Talk about ErrorWebFluxAutoConfiguration of spring boot.

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This article mainly studies the ErrorWebFluxAutoConfiguration of spring boot.

ErrorWebFluxAutoConfiguration

spring-boot-autoconfigure-2.1.5.RELEASE-sources.jar! /org/springframework/boot/autoconfigure/web/reactive/error/ErrorWebFluxAutoConfiguration.java

@Configuration
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
@ConditionalOnClass(WebFluxConfigurer.class)
@AutoConfigureBefore(WebFluxAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, ResourceProperties.class })
public class ErrorWebFluxAutoConfiguration {

    private final ServerProperties serverProperties;

    private final ApplicationContext applicationContext;

    private final ResourceProperties resourceProperties;

    private final List<ViewResolver> viewResolvers;

    private final ServerCodecConfigurer serverCodecConfigurer;

    public ErrorWebFluxAutoConfiguration(ServerProperties serverProperties,
            ResourceProperties resourceProperties,
            ObjectProvider<ViewResolver> viewResolversProvider,
            ServerCodecConfigurer serverCodecConfigurer,
            ApplicationContext applicationContext) {
        this.serverProperties = serverProperties;
        this.applicationContext = applicationContext;
        this.resourceProperties = resourceProperties;
        this.viewResolvers = viewResolversProvider.orderedStream()
                .collect(Collectors.toList());
        this.serverCodecConfigurer = serverCodecConfigurer;
    }

    @Bean
    @ConditionalOnMissingBean(value = ErrorWebExceptionHandler.class,
            search = SearchStrategy.CURRENT)
    @Order(-1)
    public ErrorWebExceptionHandler errorWebExceptionHandler(
            ErrorAttributes errorAttributes) {
        DefaultErrorWebExceptionHandler exceptionHandler = new DefaultErrorWebExceptionHandler(
                errorAttributes, this.resourceProperties,
                this.serverProperties.getError(), this.applicationContext);
        exceptionHandler.setViewResolvers(this.viewResolvers);
        exceptionHandler.setMessageWriters(this.serverCodecConfigurer.getWriters());
        exceptionHandler.setMessageReaders(this.serverCodecConfigurer.getReaders());
        return exceptionHandler;
    }

    @Bean
    @ConditionalOnMissingBean(value = ErrorAttributes.class,
            search = SearchStrategy.CURRENT)
    public DefaultErrorAttributes errorAttributes() {
        return new DefaultErrorAttributes(
                this.serverProperties.getError().isIncludeException());
    }

}
  • ErrorWebFluxAutoConfiguration has registered DefaultErrorAttributes, ErrorWebExceptionHandler

ErrorAttributes

spring-boot-2.1.5.RELEASE-sources.jar! /org/springframework/boot/web/reactive/error/ErrorAttributes.java

public interface ErrorAttributes {

    /**
     * Return a {@link Map} of the error attributes. The map can be used as the model of
     * an error page, or returned as a {@link ServerResponse} body.
     * @param request the source request
     * @param includeStackTrace if stack trace elements should be included
     * @return a map of error attributes
     */
    Map<String, Object> getErrorAttributes(ServerRequest request,
            boolean includeStackTrace);

    /**
     * Return the underlying cause of the error or {@code null} if the error cannot be
     * extracted.
     * @param request the source ServerRequest
     * @return the {@link Exception} that caused the error or {@code null}
     */
    Throwable getError(ServerRequest request);

    /**
     * Store the given error information in the current {@link ServerWebExchange}.
     * @param error the {@link Exception} that caused the error
     * @param exchange the source exchange
     */
    void storeErrorInformation(Throwable error, ServerWebExchange exchange);

}
  • ErrorAttributes interface defines three methods: getErrorAttributes, getError, storeErrorInformation

