How to globally handle errors thrown from WebFilter in Spring WebFlux? - spring

Question

How to intercept and handle errors globally in WebFlux when they are being thrown from WebFilter chain?
It is clear how to handle errors thrown from controllers: #ControllerAdvice and #ExceptionHandler help great.
This approach does not work when an exception is thrown from WebFilter components.
In the following configuration GET /first and GET /second responses intentionally induce exceptions thrown. Although #ExceptionHandler methods handleFirst, handleSecond are similar, the handleSecond is never called. I suppose that is because MyWebFilter does not let a ServerWebExchange go to the stage where GlobalErrorHandlers methods could be applied.
Response for GET /first:
HTTP 500 "hello first" // expected
HTTP 500 "hello first" // actual
Response for GET /second:
HTTP 404 "hello second" // expected
HTTP 500 {"path": "/second", "status": 500, "error": "Internal Server Error" } // actual
#RestController
class MyController {
#GetMapping("/first")
String first(){
throw new FirstException("hello first");
}
}
#Component
class MyWebFilter implements WebFilter {
#Override
public Mono<Void> filter(ServerWebExchange swe, WebFilterChain wfc) {
var path = swe.getRequest().getURI().getPath();
if (path.contains("second")){
throw new SecondException("hello second")
}
}
}
#ControllerAdvice
class GlobalErrorHandlers {
#ExceptionHandler(FirstException::class)
ResponseEntity<String> handleFirst(FirstException ex) {
return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(ex.message)
}
#ExceptionHandler(SecondException::class)
ResponseEntity<String> handleSecond(SecondException ex) {
return ResponseEntity.status(HttpStatus.NOT_FOUND).body(ex.message)
}
}

Answer 1

Three steps are required to get full control over all exceptions thrown from application endpoints handling code:
Implement org.springframework.boot.web.reactive.error.ErrorWebExceptionHandler
Annotate with #ControllerAdvice (or just #Component)
Set #Priority less than 1 to let the custom handler run before the default one (WebFluxResponseStatusExceptionHandler)
The tricky part is where we get an instance implementing
ServerResponse.Context for passing to
ServerResponse.writeTo(exchange, context). I did not find the final
answer, and comments are welcome. In the internal Spring code they always create a new instance of context for each writeTo invocation,
although in all cases (I've manged to find) the context instance is immutable.
That is why I ended up using the same ResponseContextInstance for all responses.
At the moment no problems detected with this approach.
#ControllerAdvice
#Priority(0) /* should go before WebFluxResponseStatusExceptionHandler */
class CustomWebExceptionHandler : ErrorWebExceptionHandler {
private val log = logger(CustomWebExceptionHandler::class)
override fun handle(exchange: ServerWebExchange, ex: Throwable): Mono<Void> {
log.error("handled ${ex.javaClass.simpleName}", ex)
val sr = when (ex) {
is FirstException -> handleFirst(ex)
is SecondException -> handleSecond(ex)
else -> defaultException(ex)
}
return sr.flatMap { it.writeTo(exchange, ResponseContextInstance) }.then()
}
private fun handleFirst(ex: FirstException): Mono<ServerResponse> {
return ServerResponse
.status(HttpStatus.INTERNAL_SERVER_ERROR)
.bodyValue("first")
}
private fun handleSecond(ex: SecondException): Mono<ServerResponse> {
return ServerResponse.status(HttpStatus.BAD_REQUEST).bodyValue("second")
}
private object ResponseContextInstance : ServerResponse.Context {
val strategies: HandlerStrategies = HandlerStrategies.withDefaults()
override fun messageWriters(): List<HttpMessageWriter<*>> {
return strategies.messageWriters()
}
override fun viewResolvers(): List<ViewResolver> {
return strategies.viewResolvers()
}
}
}