SOLR-18139: Migrate away from Collections.empty*() methods.

build-tools/missing-doclet/src/main/java/org/apache/lucene/missingdoclet/MissingDoclet.java

@@ -25,7 +25,6 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
-import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import javax.lang.model.element.Element;
 import javax.lang.model.element.ElementKind;
@@ -65,8 +64,8 @@ public class MissingDoclet extends StandardDoclet {
   DocletEnvironment docEnv;
   DocTrees docTrees;
   Elements elementUtils;
-  Set<String> ignored = Collections.emptySet();
-  Set<String> methodPackages = Collections.emptySet();
+  Set<String> ignored = Set.of();
+  Set<String> methodPackages = Set.of();
 
   @Override
   public Set<Doclet.Option> getSupportedOptions() {
@@ -373,7 +372,7 @@ private Stream<Element> superTypeForInheritDoc(Element type) {
         clazz.getInterfaces().stream()
             .filter(tm -> tm.getKind() == TypeKind.DECLARED)
             .map(tm -> ((DeclaredType) tm).asElement())
-            .collect(Collectors.toList());
+            .toList();
 
     Stream<Element> result = interfaces.stream();
     result = Stream.concat(result, interfaces.stream().flatMap(this::superTypeForInheritDoc));

gradle/validation/forbidden-apis/defaults.all.txt

@@ -54,6 +54,12 @@ java.io.File#delete() @ use Files.delete for real exception, IOUtils.deleteFiles
 
 java.util.Collections#shuffle(java.util.List) @ Use shuffle(List, Random) instead so that it can be reproduced
 
+@defaultMessage Use List.of()
+java.util.Collections#emptyList()
+
+@defaultMessage Use Set.of()
+java.util.Collections#emptySet()
+
 java.util.Locale#forLanguageTag(java.lang.String) @ use new Locale.Builder().setLanguageTag(...).build() which has error handling
 java.util.Locale#toString() @ use Locale#toLanguageTag() for a standardized BCP47 locale name
 
@@ -102,4 +108,4 @@ java.nio.file.Paths#get(**)
 java.nio.file.Path#startsWith(java.lang.String)
 
 @defaultMessage Use NIO Path instead of File
-java.io.File
+java.io.File

solr/benchmark/src/java/org/apache/solr/bench/generators/RandomDataHistogram.java

@@ -23,7 +23,6 @@
 import java.math.RoundingMode;
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Locale;
@@ -509,7 +508,7 @@ public String print(boolean sortByLabel, int bucketed) {
                       }
                       return Integer.compare(e2.getValue().get(), e1.getValue().get());
                     })
-                .filter(entry -> !entry.getKey().equals(Collections.emptyList()))
+                .filter(entry -> !entry.getKey().equals(List.of()))
                 .map(
                     entry -> {
                       double percentage = entry.getValue().get() * 100.0 / sum;

solr/core/src/java/org/apache/solr/api/ApiSupport.java

@@ -18,7 +18,7 @@
 package org.apache.solr.api;
 
 import java.util.Collection;
-import java.util.Collections;
+import java.util.Set;
 
 /** The interface that is implemented by a request handler to support the V2 end point */
 public interface ApiSupport {
@@ -36,7 +36,7 @@ public interface ApiSupport {
    * @see #getApis()
    */
   default Collection<Class<? extends JerseyResource>> getJerseyResources() {
-    return Collections.emptySet();
+    return Set.of();
   }
 
   /** Whether this should be made available at the regular legacy path */

solr/core/src/java/org/apache/solr/cloud/DistributedClusterStateUpdater.java

@@ -387,7 +387,7 @@ private void applyUpdate() throws KeeperException, InterruptedException {
       // For now trying to diverge as little as possible from existing data structures and code
       // given the need to support both the old way (Overseer) and new way (distributed) of handling
       // cluster state update.
-      final Set<String> liveNodes = Collections.emptySet();
+      final Set<String> liveNodes = Set.of();
 
