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-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Annotation-based setup for Spring MVC.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.config.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/config/package-info.java

@@ -2,6 +2,8 @@
  * Defines the XML configuration namespace for Spring MVC.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.config;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/handler/package-info.java

@@ -3,6 +3,8 @@
  * including abstract base classes for custom implementations.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.handler;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/i18n/package-info.java

@@ -4,6 +4,8 @@
  * and a HandlerInterceptor for locale changes.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.i18n;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Support package for annotation-based Servlet MVC controllers.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/condition/package-info.java

@@ -2,6 +2,8 @@
  * Common MVC logic for matching incoming requests based on conditions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc.condition;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/package-info.java

@@ -5,6 +5,8 @@
  * and {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc.method.annotation;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/package-info.java

@@ -3,6 +3,8 @@
  * building on the {@code org.springframework.web.method} package.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc.method;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/package-info.java

@@ -3,6 +3,8 @@
  * Spring. Provides various controller styles, including an annotation-based model.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/support/package-info.java

@@ -3,6 +3,8 @@
  * Contains a special HandlerMapping for controller conventions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.mvc.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/package-info.java

@@ -8,6 +8,8 @@
  * by Rod Johnson (Wrox, 2002).
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/package-info.java

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

spring-webmvc/src/main/java/org/springframework/web/servlet/support/package-info.java

@@ -4,6 +4,8 @@
  * and miscellaneous HandlerInterceptor implementations.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/tags/package-info.java

@@ -4,6 +4,8 @@
  * See {@code spring.tld} for descriptions of the various tags.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.tags;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/theme/package-info.java

@@ -18,6 +18,8 @@
  * </ul>
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.theme;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/document/package-info.java

@@ -3,6 +3,8 @@
  * providing View implementations for PDF and Excel.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.document;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/feed/package-info.java

@@ -3,6 +3,8 @@
  * Based on the <a href="https://github.com/rometools/rome">ROME tools</a> project.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.feed;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/freemarker/package-info.java

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

spring-webmvc/src/main/java/org/springframework/web/servlet/view/groovy/package-info.java

@@ -5,6 +5,8 @@
  * Contains a View implementation for Groovy templates.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.groovy;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/json/package-info.java

@@ -2,6 +2,8 @@
  * Support classes for providing a View implementation based on JSON serialization.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.json;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/package-info.java

@@ -9,6 +9,8 @@
  * very helpful if an application has unusual view requirements.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/script/package-info.java

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

spring-webmvc/src/main/java/org/springframework/web/servlet/view/tiles3/package-info.java

@@ -5,6 +5,8 @@
  * Contains a View implementation for Tiles definitions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.tiles3;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/xml/package-info.java

@@ -2,6 +2,8 @@
  * Support classes for providing a View implementation based on XML Marshalling.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.xml;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-webmvc/src/main/java/org/springframework/web/servlet/view/xslt/package-info.java

@@ -3,6 +3,8 @@
  * providing a View implementation for XSLT stylesheets.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.web.servlet.view.xslt;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;