DefaultErrorAttributes

spring-boot-2.1.5.RELEASE-sources.jar! /org/springframework/boot/web/reactive/error/DefaultErrorAttributes.java

public class DefaultErrorAttributes implements ErrorAttributes {

    private static final String ERROR_ATTRIBUTE = DefaultErrorAttributes.class.getName()
            + ".ERROR";

    private final boolean includeException;

    /**
     * Create a new {@link DefaultErrorAttributes} instance that does not include the
     * "exception" attribute.
     */
    public DefaultErrorAttributes() {
        this(false);
    }

    /**
     * Create a new {@link DefaultErrorAttributes} instance.
     * @param includeException whether to include the "exception" attribute
     */
    public DefaultErrorAttributes(boolean includeException) {
        this.includeException = includeException;
    }

    @Override
    public Map<String, Object> getErrorAttributes(ServerRequest request,
            boolean includeStackTrace) {
        Map<String, Object> errorAttributes = new LinkedHashMap<>();
        errorAttributes.put("timestamp", new Date());
        errorAttributes.put("path", request.path());
        Throwable error = getError(request);
        HttpStatus errorStatus = determineHttpStatus(error);
        errorAttributes.put("status", errorStatus.value());
        errorAttributes.put("error", errorStatus.getReasonPhrase());
        errorAttributes.put("message", determineMessage(error));
        handleException(errorAttributes, determineException(error), includeStackTrace);
        return errorAttributes;
    }

    private HttpStatus determineHttpStatus(Throwable error) {
        if (error instanceof ResponseStatusException) {
            return ((ResponseStatusException) error).getStatus();
        }
        ResponseStatus responseStatus = AnnotatedElementUtils
                .findMergedAnnotation(error.getClass(), ResponseStatus.class);
        if (responseStatus != null) {
            return responseStatus.code();
        }
        return HttpStatus.INTERNAL_SERVER_ERROR;
    }

    private String determineMessage(Throwable error) {
        if (error instanceof WebExchangeBindException) {
            return error.getMessage();
        }
        if (error instanceof ResponseStatusException) {
            return ((ResponseStatusException) error).getReason();
        }
        ResponseStatus responseStatus = AnnotatedElementUtils
                .findMergedAnnotation(error.getClass(), ResponseStatus.class);
        if (responseStatus != null) {
            return responseStatus.reason();
        }
        return error.getMessage();
    }

    private Throwable determineException(Throwable error) {
        if (error instanceof ResponseStatusException) {
            return (error.getCause() != null) ? error.getCause() : error;
        }
        return error;
    }

    private void addStackTrace(Map<String, Object> errorAttributes, Throwable error) {
        StringWriter stackTrace = new StringWriter();
        error.printStackTrace(new PrintWriter(stackTrace));
        stackTrace.flush();
        errorAttributes.put("trace", stackTrace.toString());
    }

    private void handleException(Map<String, Object> errorAttributes, Throwable error,
            boolean includeStackTrace) {
        if (this.includeException) {
            errorAttributes.put("exception", error.getClass().getName());
        }
        if (includeStackTrace) {
            addStackTrace(errorAttributes, error);
        }
        if (error instanceof BindingResult) {
            BindingResult result = (BindingResult) error;
            if (result.hasErrors()) {
                errorAttributes.put("errors", result.getAllErrors());
            }
        }
    }

    @Override
    public Throwable getError(ServerRequest request) {
        return (Throwable) request.attribute(ERROR_ATTRIBUTE)
                .orElseThrow(() -> new IllegalStateException(
                        "Missing exception attribute in ServerWebExchange"));
    }

    @Override
    public void storeErrorInformation(Throwable error, ServerWebExchange exchange) {
        exchange.getAttributes().putIfAbsent(ERROR_ATTRIBUTE, error);
    }

}
  • DefaulterrorAttributes implements the ErrorAttributes interface, and its getErrorAttributes method returns timestamp, path, status, Error, message, exception (includeException)、trace(includeStackTrace) and other information; The getError method gets Throwable; from the ERROR_ATTRIBUTE of ServerRequest; StoreerrorInfo stores Throwable in attributes on ServerWebExchange.

