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-messaging/src/main/java/org/springframework/messaging/converter/package-info.java

@@ -2,6 +2,8 @@
  * Provides support for message conversion.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.converter;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/core/package-info.java

@@ -2,6 +2,8 @@
  * Defines interfaces and implementation classes for messaging templates.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.core;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Annotations and support classes for handling messages.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.handler.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/package-info.java

@@ -2,6 +2,8 @@
  * Common infrastructure for invoking message handler methods.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.handler.invocation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/handler/package-info.java

@@ -2,6 +2,8 @@
  * Basic abstractions for working with message handler methods.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.handler;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -2,6 +2,8 @@
  * Support for working with messaging APIs and protocols.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Annotations and for handling messages from SImple Messaging Protocols such as STOMP.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/annotation/support/package-info.java

@@ -3,6 +3,8 @@
  * (like STOMP).
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.annotation.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/package-info.java

@@ -3,6 +3,8 @@
  * class and other supporting types such as a registry for subscriptions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.broker;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/config/package-info.java

@@ -2,6 +2,8 @@
  * Configuration support for WebSocket messaging using higher level messaging protocols.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.config;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/package-info.java

@@ -2,6 +2,8 @@
  * Generic support for SImple Messaging Protocols including protocols such as STOMP.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/package-info.java

@@ -2,6 +2,8 @@
  * Generic support for simple messaging protocols (like STOMP).
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.stomp;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/simp/user/package-info.java

@@ -7,6 +7,8 @@
  * for keeping track of connected user sessions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.simp.user;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/support/package-info.java

@@ -5,6 +5,8 @@
  * implementations and channel interceptor support.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/tcp/package-info.java

@@ -7,6 +7,8 @@
  * {@link org.springframework.messaging.tcp.TcpConnection TcpConnection}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.tcp;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-messaging/src/main/java/org/springframework/messaging/tcp/reactor/package-info.java

@@ -2,6 +2,8 @@
  * Contains support for TCP messaging based on Reactor.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.messaging.tcp.reactor;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;