       // Per Replica States updates are done before all other updates and not subject to the number
       // of attempts of CAS made here, given they have their own CAS strategy and implementation
@@ -586,7 +586,7 @@ private ClusterState fetchStateForCollection() throws KeeperException, Interrupt
           ZkClientClusterStateProvider.createFromJsonSupportingLegacyConfigName(
               stat.getVersion(),
               data,
-              Collections.emptySet(),
+              Set.of(),
               updater.getCollectionName(),
               zkStateReader.getZkClient(),
               Instant.ofEpochMilli(stat.getCtime()));

solr/core/src/java/org/apache/solr/cloud/ZkConfigSetService.java

@@ -261,7 +261,7 @@ public List<String> listConfigs() throws IOException {
     try {
       return zkClient.getChildren(CONFIGS_ZKNODE, null);
     } catch (KeeperException.NoNodeException e) {
-      return Collections.emptyList();
+      return List.of();
     } catch (KeeperException | InterruptedException e) {
       throw new IOException("Error listing configs", SolrZkClient.checkInterrupted(e));
     }

solr/core/src/java/org/apache/solr/cloud/api/collections/BackupCmd.java

@@ -26,10 +26,10 @@
 import java.net.URI;
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 import org.apache.solr.cloud.api.collections.CollectionHandlingUtils.ShardRequestTracker;
 import org.apache.solr.common.SolrException;
 import org.apache.solr.common.SolrException.ErrorCode;
@@ -168,7 +168,7 @@ public void call(AdminCmdContext adminCmdContext, ZkNodeProps message, NamedList
         backupMgr.downloadConfigDir(configName, cc.getConfigSetService());
       }
 
-      // Save the collection's state. Can be part of the monolithic clusterstate.json or a
+      // Save the collection's state. Can be part of the monolithic clusterstate.json or an
       // individual state.json. Since we don't want to distinguish we extract the state and back it
       // up as a separate json
       DocCollection collectionState =
@@ -285,7 +285,7 @@ private Replica selectReplicaWithSnapshot(CollectionSnapshotMetaData snapshotMet
                     x.getState() != State.DOWN && snapshotMeta.isSnapshotExists(slice.getName(), x))
             .findFirst();
 
-    if (!r.isPresent()) {
+    if (r.isEmpty()) {
       throw new SolrException(
           ErrorCode.SERVER_ERROR,
           "Unable to find any live replica with a snapshot named "
@@ -462,7 +462,7 @@ private void copyIndexFiles(
       SolrZkClient zkClient = ccc.getZkStateReader().getZkClient();
       snapshotMeta =
           SolrSnapshotManager.getCollectionLevelSnapshot(zkClient, collectionName, commitName);
-      if (!snapshotMeta.isPresent()) {
+      if (snapshotMeta.isEmpty()) {
         throw new SolrException(
             ErrorCode.BAD_REQUEST,
             "Snapshot with name "
@@ -488,7 +488,7 @@ private void copyIndexFiles(
         backupName,
         backupPath);
 
-    Collection<String> shardsToConsider = Collections.emptySet();
+    Collection<String> shardsToConsider = Set.of();
     if (snapshotMeta.isPresent()) {
       shardsToConsider = snapshotMeta.get().getShards();
     }

solr/core/src/java/org/apache/solr/cloud/api/collections/BalanceReplicasCmd.java

@@ -18,7 +18,6 @@
 package org.apache.solr.cloud.api.collections;
 
 import java.util.Collection;
-import java.util.Collections;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
@@ -43,7 +42,7 @@ public void call(AdminCmdContext adminCmdContext, ZkNodeProps message, NamedList
     Set<String> nodes;
     Object nodesRaw = message.get(CollectionParams.NODES);
     if (nodesRaw == null) {
-      nodes = Collections.emptySet();
+      nodes = Set.of();
     } else if (nodesRaw instanceof Set) {
       nodes = (Set<String>) nodesRaw;
     } else if (nodesRaw instanceof Collection) {

solr/core/src/java/org/apache/solr/cloud/api/collections/CategoryRoutedAlias.java

@@ -21,7 +21,6 @@
 
 import java.lang.invoke.MethodHandles;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -260,7 +259,7 @@ protected List<Action> calculateActions(String targetCol) {
       }
       return actionList;
     } else {
-      return Collections.emptyList();
+      return List.of();
     }
   }
 }

solr/core/src/java/org/apache/solr/cloud/api/collections/CollApiCmds.java

@@ -70,10 +70,10 @@
 import io.opentelemetry.api.trace.Span;
 import io.opentelemetry.context.Context;
 import java.lang.invoke.MethodHandles;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import org.apache.solr.cloud.DistributedClusterStateUpdater;
@@ -275,13 +275,7 @@ public void call(
       params.set(CoreAdminParams.ACTION, CoreAdminParams.CoreAdminAction.RELOAD.toString());
 
