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-websocket/src/main/java/org/springframework/web/socket/adapter/jetty/package-info.java

@@ -2,6 +2,8 @@
  * Adapter classes for the Jetty WebSocket API.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.adapter.jetty;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

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

spring-websocket/src/main/java/org/springframework/web/socket/adapter/standard/package-info.java

@@ -2,6 +2,8 @@
  * Adapter classes for the standard Java WebSocket API.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.adapter.standard;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/client/jetty/package-info.java

@@ -2,6 +2,8 @@
  * Client-side support for the Jetty WebSocket API.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.client.jetty;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

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

spring-websocket/src/main/java/org/springframework/web/socket/client/standard/package-info.java

@@ -2,6 +2,8 @@
  * Client-side classes for use with standard Java WebSocket endpoints.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.client.standard;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/config/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Support for annotation-based WebSocket setup in configuration classes.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.config.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/config/package-info.java

@@ -2,6 +2,8 @@
  * Configuration support for WebSocket request handling.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.config;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/handler/package-info.java

@@ -3,6 +3,8 @@
  * implementations and decorators.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.handler;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/messaging/package-info.java

@@ -2,6 +2,8 @@
  * WebSocket integration for Spring's messaging module.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.messaging;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -2,6 +2,8 @@
  * Common abstractions and Spring configuration support for WebSocket applications.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

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

spring-websocket/src/main/java/org/springframework/web/socket/server/standard/package-info.java

@@ -2,6 +2,8 @@
  * Server-side classes for use with standard JSR-356 WebSocket endpoints.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.server.standard;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -3,6 +3,8 @@
  * for upgrading a request.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.server.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/package-info.java

@@ -3,6 +3,8 @@
  * {@link org.springframework.web.socket.client.WebSocketClient}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.client;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/package-info.java

@@ -3,6 +3,8 @@
  * of SockJS message frames.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.frame;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/package-info.java

@@ -2,6 +2,8 @@
  * Top-level SockJS types.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/support/package-info.java

@@ -4,6 +4,8 @@
  * implementation.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/handler/package-info.java

@@ -4,6 +4,8 @@
  * {@link org.springframework.web.socket.sockjs.SockJsService}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.transport.handler;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/package-info.java

@@ -7,6 +7,8 @@
  * {@link org.springframework.web.socket.sockjs.transport.handler.DefaultSockJsService}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.transport;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/package-info.java

@@ -3,6 +3,8 @@
  * {@link org.springframework.web.socket.WebSocketSession}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.socket.sockjs.transport.session;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;