Revisit nullability annotations

This commit introduces the following changes.

1) It adds a new Spring @NonNull annotation which allows to apply
@NonNullApi semantic on a specific element, like @Nullable does.
Combined with @Nullable, it allows partial null-safety support when
package granularity is too broad.

2) @Nullable and @NonNull can apply to ElementType.TYPE_USE in order
to be used on generic type arguments (SPR-15942).

3) Annotations does not apply to ElementType.TYPE_PARAMETER anymore
since it is not supported yet (applicability for such use case is
controversial and need to be discussed).

4) @NonNullApi does not apply to ElementType.FIELD anymore since in a
lot of use cases (private, protected) it is not part for the public API
+ its usage should remain opt-in. A dedicated @NonNullFields annotation
has been added in order to set fields default to non-nullable.

5) Updated Javadoc and reference documentation.

Issue: SPR-15756

spring-webflux/src/main/java/org/springframework/web/reactive/accept/package-info.java

@@ -4,6 +4,8 @@
  * given request.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.accept;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/config/package-info.java

@@ -2,6 +2,8 @@
  * Spring WebFlux configuration infrastructure.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.config;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/package-info.java

@@ -4,6 +4,8 @@
  * {@code org.springframework.http.client.reactive} reactive HTTP adapter layer.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.function.client;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/function/package-info.java

@@ -2,6 +2,8 @@
  * Provides a foundation for both the reactive client and server subpackages.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.function;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/package-info.java

@@ -2,6 +2,8 @@
  * Provides the types that make up Spring's functional web framework.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.function.server;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/support/package-info.java

@@ -5,6 +5,8 @@
  * a {@code ServerRequest} wrapper to adapt a request.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.function.server.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/handler/package-info.java

@@ -2,6 +2,8 @@
  * Provides HandlerMapping implementations including abstract base classes.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.handler;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/package-info.java

@@ -10,6 +10,8 @@
  * {@code WebClient} as well as client and server, reactive WebSocket support.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/resource/package-info.java

@@ -2,6 +2,8 @@
  * Support classes for serving static resources.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.resource;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -2,6 +2,8 @@
  * Infrastructure for annotation-based handler method processing.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result.method.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/package-info.java

@@ -2,6 +2,8 @@
  * Infrastructure for handler method processing.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result.method;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/result/package-info.java

@@ -5,6 +5,8 @@
  * resolution, and so on.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/freemarker/package-info.java

@@ -5,6 +5,8 @@
  * Contains a View implementation for FreeMarker templates.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result.view.freemarker;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/package-info.java

@@ -2,6 +2,8 @@
  * Support for result handling through view resolution.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result.view;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/script/package-info.java

@@ -4,6 +4,8 @@
  * Contains a View implementation for scripted templates.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.result.view.script;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/package-info.java

@@ -2,6 +2,8 @@
  * Classes adapting Spring's Reactive WebSocket API to and from WebSocket runtimes.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket.adapter;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/client/package-info.java

@@ -2,6 +2,8 @@
  * Client support for WebSocket interactions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket.client;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/package-info.java

@@ -2,6 +2,8 @@
  * Abstractions and support classes for reactive WebSocket interactions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/package-info.java

@@ -2,6 +2,8 @@
  * Server support for WebSocket interactions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket.server;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/support/package-info.java

@@ -2,6 +2,8 @@
  * Server-side support classes for WebSocket requests.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket.server.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/upgrade/package-info.java

@@ -3,6 +3,8 @@
  * {@link org.springframework.web.reactive.socket.server.RequestUpgradeStrategy}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.socket.server.upgrade;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webflux/src/main/java/org/springframework/web/reactive/support/package-info.java

@@ -2,6 +2,8 @@
  * Support classes for Spring WebFlux setup.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.reactive.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;