       CollectionHandlingUtils.collectionCmd(
-          adminCmdContext,
-          message,
-          params,
-          results,
-          Replica.State.ACTIVE,
-          Collections.emptySet(),
-          ccc);
+          adminCmdContext, message, params, results, Replica.State.ACTIVE, Set.of(), ccc);
     }
   }
 

solr/core/src/java/org/apache/solr/cloud/api/collections/CollectionHandlingUtils.java

@@ -237,7 +237,7 @@ static void commit(
           parentShardLeader.getName(),
           updateResponse,
           slice,
-          Collections.emptySet(),
+          Set.of(),
           null);
     } catch (Exception e) {
       CollectionHandlingUtils.processResponse(
@@ -247,7 +247,7 @@ static void commit(
           parentShardLeader.getCoreName(),
           updateResponse,
           slice,
-          Collections.emptySet(),
+          Set.of(),
           null);
       throw new SolrException(
           SolrException.ErrorCode.SERVER_ERROR,
@@ -742,7 +742,7 @@ void processResponses(
         ShardHandler shardHandler,
         boolean abortOnError,
         String msgOnError) {
-      processResponses(results, shardHandler, abortOnError, msgOnError, Collections.emptySet());
+      processResponses(results, shardHandler, abortOnError, msgOnError, Set.of());
     }
 
     void processResponses(

solr/core/src/java/org/apache/solr/cloud/api/collections/CreateCollectionCmd.java

@@ -39,6 +39,7 @@
 import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Objects;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
@@ -415,8 +416,7 @@ public void call(AdminCmdContext adminCmdContext, ZkNodeProps message, NamedList
             nodeName, replicas.get(e.getKey()).getCoreName(), params, shardHandler);
       }
 
-      shardRequestTracker.processResponses(
-          results, shardHandler, false, null, Collections.emptySet());
+      shardRequestTracker.processResponses(results, shardHandler, false, null, Set.of());
       boolean failure =
           results.get("failure") != null
               && ((SimpleOrderedMap<?>) results.get("failure")).size() > 0;

solr/core/src/java/org/apache/solr/cloud/api/collections/MigrateReplicasCmd.java

@@ -134,7 +134,7 @@ public void call(AdminCmdContext adminCmdContext, ZkNodeProps message, NamedList
   protected Set<String> getNodesFromParam(ZkNodeProps message, String paramName) {
     Object rawParam = message.get(paramName);
     if (rawParam == null) {
-      return Collections.emptySet();
+      return Set.of();
     } else if (rawParam instanceof Set) {
       return (Set<String>) rawParam;
     } else if (rawParam instanceof Collection) {

solr/core/src/java/org/apache/solr/cloud/api/collections/TimeRoutedAlias.java

@@ -590,7 +590,7 @@ private CandidateCollection calcCandidateCollection(Instant docTimestamp) {
   private List<Action> calcDeletes(List<Action> actions) {
     final String autoDeleteAgeMathStr = this.getAutoDeleteAgeMath();
     if (autoDeleteAgeMathStr == null || actions.size() == 0) {
-      return Collections.emptyList();
+      return List.of();
     }
     if (actions.size() > 1) {
       throw new IllegalStateException(
@@ -675,7 +675,7 @@ private List<Action> calcAdd(String targetCol) {
       String nextCol = calcNextCollection(parsed);
       return Collections.singletonList(new Action(this, ActionType.ENSURE_EXISTS, nextCol));
     } else {
-      return Collections.emptyList();
+      return List.of();
     }
   }
 

solr/core/src/java/org/apache/solr/cluster/events/ClusterEventProducerBase.java

@@ -18,7 +18,6 @@
 
 import java.io.IOException;
 import java.lang.invoke.MethodHandles;
-import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -96,7 +95,7 @@ public void close() throws IOException {
   protected void fireEvent(ClusterEvent event) {
     synchronized (listeners) {
       listeners
-          .getOrDefault(event.getType(), Collections.emptySet())
+          .getOrDefault(event.getType(), Set.of())
           .forEach(
               listener -> {
                 if (log.isDebugEnabled()) {

solr/core/src/java/org/apache/solr/cluster/placement/impl/AttributeFetcherImpl.java

