Use List.of convenience methods in 7.x autoscaling

We do not have access to JDK 9 collection convenience methods in 7.x
because we are compatible with JDK 8 there. Yet, we have recently added
a substitute for these convenience methods that even delegate to the
right places when running on JDK 9, to make backporting easier. This
commit utilizes these new methods in the autoscaling codebase.

Author: jasontedor

x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/Autoscaling.java

@@ -36,8 +36,6 @@
 import org.elasticsearch.xpack.autoscaling.rest.RestPutAutoscalingPolicyHandler;
 import org.elasticsearch.xpack.core.XPackPlugin;
 
-import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 import java.util.function.Supplier;
 
@@ -86,9 +84,9 @@ public Autoscaling(final Settings settings) {
     @Override
     public List<Setting<?>> getSettings() {
         if (isSnapshot() || (AUTOSCALING_FEATURE_FLAG_REGISTERED != null && AUTOSCALING_FEATURE_FLAG_REGISTERED)) {
-            return Collections.singletonList(AUTOSCALING_ENABLED_SETTING);
+            return org.elasticsearch.common.collect.List.of(AUTOSCALING_ENABLED_SETTING);
         } else {
-            return Collections.emptyList();
+            return org.elasticsearch.common.collect.List.of();
         }
     }
 
@@ -99,14 +97,12 @@ boolean isSnapshot() {
     @Override
     public List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> getActions() {
         if (enabled) {
-            return Collections.unmodifiableList(
-                Arrays.asList(
-                    new ActionHandler<>(GetAutoscalingDecisionAction.INSTANCE, TransportGetAutoscalingDecisionAction.class),
-                    new ActionHandler<>(PutAutoscalingPolicyAction.INSTANCE, TransportPutAutoscalingPolicyAction.class)
-                )
+            return org.elasticsearch.common.collect.List.of(
+                new ActionHandler<>(GetAutoscalingDecisionAction.INSTANCE, TransportGetAutoscalingDecisionAction.class),
+                new ActionHandler<>(PutAutoscalingPolicyAction.INSTANCE, TransportPutAutoscalingPolicyAction.class)
             );
         } else {
-            return Collections.emptyList();
+            return org.elasticsearch.common.collect.List.of();
         }
     }
 
@@ -121,43 +117,29 @@ public List<RestHandler> getRestHandlers(
         final Supplier<DiscoveryNodes> nodesInCluster
     ) {
         if (enabled) {
-            return Collections.unmodifiableList(
-                Arrays.asList(new RestGetAutoscalingDecisionHandler(), new RestPutAutoscalingPolicyHandler())
-            );
+            return org.elasticsearch.common.collect.List.of(new RestGetAutoscalingDecisionHandler(), new RestPutAutoscalingPolicyHandler());
         } else {
-            return Collections.emptyList();
+            return org.elasticsearch.common.collect.List.of();
         }
     }
 
     @Override
     public List<NamedWriteableRegistry.Entry> getNamedWriteables() {
-        return Collections.unmodifiableList(
-            Arrays.asList(
-                new NamedWriteableRegistry.Entry(Metadata.Custom.class, AutoscalingMetadata.NAME, AutoscalingMetadata::new),
-                new NamedWriteableRegistry.Entry(
-                    NamedDiff.class,
-                    AutoscalingMetadata.NAME,
-                    AutoscalingMetadata.AutoscalingMetadataDiff::new
-                ),
-                new NamedWriteableRegistry.Entry(AutoscalingDecider.class, AlwaysAutoscalingDecider.NAME, AlwaysAutoscalingDecider::new)
-            )
+        return org.elasticsearch.common.collect.List.of(
+            new NamedWriteableRegistry.Entry(Metadata.Custom.class, AutoscalingMetadata.NAME, AutoscalingMetadata::new),
+            new NamedWriteableRegistry.Entry(NamedDiff.class, AutoscalingMetadata.NAME, AutoscalingMetadata.AutoscalingMetadataDiff::new),
+            new NamedWriteableRegistry.Entry(AutoscalingDecider.class, AlwaysAutoscalingDecider.NAME, AlwaysAutoscalingDecider::new)
         );
     }
 
     @Override
     public List<NamedXContentRegistry.Entry> getNamedXContent() {
-        return Collections.unmodifiableList(
-            Arrays.asList(
-                new NamedXContentRegistry.Entry(
-                    Metadata.Custom.class,
-                    new ParseField(AutoscalingMetadata.NAME),
-                    AutoscalingMetadata::parse
-                ),
-                new NamedXContentRegistry.Entry(
-                    AutoscalingDecider.class,
-                    new ParseField(AlwaysAutoscalingDecider.NAME),
-                    AlwaysAutoscalingDecider::parse
-                )
+        return org.elasticsearch.common.collect.List.of(
+            new NamedXContentRegistry.Entry(Metadata.Custom.class, new ParseField(AutoscalingMetadata.NAME), AutoscalingMetadata::parse),
+            new NamedXContentRegistry.Entry(
+                AutoscalingDecider.class,
+                new ParseField(AlwaysAutoscalingDecider.NAME),
+                AlwaysAutoscalingDecider::parse
             )
         );
     }

x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/rest/RestGetAutoscalingDecisionHandler.java