WebExceptionHandler

spring-web-5.1.7.RELEASE-sources.jar! /org/springframework/web/server/WebExceptionHandler.java

public interface WebExceptionHandler {

    /**
     * Handle the given exception. A completion signal through the return value
     * indicates error handling is complete while an error signal indicates the
     * exception is still not handled.
     * @param exchange the current exchange
     * @param ex the exception to handle
     * @return {@code Mono<Void>} to indicate when exception handling is complete
     */
    Mono<Void> handle(ServerWebExchange exchange, Throwable ex);

}
  • WebExceptionHandler defines the handle method

ErrorWebExceptionHandler

spring-boot-2.1.5.RELEASE-sources.jar! /org/springframework/boot/web/reactive/error/ErrorWebExceptionHandler.java

@FunctionalInterface
public interface ErrorWebExceptionHandler extends WebExceptionHandler {

}
  • ErrorWebExceptionHandler inherits the WebExceptionHandler interface and only uses the class name to identify it for render errors.

AbstractErrorWebExceptionHandler

public abstract class AbstractErrorWebExceptionHandler
        implements ErrorWebExceptionHandler, InitializingBean {

    /**
     * Currently duplicated from Spring WebFlux HttpWebHandlerAdapter.
     */
    private static final Set<String> DISCONNECTED_CLIENT_EXCEPTIONS;
    static {
        Set<String> exceptions = new HashSet<>();
        exceptions.add("AbortedException");
        exceptions.add("ClientAbortException");
        exceptions.add("EOFException");
        exceptions.add("EofException");
        DISCONNECTED_CLIENT_EXCEPTIONS = Collections.unmodifiableSet(exceptions);
    }

    private static final Log logger = HttpLogging
            .forLogName(AbstractErrorWebExceptionHandler.class);

    private final ApplicationContext applicationContext;

    private final ErrorAttributes errorAttributes;

    private final ResourceProperties resourceProperties;

    private final TemplateAvailabilityProviders templateAvailabilityProviders;

    private List<HttpMessageReader<?>> messageReaders = Collections.emptyList();

    private List<HttpMessageWriter<?>> messageWriters = Collections.emptyList();

    private List<ViewResolver> viewResolvers = Collections.emptyList();

    public AbstractErrorWebExceptionHandler(ErrorAttributes errorAttributes,
            ResourceProperties resourceProperties,
            ApplicationContext applicationContext) {
        Assert.notNull(errorAttributes, "ErrorAttributes must not be null");
        Assert.notNull(resourceProperties, "ResourceProperties must not be null");
        Assert.notNull(applicationContext, "ApplicationContext must not be null");
        this.errorAttributes = errorAttributes;
        this.resourceProperties = resourceProperties;
        this.applicationContext = applicationContext;
        this.templateAvailabilityProviders = new TemplateAvailabilityProviders(
                applicationContext);
    }

    //......

    @Override
    public void afterPropertiesSet() throws Exception {
        if (CollectionUtils.isEmpty(this.messageWriters)) {
            throw new IllegalArgumentException("Property 'messageWriters' is required");
        }
    }

    /**
     * Create a {@link RouterFunction} that can route and handle errors as JSON responses
     * or HTML views.
     * <p>
     * If the returned {@link RouterFunction} doesn't route to a {@code HandlerFunction},
     * the original exception is propagated in the pipeline and can be processed by other
     * {@link org.springframework.web.server.WebExceptionHandler}s.
     * @param errorAttributes the {@code ErrorAttributes} instance to use to extract error
     * information
     * @return a {@link RouterFunction} that routes and handles errors
     */
    protected abstract RouterFunction<ServerResponse> getRoutingFunction(
            ErrorAttributes errorAttributes);

    @Override
    public Mono<Void> handle(ServerWebExchange exchange, Throwable throwable) {
        if (exchange.getResponse().isCommitted()
                || isDisconnectedClientError(throwable)) {
            return Mono.error(throwable);
        }
        this.errorAttributes.storeErrorInformation(throwable, exchange);
        ServerRequest request = ServerRequest.create(exchange, this.messageReaders);
        return getRoutingFunction(this.errorAttributes).route(request)
                .switchIfEmpty(Mono.error(throwable))
                .flatMap((handler) -> handler.handle(request))
                .doOnNext((response) -> logError(request, response, throwable))
                .flatMap((response) -> write(exchange, response));
    }

    //......

