Make inner classes static when feasible

A static nested class does not keep an implicit reference to its
enclosing instance.

This prevents a common cause of memory leaks and uses less memory per
instance of the class.

Closes gh-28433

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java

@@ -88,7 +88,7 @@ public class UndertowHttpHandlerAdapter implements io.undertow.server.HttpHandle
 	}
 
 
-	private class HandlerResultSubscriber implements Subscriber<Void> {
+	private static class HandlerResultSubscriber implements Subscriber<Void> {
 
 		private final HttpServerExchange exchange;
 

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

@@ -236,7 +236,7 @@ public class VersionResourceResolver extends AbstractResourceResolver {
 	}
 
 
-	private class FileNameVersionedResource extends AbstractResource implements HttpResource {
+	private static class FileNameVersionedResource extends AbstractResource implements HttpResource {
 
 		private final Resource original;
 

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

@@ -82,7 +82,7 @@ public class UndertowRequestUpgradeStrategy implements RequestUpgradeStrategy {
 	}
 
 
-	private class DefaultCallback implements WebSocketConnectionCallback {
+	private static class DefaultCallback implements WebSocketConnectionCallback {
 
 		private final HandshakeInfo handshakeInfo;
 

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

@@ -232,7 +232,7 @@ public class VersionResourceResolver extends AbstractResourceResolver {
 	}
 
 
-	private class FileNameVersionedResource extends AbstractResource implements HttpResource {
+	private static class FileNameVersionedResource extends AbstractResource implements HttpResource {
 
 		private final Resource original;
 

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

@@ -75,7 +75,7 @@ public class XhrStreamingTransportHandler extends AbstractHttpSendingTransportHa
 	}
 
 
-	private class XhrStreamingSockJsSession extends StreamingSockJsSession {
+	private static class XhrStreamingSockJsSession extends StreamingSockJsSession {
 
 		public XhrStreamingSockJsSession(String sessionId, SockJsServiceConfig config,
 				WebSocketHandler wsHandler, Map<String, Object> attributes) {