add some javadoc and cleanup to Request.get

Signed-off-by: Lachlan Roberts <lachlan@webtide.com>

jetty-core/jetty-security/src/main/java/org/eclipse/jetty/security/AuthenticationState.java

@@ -271,6 +271,10 @@ public interface AuthenticationState extends Request.AuthenticationState
         }
     };
 
+    /**
+     * The {@link SecurityHandler} will use this to wrap the {@link Request}.
+     * And then will return a {@link Deferred} authentication to bypass security constraints.
+     */
     class ServeAs implements AuthenticationState
     {
         private final HttpURI _uri;
@@ -295,6 +299,11 @@ public interface AuthenticationState extends Request.AuthenticationState
             };
         }
 
+        /**
+         * This interface can be used inside a {@link ServeAs} implementation to wrap the request
+         * changing its target to a given path. If a {@link Request} implements this interface it can
+         * be obtained with the {@link Request#as(Request, Class)} method.
+         */
         public interface Path
         {
             Request serveAs(Request request, String path);

jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/Request.java

@@ -790,15 +790,10 @@ public interface Request extends Attributes, Content.Source
     }
 
     @SuppressWarnings("unchecked")
-    static <T extends Request.Wrapper, R> R get(Request request, Class<T> type, Function<T, R> getter)
+    static <T, R> R get(Request request, Class<T> type, Function<T, R> getter)
     {
-        while (request instanceof Request.Wrapper wrapper)
-        {
-            if (type.isInstance(wrapper))
-                return getter.apply((T)wrapper);
-            request = wrapper.getWrapped();
-        }
-        return null;
+        T t = Request.as(request, type);
+        return (t == null) ? null : getter.apply(t);
     }
 
     static Request unWrap(Request request)