException ex, HttpServletRequest request, HttpServletResponse response) {
response.sendError(HttpServletResponse.SC_FORBIDDEN);
System.out.println("It worked!");
return new ModelAndView();
}
#RequestMapping(value = "/json/remove-service/{id}", method = RequestMethod.DELETE)
public void remove(#PathVariable("id") Long id) throws IOException {
throw new IOException("The handler should take over from here!");
}

Frustratingly, I have suffered from this as well. I discovered that if you mistakenly implement Throwable instead of Exception the Exception resolver will just rethrow your Throwable as a IllegalStateException. This will fail to invoke your #ExceptionHandler.
If you've implemented Throwable instead of Exception try changing it to Exception instead.
Here's the code in question from InvocableHandlerMethod
catch (InvocationTargetException e) {
// Unwrap for HandlerExceptionResolvers ...
Throwable targetException = e.getTargetException();
if (targetException instanceof RuntimeException) {
throw (RuntimeException) targetException;
}
else if (targetException instanceof Error) {
throw (Error) targetException;
}
else if (targetException instanceof Exception) {
throw (Exception) targetException;
}
else {
String msg = getInvocationErrorMessage("Failed to invoke controller method", args);
throw new IllegalStateException(msg, targetException);
}
}

This tip on the Spring forum may help you.
Likely you have configured the beans for your DispatchServlet in a webmvc-servlet.xml file (the *-servlet.xml file may be named differently)
If the XML file already includes another ExceptionResolver (like SimpleMappingExceptionResovler Spring wont automatically add any other resolvers for you. So manually adding the annotation resolver like so:
<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver" />
Should enable the #HandlerException processing.

I have found that #ExceptionHandler works with Throwable when you define method in such a manner:
#ExceptionHandler(Throwable.class)
#ResponseBody
public String handleException(Throwable e) {
}
In this case method has only one argument of type Throwable. If I try to use in this method some additional parameters (I tried to use Model), I receive 500 exception (this method isn't call). However this still works when additional parameters are HttpServlerRequest or HttpServlerResponse.

It will not work, because when you return String you return View name.
Now your Spring MVC controller is searching for This method is never called! Why not?! view and can find.
Make sure that #ExceptionHandler is mapped to existing view.