@@ -17,7 +17,6 @@
 
 package org.apache.solr.cluster.placement.impl;
 
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -46,7 +45,7 @@ public class AttributeFetcherImpl implements AttributeFetcher {
   Set<NodeMetric<?>> requestedNodeMetricSnitchTags = new HashSet<>();
   Map<SolrCollection, Set<ReplicaMetric<?>>> requestedCollectionMetrics = new HashMap<>();
 
-  Set<Node> nodes = Collections.emptySet();
+  Set<Node> nodes = Set.of();
 
   private final SolrCloudManager cloudManager;
 

solr/core/src/java/org/apache/solr/cluster/placement/plugins/AffinityPlacementFactory.java

@@ -338,7 +338,7 @@ AffinityNode newNodeFromMetrics(
       Optional<String> nodePropOpt =
           attrValues.getSystemProperty(node, AffinityPlacementConfig.NODE_TYPE_SYSPROP);
       if (nodePropOpt.isEmpty()) {
-        nodeType = Collections.emptySet();
+        nodeType = Set.of();
       } else {
         nodeType = new HashSet<>(StrUtils.splitSmart(nodePropOpt.get(), ','));
       }

solr/core/src/java/org/apache/solr/cluster/placement/plugins/OrderedNodePlacementPlugin.java

@@ -451,7 +451,7 @@ public boolean hasShardOnNode(Shard shard) {
     public Set<Replica> getReplicasForShardOnNode(Shard shard) {
       return Optional.ofNullable(replicas.get(shard.getCollection().getName()))
           .map(m -> m.get(shard.getShardName()))
-          .orElseGet(Collections::emptySet);
+          .orElseGet(Set::of);
     }
 
     public abstract int calcWeight();
@@ -899,7 +899,7 @@ public Collection<Replica.ReplicaType> getOutstandingReplicaTypes(String shard)
           .map(Map::keySet)
           // Use a sorted TreeSet to make sure that tests are repeatable
           .<Collection<Replica.ReplicaType>>map(TreeSet::new)
-          .orElseGet(Collections::emptyList);
+          .orElseGet(List::of);
     }
 
     /**

solr/core/src/java/org/apache/solr/core/CoreSorter.java

@@ -17,7 +17,6 @@
 
 package org.apache.solr.core;
 
-import static java.util.Collections.emptyList;
 import static java.util.stream.Collectors.toList;
 
 import java.util.Collection;
@@ -47,7 +46,7 @@ public class CoreSorter {
         if (c1 == null) c1 = zero; // just to avoid  NPE
         if (c2 == null) c2 = zero;
         if (c1.totalReplicasInDownNodes < c2.totalReplicasInDownNodes) {
-          // Prioritize replicas with least no:of down nodes waiting.
+          // Prioritize replicas with the least number of down nodes waiting.
           // It's better to bring up a node that is a member of a shard
           // with 0 down nodes than 1 down node because it will make the shard
           // complete earlier and avoid waiting by the other live nodes
@@ -64,9 +63,9 @@ public class CoreSorter {
           }
         }
 
-        // Prioritize replicas where most no:of other nodes are waiting for
-        // For example if 1 other replicas are waiting for this replica, then
-        // prioritize that over the replica were zero other nodes are waiting
+        // Prioritize replicas where the greatest number of other nodes are waiting.
+        // For example, if one other replica is waiting for this replica, then
+        // prioritize that over a replica where zero other nodes are waiting
         if (c1.totalReplicasInLiveNodes > c2.totalReplicasInLiveNodes) return -1;
         if (c2.totalReplicasInLiveNodes > c1.totalReplicasInLiveNodes) return 1;
 
@@ -176,9 +175,9 @@ private static String getShardName(CloudDescriptor cd) {
   /** Return all replicas for a given collection+slice combo */
   private Collection<Replica> getReplicas(ClusterState cs, String coll, String slice) {
     DocCollection c = cs.getCollectionOrNull(coll);
-    if (c == null) return emptyList();
+    if (c == null) return List.of();
     Slice s = c.getSlice(slice);
-    if (s == null) return emptyList();
+    if (s == null) return List.of();
     return s.getReplicas();
   }
 }