Refine exception handling

Refine exception handling responsibilities between the
DispatcherHandler and the HandlerAdapter.

spring-web-reactive/src/main/java/org/springframework/web/reactive/DispatcherHandler.java

@@ -117,35 +117,29 @@ public class DispatcherHandler implements HttpHandler, ApplicationContextAware {
 			logger.debug("Processing " + request.getMethod() + " request for [" + request.getURI() + "]");
 		}
 		return Flux.fromIterable(this.handlerMappings)
-				.concatMap(m -> m.getHandler(request))
+				.concatMap(mapping -> mapping.getHandler(request))
 				.next()
-				.then(handler -> getHandlerAdapter(handler).handle(request, response, handler))
-				.then(result -> {
-					Mono<Void> mono = (result.hasError() ? Mono.error(result.getError()) :
-							handleResult(request, response, result));
-					if (result.hasExceptionMapper()) {
-						return mono.otherwise(ex -> result.getExceptionMapper().apply(ex)
-								.then(exResult -> handleResult(request, response, exResult)));
-					}
-					return mono;
-				})
+				.then(handler -> invokeHandler(request, response, handler))
+				.then(result -> handleResult(request, response, result))
 				.otherwise(ex -> Mono.error(this.errorMapper.apply(ex)));
 	}
 
-	protected HandlerAdapter getHandlerAdapter(Object handler) {
+	private Mono<HandlerResult> invokeHandler(ServerHttpRequest request, ServerHttpResponse response, Object handler) {
 		for (HandlerAdapter handlerAdapter : this.handlerAdapters) {
 			if (handlerAdapter.supports(handler)) {
-				return handlerAdapter;
+				return handlerAdapter.handle(request, response, handler);
 			}
 		}
-		throw new IllegalStateException("No HandlerAdapter: " + handler);
+		return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
 	}
 
-	protected Mono<Void> handleResult(ServerHttpRequest request, ServerHttpResponse response, HandlerResult result) {
-		return getResultHandler(result).handleResult(request, response, result);
+	private Mono<Void> handleResult(ServerHttpRequest request, ServerHttpResponse response, HandlerResult result) {
+		return getResultHandler(result).handleResult(request, response, result)
+				.otherwise(ex -> result.applyExceptionHandler(ex).then(exceptionResult ->
+						getResultHandler(result).handleResult(request, response, exceptionResult)));
 	}
 
-	protected HandlerResultHandler getResultHandler(HandlerResult handlerResult) {
+	private HandlerResultHandler getResultHandler(HandlerResult handlerResult) {
 		for (HandlerResultHandler resultHandler : resultHandlers) {
 			if (resultHandler.supports(handlerResult)) {
 				return resultHandler;

spring-web-reactive/src/main/java/org/springframework/web/reactive/HandlerAdapter.java

@@ -16,17 +16,16 @@
 
 package org.springframework.web.reactive;
 
-import org.reactivestreams.Publisher;
+import java.util.function.Function;
+
 import reactor.Mono;
 
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
 
 /**
- * Interface that must be implemented for each handler type to handle an HTTP request.
- * This interface is used to allow the {@link DispatcherHandler} to be indefinitely
- * extensible. The {@code DispatcherHandler} accesses all installed handlers through
- * this interface, meaning that it does not contain code specific to any handler type.
+ * Contract that decouples the {@link DispatcherHandler} from the details of
+ * invoking a handler and makes it possible to support any handler type.
  *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
@@ -34,26 +33,32 @@ import org.springframework.http.server.reactive.ServerHttpResponse;
 public interface HandlerAdapter {
 
 	/**
-	 * Given a handler instance, return whether or not this {@code HandlerAdapter}
-	 * can support it. Typical HandlerAdapters will base the decision on the handler
-	 * type. HandlerAdapters will usually only support one handler type each.
-	 * <p>A typical implementation:
-	 * <p>{@code
-	 * return (handler instanceof MyHandler);
-	 * }
+	 * Whether this {@code HandlerAdapter} supports the given {@code handler}.
+	 *
 	 * @param handler handler object to check
-	 * @return whether or not this object can use the given handler
+	 * @return whether or not the handler is supported
 	 */
 	boolean supports(Object handler);
 