@@ -14,14 +14,13 @@
 
 import java.util.List;
 
-import static java.util.Collections.singletonList;
 import static org.elasticsearch.rest.RestRequest.Method.GET;
 
 public class RestGetAutoscalingDecisionHandler extends BaseRestHandler {
 
     @Override
     public List<Route> routes() {
-        return singletonList(new Route(GET, "/_autoscaling/decision"));
+        return org.elasticsearch.common.collect.List.of(new Route(GET, "/_autoscaling/decision"));
     }
 
     @Override

x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/rest/RestPutAutoscalingPolicyHandler.java

@@ -14,7 +14,6 @@
 import org.elasticsearch.xpack.autoscaling.action.PutAutoscalingPolicyAction;
 
 import java.io.IOException;
-import java.util.Collections;
 import java.util.List;
 
 import static org.elasticsearch.rest.RestRequest.Method.PUT;
@@ -23,7 +22,7 @@ public class RestPutAutoscalingPolicyHandler extends BaseRestHandler {
 
     @Override
     public List<Route> routes() {
-        return Collections.singletonList(new Route(PUT, "/_autoscaling/policy/{name}"));
+        return org.elasticsearch.common.collect.List.of(new Route(PUT, "/_autoscaling/policy/{name}"));
     }
 
     @Override

x-pack/plugin/autoscaling/src/test/java/org/elasticsearch/xpack/autoscaling/AutoscalingIntegTestCase.java

@@ -6,20 +6,19 @@
 
 package org.elasticsearch.xpack.autoscaling;
 
+import org.elasticsearch.common.collect.List;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.xpack.core.XPackSettings;
 
-import java.util.Arrays;
 import java.util.Collection;
-import java.util.Collections;
 
 public abstract class AutoscalingIntegTestCase extends ESIntegTestCase {
 
     @Override
     protected Collection<Class<? extends Plugin>> nodePlugins() {
-        return Collections.singletonList(LocalStateAutoscaling.class);
+        return List.of(LocalStateAutoscaling.class);
     }
 
     @Override
@@ -32,7 +31,7 @@ protected Settings nodeSettings(final int nodeOrdinal) {
 
     @Override
     protected Collection<Class<? extends Plugin>> transportClientPlugins() {
-        return Collections.unmodifiableList(Arrays.asList(LocalStateAutoscaling.class, getTestTransportPlugin()));
+        return List.of(LocalStateAutoscaling.class, getTestTransportPlugin());
     }
 
     @Override

x-pack/plugin/autoscaling/src/test/java/org/elasticsearch/xpack/autoscaling/AutoscalingTestCase.java

@@ -17,7 +17,6 @@
 import org.elasticsearch.xpack.autoscaling.policy.AutoscalingPolicyMetadata;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.SortedMap;
@@ -68,7 +67,7 @@ public static AutoscalingDecisions randomAutoscalingDecisions(
 
     public static SortedMap<String, AutoscalingDecider> randomAutoscalingDeciders() {
         return new TreeMap<>(
-            Collections.singletonList(new AlwaysAutoscalingDecider())
+            org.elasticsearch.common.collect.List.of(new AlwaysAutoscalingDecider())
                 .stream()
                 .collect(Collectors.toMap(AutoscalingDecider::name, Function.identity()))
         );

x-pack/plugin/autoscaling/src/test/java/org/elasticsearch/xpack/autoscaling/decision/AutoscalingDecisionsTests.java

@@ -6,19 +6,15 @@
 
 package org.elasticsearch.xpack.autoscaling.decision;
 
+import org.elasticsearch.common.collect.List;
 import org.elasticsearch.xpack.autoscaling.AutoscalingTestCase;
 
-import java.util.Collections;
-
 import static org.hamcrest.Matchers.equalTo;
 
 public class AutoscalingDecisionsTests extends AutoscalingTestCase {
 
     public void testAutoscalingDecisionsRejectsEmptyDecisions() {
-        final IllegalArgumentException e = expectThrows(
-            IllegalArgumentException.class,
-            () -> new AutoscalingDecisions(Collections.emptyList())
-        );
+        final IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> new AutoscalingDecisions(List.of()));
         assertThat(e.getMessage(), equalTo("decisions can not be empty"));
     }