}
  • AbstractErrorWebExceptionHandler declaration implements errorwebexceptionhandler and InitializingBean interfaces; Its handle method first stores throwable into errorAttributes for summary, and then route; through getRoutingFunction; AfterPropertiesSet mainly ensures that messageWriters are not empty; It defines the subclass of getRoutingFunction to implement

DefaultErrorWebExceptionHandler

spring-boot-autoconfigure-2.1.5.RELEASE-sources.jar! /org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandler.java

public class DefaultErrorWebExceptionHandler extends AbstractErrorWebExceptionHandler {

    private static final Map<HttpStatus.Series, String> SERIES_VIEWS;

    static {
        Map<HttpStatus.Series, String> views = new EnumMap<>(HttpStatus.Series.class);
        views.put(HttpStatus.Series.CLIENT_ERROR, "4xx");
        views.put(HttpStatus.Series.SERVER_ERROR, "5xx");
        SERIES_VIEWS = Collections.unmodifiableMap(views);
    }

    private final ErrorProperties errorProperties;

    /**
     * Create a new {@code DefaultErrorWebExceptionHandler} instance.
     * @param errorAttributes the error attributes
     * @param resourceProperties the resources configuration properties
     * @param errorProperties the error configuration properties
     * @param applicationContext the current application context
     */
    public DefaultErrorWebExceptionHandler(ErrorAttributes errorAttributes,
            ResourceProperties resourceProperties, ErrorProperties errorProperties,
            ApplicationContext applicationContext) {
        super(errorAttributes, resourceProperties, applicationContext);
        this.errorProperties = errorProperties;
    }

    @Override
    protected RouterFunction<ServerResponse> getRoutingFunction(
            ErrorAttributes errorAttributes) {
        return route(acceptsTextHtml(), this::renderErrorView).andRoute(all(),
                this::renderErrorResponse);
    }

    /**
     * Render the error information as an HTML view.
     * @param request the current request
     * @return a {@code Publisher} of the HTTP response
     */
    protected Mono<ServerResponse> renderErrorView(ServerRequest request) {
        boolean includeStackTrace = isIncludeStackTrace(request, MediaType.TEXT_HTML);
        Map<String, Object> error = getErrorAttributes(request, includeStackTrace);
        HttpStatus errorStatus = getHttpStatus(error);
        ServerResponse.BodyBuilder responseBody = ServerResponse.status(errorStatus)
                .contentType(MediaType.TEXT_HTML);
        return Flux
                .just("error/" + errorStatus.value(),
                        "error/" + SERIES_VIEWS.get(errorStatus.series()), "error/error")
                .flatMap((viewName) -> renderErrorView(viewName, responseBody, error))
                .switchIfEmpty(this.errorProperties.getWhitelabel().isEnabled()
                        ? renderDefaultErrorView(responseBody, error)
                        : Mono.error(getError(request)))
                .next();
    }

    /**
     * Render the error information as a JSON payload.
     * @param request the current request
     * @return a {@code Publisher} of the HTTP response
     */
    protected Mono<ServerResponse> renderErrorResponse(ServerRequest request) {
        boolean includeStackTrace = isIncludeStackTrace(request, MediaType.ALL);
        Map<String, Object> error = getErrorAttributes(request, includeStackTrace);
        return ServerResponse.status(getHttpStatus(error))
                .contentType(MediaType.APPLICATION_JSON_UTF8)
                .body(BodyInserters.fromObject(error));
    }