 	/**
-	 * Use the given handler to handle this request.
-	 * @param request current HTTP request
-	 * @param response current HTTP response
-	 * @param handler handler to use. This object must have previously been passed
-	 * to the {@code supports} method of this interface, which must have
-	 * returned {@code true}.
-	 * @return A {@link Mono} that emits a single {@link HandlerResult} element
+	 * Handle the request with the given handler.
+	 *
+	 * <p>Implementations are encouraged to handle exceptions resulting from the
+	 * invocation of a handler in order and if necessary to return an alternate
+	 * result that represents an error response.
+	 *
+	 * <p>Furthermore since an async {@code HandlerResult} may produce an error
+	 * later during result handling implementations are also encouraged to
+	 * {@link HandlerResult#setExceptionHandler(Function) set an exception
+	 * handler} on the {@code HandlerResult} so that may also be applied later
+	 * after result handling.
+	 *
+	 * @param request current request
+	 * @param response current response
+	 * @param handler the selected handler which must have been previously
+	 * checked via {@link #supports(Object)}
+	 * @return {@link Mono} that emits a single {@code HandlerResult} or none if
+	 * the request has been fully handled and doesn't require further handling.
 	 */
 	Mono<HandlerResult> handle(ServerHttpRequest request, ServerHttpResponse response,
 			Object handler);

spring-web-reactive/src/main/java/org/springframework/web/reactive/HandlerResult.java

@@ -36,9 +36,7 @@ public class HandlerResult {
 
 	private final ResolvableType resultType;
 
-	private final Throwable error;
-
-	private Function<Throwable, Mono<HandlerResult>> exceptionMapper;
+	private Function<Throwable, Mono<HandlerResult>> exceptionHandler;
 
 
 	public HandlerResult(Object handler, Object result, ResolvableType resultType) {
@@ -47,16 +45,6 @@ public class HandlerResult {
 		this.handler = handler;
 		this.result = result;
 		this.resultType = resultType;
-		this.error = null;
-	}
-
-	public HandlerResult(Object handler, Throwable error) {
-		Assert.notNull(handler, "'handler' is required");
-		Assert.notNull(error, "'error' is required");
-		this.handler = handler;
-		this.result = null;
-		this.resultType = null;
-		this.error = error;
 	}
 
 
@@ -72,38 +60,25 @@ public class HandlerResult {
 		return this.resultType;
 	}
 
-	public Throwable getError() {
-		return this.error;
-	}
-
-	/**
-	 * Whether handler invocation produced a result or failed with an error.
-	 * <p>If {@code true} the {@link #getError()} returns the error while
-	 * {@link #getResult()} and {@link #getResultType()} return {@code null}
-	 * and vice versa.
-	 * @return whether this instance contains a result or an error.
-	 */
-	public boolean hasError() {
-		return (this.error != null);
-	}
-
 	/**
-	 * Configure a function for selecting an alternate {@code HandlerResult} in
-	 * case of an {@link #hasError() error result} or in case of an async result
-	 * that results in an error.
-	 * @param function the exception resolving function
+	 * For an async result, failures may occur later during result handling.
+	 * Use this property to configure an exception handler to be invoked if
+	 * result handling fails.
+	 *
+	 * @param function a function to map the the error to an alternative result.
+	 * @return the current instance
 	 */
-	public HandlerResult setExceptionMapper(Function<Throwable, Mono<HandlerResult>> function) {
-		this.exceptionMapper = function;
+	public HandlerResult setExceptionHandler(Function<Throwable, Mono<HandlerResult>> function) {
+		this.exceptionHandler = function;
 		return this;
 	}
 
-	public Function<Throwable, Mono<HandlerResult>> getExceptionMapper() {
-		return this.exceptionMapper;
+	public boolean hasExceptionHandler() {
+		return (this.exceptionHandler != null);
 	}
 
-	public boolean hasExceptionMapper() {
-		return (this.exceptionMapper != null);
+	public Mono<HandlerResult> applyExceptionHandler(Throwable ex) {
+		return (hasExceptionHandler() ? this.exceptionHandler.apply(ex) : Mono.error(ex));
 	}
 
 }

spring-web-reactive/src/main/java/org/springframework/web/reactive/method/annotation/RequestMappingHandlerAdapter.java

@@ -109,17 +109,15 @@ public class RequestMappingHandlerAdapter implements HandlerAdapter, Initializin
 			Object handler) {
 
 		HandlerMethod handlerMethod = (HandlerMethod) handler;
-
 		InvocableHandlerMethod invocable = new InvocableHandlerMethod(handlerMethod);
 		invocable.setHandlerMethodArgumentResolvers(this.argumentResolvers);
 
 		return invocable.invokeForRequest(request)
-				.otherwise(ex -> Mono.just(new HandlerResult(handler, ex)))
-				.map(result -> result.setExceptionMapper(
-						ex -> mapException(ex, handlerMethod, request, response)));
+				.map(result -> result.setExceptionHandler(ex -> handleException(ex, handlerMethod, request, response)))
+				.otherwise(ex -> handleException(ex, handlerMethod, request, response));
 	}
 
-	private Mono<HandlerResult> mapException(Throwable ex, HandlerMethod handlerMethod,
+	private Mono<HandlerResult> handleException(Throwable ex, HandlerMethod handlerMethod,
 			ServerHttpRequest request, ServerHttpResponse response) {
 
 		if (ex instanceof Exception) {