Avoid null signals when resolving handler arguments

Prior to this commit, resolving an argument for a WebFlux controller
that's missing from the request and not required by the handler would
throw a NullPointerException in some cases.

This involves the conversion of the parameter (a `String` parameter type
might not trigger this behavior) and sending a `null` within a reactive
stream, which is illegal per the RS spec.

We now rely on a `Mono.justOrEmpty()` to handle those specific cases.

Issue: SPR-17050

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractNamedValueArgumentResolver.java

@@ -101,13 +101,13 @@ public Mono<Object> resolveArgument(
 		Model model = bindingContext.getModel();
 
 		return resolveName(resolvedName.toString(), nestedParameter, exchange)
-				.map(arg -> {
+				.flatMap(arg -> {
 					if ("".equals(arg) && namedValueInfo.defaultValue != null) {
 						arg = resolveStringValue(namedValueInfo.defaultValue);
 					}
 					arg = applyConversion(arg, namedValueInfo, parameter, bindingContext, exchange);
 					handleResolvedValue(arg, namedValueInfo.name, parameter, model, exchange);
-					return arg;
+					return Mono.justOrEmpty(arg);
 				})
 				.switchIfEmpty(getDefaultValue(
 						namedValueInfo, parameter, bindingContext, model, exchange));

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestParamMethodArgumentResolverTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -133,14 +133,14 @@ public void doesNotSupportReactiveWrapper() {
 	}
 
 	@Test
-	public void resolveWithQueryString() throws Exception {
+	public void resolveWithQueryString() {
 		MethodParameter param = this.testMethod.annot(requestParam().notRequired("bar")).arg(String.class);
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path?name=foo"));
 		assertEquals("foo", resolve(param, exchange));
 	}
 
 	@Test
-	public void resolveStringArray() throws Exception {
+	public void resolveStringArray() {
 		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(String[].class);
 		MockServerHttpRequest request = MockServerHttpRequest.get("/path?name=foo&name=bar").build();
 		Object result = resolve(param, MockServerWebExchange.from(request));
@@ -149,13 +149,21 @@ public void resolveStringArray() throws Exception {
 	}
 
 	@Test
-	public void resolveDefaultValue() throws Exception {
+	public void resolveDefaultValue() {
 		MethodParameter param = this.testMethod.annot(requestParam().notRequired("bar")).arg(String.class);
 		assertEquals("bar", resolve(param, MockServerWebExchange.from(MockServerHttpRequest.get("/"))));
 	}
 
+	@Test // SPR-17050
+	public void resolveAndConvertNullValue() {
+		MethodParameter param = this.testMethod
+				.annot(requestParam().notRequired())
+				.arg(Integer.class);
+		assertNull(resolve(param, MockServerWebExchange.from(MockServerHttpRequest.get("/?nullParam="))));
+	}
+
 	@Test
-	public void missingRequestParam() throws Exception {
+	public void missingRequestParam() {
 
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/"));
 		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(String[].class);
@@ -168,7 +176,7 @@ public void missingRequestParam() throws Exception {
 	}
 
 	@Test
-	public void resolveSimpleTypeParam() throws Exception {
+	public void resolveSimpleTypeParam() {
 		MockServerHttpRequest request = MockServerHttpRequest.get("/path?stringNotAnnot=plainValue").build();
 		ServerWebExchange exchange = MockServerWebExchange.from(request);
 		MethodParameter param = this.testMethod.annotNotPresent(RequestParam.class).arg(String.class);
@@ -177,35 +185,35 @@ public void resolveSimpleTypeParam() throws Exception {
 	}
 
 	@Test  // SPR-8561
-	public void resolveSimpleTypeParamToNull() throws Exception {
+	public void resolveSimpleTypeParamToNull() {
 		MethodParameter param = this.testMethod.annotNotPresent(RequestParam.class).arg(String.class);
 		assertNull(resolve(param, MockServerWebExchange.from(MockServerHttpRequest.get("/"))));
 	}
 
 	@Test  // SPR-10180
-	public void resolveEmptyValueToDefault() throws Exception {
+	public void resolveEmptyValueToDefault() {
 		ServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path?name="));
 		MethodParameter param = this.testMethod.annot(requestParam().notRequired("bar")).arg(String.class);
 		Object result = resolve(param, exchange);
 		assertEquals("bar", result);
 	}
 
 	@Test
-	public void resolveEmptyValueWithoutDefault() throws Exception {
+	public void resolveEmptyValueWithoutDefault() {
 		MethodParameter param = this.testMethod.annotNotPresent(RequestParam.class).arg(String.class);
 		MockServerHttpRequest request = MockServerHttpRequest.get("/path?stringNotAnnot=").build();
 		assertEquals("", resolve(param, MockServerWebExchange.from(request)));
 	}
 
 	@Test
-	public void resolveEmptyValueRequiredWithoutDefault() throws Exception {
+	public void resolveEmptyValueRequiredWithoutDefault() {
 		MethodParameter param = this.testMethod.annot(requestParam()).arg(String.class);
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path?name="));
 		assertEquals("", resolve(param, exchange));
 	}
 
 	@Test
-	public void resolveOptionalParamValue() throws Exception {
+	public void resolveOptionalParamValue() {
 		ServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/"));
 		MethodParameter param = this.testMethod.arg(forClassWithGenerics(Optional.class, Integer.class));
 		Object result = resolve(param, exchange);
@@ -237,7 +245,8 @@ public void handle(
 			@RequestParam("name") String paramRequired,
 			@RequestParam(name = "name", required = false) String paramNotRequired,
 			@RequestParam("name") Optional<Integer> paramOptional,
-			@RequestParam Mono<String> paramMono) {
+			@RequestParam Mono<String> paramMono,
+			@RequestParam(required = false) Integer nullParam) {
 	}
 
 }