Add experimental API label. Allow null value.

api/src/main/java/io/grpc/NameResolver.java

@@ -219,7 +219,7 @@ public abstract static class Listener2 implements Listener {
     @Deprecated
     @InlineMe(
         replacement = "this.onResult2(ResolutionResult.newBuilder().setAddressesOrError("
-            + "StatusOr.of(servers)).setAttributes(attributes).build())",
+            + "StatusOr.fromValue(servers)).setAttributes(attributes).build())",
         imports = {"io.grpc.NameResolver.ResolutionResult", "io.grpc.StatusOr"})
     public final void onAddresses(
         List<EquivalentAddressGroup> servers, @ResolutionResultAttr Attributes attributes) {

api/src/main/java/io/grpc/StatusOr.java

@@ -20,17 +20,19 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.base.Objects;
+import javax.annotation.Nullable;
 
 /** Either a Status or a value. */
+@ExperimentalApi("https://github.com/grpc/grpc-java/issues/11563")
 public class StatusOr<T> {
   private StatusOr(Status status, T value) {
     this.status = status;
     this.value = value;
   }
 
   /** Construct from a value. */
-  public static <T> StatusOr<T> fromValue(T value) {
-    StatusOr<T> result = new StatusOr<T>(null, checkNotNull(value, "value"));
+  public static <T> StatusOr<T> fromValue(@Nullable T value) {
+    StatusOr<T> result = new StatusOr<T>(null, value);
     return result;
   }
 
@@ -50,8 +52,8 @@ public boolean hasValue() {
    * Returns the value if set or throws exception if there is no value set. This method is meant
    * to be called after checking the return value of hasValue() first.
    */
-  public T getValue() {
-    if (value == null) {
+  public @Nullable T getValue() {
+    if (status != null) {
       throw new IllegalStateException("No value present.");
     }
     return value;