Don't wrap resolver exceptions in InvocableHandlerMethod

Prior to this commit, exceptions thrown by the
`HandlerMethodArgumentResolver` would be wrapped into
`IllegalStateException`s.

This commit makes sure that a DEBUG log is written with the relevant
information and that the root cause is not wrapped into another
exception, so that the appropriate `ExceptionHandler` can be used to
deal with this error.

Issue: SPR-14618

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -129,7 +129,11 @@ public class InvocableHandlerMethod extends HandlerMethod {
 						try {
 							return resolver.resolveArgument(param, model, exchange)
 									.defaultIfEmpty(NO_VALUE)
-									.otherwise(ex -> Mono.error(getArgError("Error resolving ", param, ex)))
+									.doOnError(cause -> {
+										if(logger.isDebugEnabled()) {
+											logger.debug(getDetailedErrorMessage("Error resolving ", param), cause);
+										}
+									})
 									.log("reactor.unresolved");
 						}
 						catch (Exception ex) {
@@ -148,10 +152,15 @@ public class InvocableHandlerMethod extends HandlerMethod {
 	}
 
 	private IllegalStateException getArgError(String message, MethodParameter param, Throwable cause) {
-		return new IllegalStateException(message +
-				"argument [" + param.getParameterIndex() + "] " +
-				"of type [" + param.getParameterType().getName() + "] " +
-				"on method [" + getBridgedMethod().toGenericString() + "]", cause);
+		return new IllegalStateException(getDetailedErrorMessage(message, param), cause);
+	}
+
+	private String getDetailedErrorMessage(String message, MethodParameter param) {
+		StringBuilder sb = new StringBuilder(message);
+		sb.append("argument [" + param.getParameterIndex() + "] ");
+		sb.append("of type [" + param.getParameterType().getName() + "] ");
+		sb.append("on method [" + getBridgedMethod().toGenericString() + "]");
+		return sb.toString();
 	}
 
 	private Object doInvoke(Object[] args) throws Exception {

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/InvocableHandlerMethodTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@ import org.springframework.ui.ModelMap;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.result.ResolvableMethod;
 import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.UnsupportedMediaTypeStatusException;
 import org.springframework.web.server.adapter.DefaultServerWebExchange;
 import org.springframework.web.server.session.MockWebSessionManager;
 
@@ -102,25 +103,12 @@ public class InvocableHandlerMethodTests {
 	@Test
 	public void resolverThrowsException() throws Exception {
 		InvocableHandlerMethod hm = handlerMethod("singleArg");
-		addResolver(hm, Mono.error(new IllegalStateException("boo")));
+		addResolver(hm, Mono.error(new UnsupportedMediaTypeStatusException("boo")));
 		Mono<HandlerResult> mono = hm.invokeForRequest(this.exchange, this.model);
 
 		TestSubscriber.subscribe(mono)
-				.assertError(IllegalStateException.class)
-				.assertErrorMessage("Error resolving argument [0] of type [java.lang.String] " +
-						"on method [" + hm.getMethod().toGenericString() + "]");
-	}
-
-	@Test
-	public void resolverWithErrorSignal() throws Exception {
-		InvocableHandlerMethod hm = handlerMethod("singleArg");
-		addResolver(hm, Mono.error(new IllegalStateException("boo")));
-		Mono<HandlerResult> mono = hm.invokeForRequest(this.exchange, this.model);
-
-		TestSubscriber.subscribe(mono)
-				.assertError(IllegalStateException.class)
-				.assertErrorMessage("Error resolving argument [0] of type [java.lang.String] " +
-						"on method [" + hm.getMethod().toGenericString() + "]");
+				.assertError(UnsupportedMediaTypeStatusException.class)
+				.assertErrorMessage("Request failure [status: 415, reason: \"boo\"]");
 	}
 
 	@Test