Add DispatcherHandlerErrorTests

The tests demonstrate failures at various phases of request processing
and the resulting error signals.

spring-web-reactive/src/main/java/org/springframework/core/codec/support/AbstractDecoder.java

@@ -44,6 +44,9 @@ public abstract class AbstractDecoder<T> implements Decoder<T> {
 
 	@Override
 	public boolean canDecode(ResolvableType type, MimeType mimeType, Object... hints) {
+		if (mimeType == null) {
+			return true;
+		}
 		for (MimeType supportedMimeType : this.supportedMimeTypes) {
 			if (supportedMimeType.isCompatibleWith(mimeType)) {
 				return true;

spring-web-reactive/src/main/java/org/springframework/core/codec/support/AbstractEncoder.java

@@ -44,6 +44,9 @@ public abstract class AbstractEncoder<T> implements Encoder<T> {
 
 	@Override
 	public boolean canEncode(ResolvableType type, MimeType mimeType, Object... hints) {
+		if (mimeType == null) {
+			return true;
+		}
 		for (MimeType supportedMimeType : this.supportedMimeTypes) {
 			if (supportedMimeType.isCompatibleWith(mimeType)) {
 				return true;

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

@@ -128,7 +128,7 @@ public class DispatcherHandler implements HttpHandler, ApplicationContextAware {
 				return resultHandler;
 			}
 		}
-		throw new IllegalStateException("No HandlerResultHandler: " + handlerResult.getValue());
+		throw new IllegalStateException("No HandlerResultHandler for " + handlerResult.getValue());
 	}
 
 

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

@@ -26,6 +26,7 @@ import reactor.Publishers;
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.core.codec.support.ByteBufferDecoder;
@@ -33,6 +34,7 @@ import org.springframework.core.codec.Decoder;
 import org.springframework.core.codec.support.JacksonJsonDecoder;
 import org.springframework.core.codec.support.JsonObjectDecoder;
 import org.springframework.core.codec.support.StringDecoder;
+import org.springframework.util.ObjectUtils;
 import org.springframework.web.reactive.HandlerAdapter;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.method.HandlerMethodArgumentResolver;
@@ -45,9 +47,9 @@ import org.springframework.web.method.HandlerMethod;
  */
 public class RequestMappingHandlerAdapter implements HandlerAdapter, InitializingBean {
 
-	private List<HandlerMethodArgumentResolver> argumentResolvers;
+	private final List<HandlerMethodArgumentResolver> argumentResolvers = new ArrayList<>();
 
-	private ConversionService conversionService;
+	private ConversionService conversionService = new DefaultConversionService();
 
 
 	public void setArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
@@ -70,12 +72,11 @@ public class RequestMappingHandlerAdapter implements HandlerAdapter, Initializin
 
 	@Override
 	public void afterPropertiesSet() throws Exception {
-		if (this.argumentResolvers == null) {
+		if (ObjectUtils.isEmpty(this.argumentResolvers)) {
 
 			List<Decoder<?>> decoders = Arrays.asList(new ByteBufferDecoder(),
 					new StringDecoder(), new JacksonJsonDecoder(new JsonObjectDecoder()));
 
-			this.argumentResolvers = new ArrayList<>();
 			this.argumentResolvers.add(new RequestParamArgumentResolver());
 			this.argumentResolvers.add(new RequestBodyArgumentResolver(decoders, this.conversionService));
 		}

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

@@ -135,10 +135,20 @@ public class ResponseBodyResultHandler implements HandlerResultHandler, Ordered
 			return Publishers.empty();
 		}
 
+		Publisher<?> publisher;
+		ResolvableType elementType;
 		ResolvableType returnType = result.getValueType();
+		if (this.conversionService.canConvert(returnType.getRawClass(), Publisher.class)) {
+			publisher = this.conversionService.convert(value, Publisher.class);
+			elementType = returnType.getGeneric(0);
+		}
+		else {
+			publisher = Publishers.just(value);
+			elementType = returnType;
+		}
 
 		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
-		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(returnType);
+		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(elementType);
 
 		if (producibleMediaTypes.isEmpty()) {
 			producibleMediaTypes.add(MediaType.ALL);
@@ -172,16 +182,6 @@ public class ResponseBodyResultHandler implements HandlerResultHandler, Ordered
 		}
 
 		if (selectedMediaType != null) {
-			Publisher<?> publisher;
-			ResolvableType elementType;
-			if (this.conversionService.canConvert(returnType.getRawClass(), Publisher.class)) {
-				publisher = this.conversionService.convert(value, Publisher.class);
-				elementType = returnType.getGeneric(0);
-			}
-			else {
-				publisher = Publishers.just(value);
-				elementType = returnType;
-			}
 			Encoder<?> encoder = resolveEncoder(elementType, selectedMediaType);
 			if (encoder != null) {
 				response.getHeaders().setContentType(selectedMediaType);