    /**
     * Determine if the stacktrace attribute should be included.
     * @param request the source request
     * @param produces the media type produced (or {@code MediaType.ALL})
     * @return if the stacktrace attribute should be included
     */
    protected boolean isIncludeStackTrace(ServerRequest request, MediaType produces) {
        ErrorProperties.IncludeStacktrace include = this.errorProperties
                .getIncludeStacktrace();
        if (include == ErrorProperties.IncludeStacktrace.ALWAYS) {
            return true;
        }
        if (include == ErrorProperties.IncludeStacktrace.ON_TRACE_PARAM) {
            return isTraceEnabled(request);
        }
        return false;
    }

    /**
     * Get the HTTP error status information from the error map.
     * @param errorAttributes the current error information
     * @return the error HTTP status
     */
    protected HttpStatus getHttpStatus(Map<String, Object> errorAttributes) {
        int statusCode = (int) errorAttributes.get("status");
        return HttpStatus.valueOf(statusCode);
    }

    /**
     * Predicate that checks whether the current request explicitly support
     * {@code "text/html"} media type.
     * <p>
     * The "match-all" media type is not considered here.
     * @return the request predicate
     */
    protected RequestPredicate acceptsTextHtml() {
        return (serverRequest) -> {
            try {
                List<MediaType> acceptedMediaTypes = serverRequest.headers().accept();
                acceptedMediaTypes.remove(MediaType.ALL);
                MediaType.sortBySpecificityAndQuality(acceptedMediaTypes);
                return acceptedMediaTypes.stream()
                        .anyMatch(MediaType.TEXT_HTML::isCompatibleWith);
            }
            catch (InvalidMediaTypeException ex) {
                return false;
            }
        };
    }

}
  • Defaulterrorwebexceptionhandler inherits abstracterrorwebexceptionhandler; Its getRoutingFunction method returns json-formatted error messages to renderErrorView of acceptsTextHtml and others via renderErrorResponse

ExceptionHandlingWebHandler

spring-web-5.1.7.RELEASE-sources.jar! /org/springframework/web/server/handler/ExceptionHandlingWebHandler.java

public class ExceptionHandlingWebHandler extends WebHandlerDecorator {


    private final List<WebExceptionHandler> exceptionHandlers;


    public ExceptionHandlingWebHandler(WebHandler delegate, List<WebExceptionHandler> handlers) {
        super(delegate);
        this.exceptionHandlers = Collections.unmodifiableList(new ArrayList<>(handlers));
    }


    /**
     * Return a read-only list of the configured exception handlers.
     */
    public List<WebExceptionHandler> getExceptionHandlers() {
        return this.exceptionHandlers;
    }


    @Override
    public Mono<Void> handle(ServerWebExchange exchange) {

        Mono<Void> completion;
        try {
            completion = super.handle(exchange);
        }
        catch (Throwable ex) {
            completion = Mono.error(ex);
        }

        for (WebExceptionHandler handler : this.exceptionHandlers) {
            completion = completion.onErrorResume(ex -> handler.handle(exchange, ex));
        }

        return completion;
    }

}
  • ExceptionHandlingWebHandler inherits WebHandlerDecorator and calls WebExceptionHandler’s handle method one by one.

Summary

  • ErrorWebFluxAutoConfiguration has registered DefaultErrorAttributes, ErrorWebExceptionHandler
  • DefaulterrorAttributes implements the ErrorAttributes interface, and its getErrorAttributes method returns timestamp, path, status, Error, message, exception (includeException)、trace(includeStackTrace) and other information; The getError method gets Throwable; from the ERROR_ATTRIBUTE of ServerRequest; StoreerrorInfo stores Throwable in attributes on ServerWebExchange.
  • Defaulterrorwebexceptionhandler inherits abstracterrorwebexceptionhandler; Its getRoutingFunction method returns json-formatted error messages to renderErrorView of acceptsTextHtml and others via renderErrorResponse

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