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

gradle/validation/forbidden-apis/java.solr.txt

@@ -26,3 +26,6 @@ java.util.LinkedHashMap#<init>(int)
 
 @defaultMessage Use CollectionUtil.newHashSet(int)
 java.util.HashSet#<init>(int)
+
+@defaultMessage Use List.of()
+java.util.Collections#emptyList()

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/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/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/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/placement/plugins/OrderedNodePlacementPlugin.java

@@ -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
@@ -64,7 +63,7 @@ public class CoreSorter {
           }
         }
 
-        // Prioritize replicas where most no:of other nodes are waiting for
+        // Prioritize replicas where the greatest number of other nodes waiting.
         // For example if 1 other replicas are waiting for this replica, then
         // prioritize that over the replica were zero other nodes are waiting
         if (c1.totalReplicasInLiveNodes > c2.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();
   }
 }

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

@@ -600,7 +600,7 @@ public static class NodeConfigBuilder {
     private PluginInfo[] clusterPlugins;
     private String defaultZkHost;
     private Set<Path> allowPaths = Collections.emptySet();
-    private List<String> allowUrls = Collections.emptyList();
+    private List<String> allowUrls = List.of();
     private boolean hideStackTrace =
         !EnvUtils.getPropertyAsBool("solr.responses.stacktrace.enabled", true);
 

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

@@ -59,7 +59,7 @@ public PluginInfo(
       }
     }
     attributes = unmodifiableMap(attrs);
-    this.children = children == null ? Collections.emptyList() : unmodifiableList(children);
+    this.children = children == null ? List.of() : unmodifiableList(children);
     isFromSolrConfig = false;
   }
 
@@ -148,7 +148,7 @@ public PluginInfo(String type, Map<String, Object> map) {
     // TODO This is not type-safe and needs to be fixed -
     // https://issues.apache.org/jira/browse/SOLR-14696
     attributes = unmodifiableMap(m);
-    this.children = Collections.emptyList();
+    this.children = List.of();
     isFromSolrConfig = true;
   }
 
@@ -162,7 +162,7 @@ private List<PluginInfo> loadSubPlugins(ConfigNode node) {
           if (pluginInfo.isEnabled()) children.add(pluginInfo);
           return null;
         });
-    return children.isEmpty() ? Collections.emptyList() : unmodifiableList(children);
+    return children.isEmpty() ? List.of() : unmodifiableList(children);
   }
 
   @Override
@@ -230,7 +230,7 @@ public List<PluginInfo> getChildren(String type) {
   }
 
   public static final PluginInfo EMPTY_INFO =
-      new PluginInfo("", Collections.emptyMap(), new NamedList<>(), Collections.emptyList());
+      new PluginInfo("", Collections.emptyMap(), new NamedList<>(), List.of());
 
   private static final HashSet<String> NL_TAGS =
       new HashSet<>(asList("lst", "arr", "bool", "str", "int", "long", "float", "double"));

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

@@ -900,7 +900,7 @@ public List<PluginInfo> getPluginInfos(String type) {
         result = new ArrayList<>(map.values());
       }
     }
-    return result == null ? Collections.emptyList() : result;
+    return result == null ? List.of() : result;
   }
 
   public PluginInfo getPluginInfo(String type) {

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

@@ -3231,7 +3231,7 @@ public void initDefaultPlugin(Object plugin, Class<?> type) {
    * @return The instances initialized
    */
   public <T> List<T> initPlugins(List<PluginInfo> pluginInfos, Class<T> type, String defClassName) {
-    if (pluginInfos.isEmpty()) return Collections.emptyList();
+    if (pluginInfos.isEmpty()) return List.of();
     List<T> result = new ArrayList<>(pluginInfos.size());
     for (PluginInfo info : pluginInfos)
       result.add(createInitInstance(info, type, type.getSimpleName(), defClassName));
@@ -3623,7 +3623,7 @@ private ImplicitPluginsHolder() {}
     }
 
     public static List<PluginInfo> getImplicitPlugins(String type) {
-      return ALL_IMPLICIT_PLUGINS.getOrDefault(type, Collections.emptyList());
+      return ALL_IMPLICIT_PLUGINS.getOrDefault(type, List.of());
     }
   }
 

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

@@ -394,7 +394,7 @@ private static NodeConfig fillSolrSection(NodeConfig.NodeConfigBuilder builder,
 
   private static List<String> separateStrings(String commaSeparatedString) {
     if (StrUtils.isNullOrEmpty(commaSeparatedString)) {
-      return Collections.emptyList();
+      return List.of();
     }
     return Arrays.asList(COMMA_SEPARATED_PATTERN.split(commaSeparatedString));
   }

solr/core/src/java/org/apache/solr/core/snapshots/CollectionSnapshotMetaData.java

@@ -18,7 +18,6 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.Date;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
@@ -125,8 +124,7 @@ public static enum SnapshotStatus {
   private final List<CoreSnapshotMetaData> replicaSnapshots;
 
   public CollectionSnapshotMetaData(String name) {
-    this(
-        name, SnapshotStatus.InProgress, new Date(), Collections.<CoreSnapshotMetaData>emptyList());
+    this(name, SnapshotStatus.InProgress, new Date(), List.of());
   }
 
   public CollectionSnapshotMetaData(

solr/core/src/java/org/apache/solr/handler/ClusterAPI.java

@@ -190,7 +190,7 @@ public static List<String> getNodesByRole(
     try {
       return zk.listData(ZkStateReader.NODE_ROLES + "/" + role + "/" + mode);
     } catch (NoSuchElementException e) {
-      return Collections.emptyList();
+      return List.of();
     }
   }
 

solr/core/src/java/org/apache/solr/handler/IndexFetcher.java

@@ -381,7 +381,7 @@ private void fetchFileList(long gen) throws IOException {
       List<Map<String, Object>> files = (List<Map<String, Object>>) response.get(CMD_GET_FILE_LIST);
       if (files != null) filesToDownload = Collections.synchronizedList(files);
       else {
-        filesToDownload = Collections.emptyList();
+        filesToDownload = List.of();
         log.error("No files to download for index generation: {}", gen);
       }
 
@@ -1473,7 +1473,7 @@ private List<Path> makeTmpConfDirFileList(Path dir) {
       return Files.walk(dir).filter(Files::isRegularFile).collect(Collectors.toList());
     } catch (IOException e) {
       log.warn("Could not walk file tree", e);
-      return Collections.emptyList();
+      return List.of();
     }
   }
 
@@ -1532,8 +1532,7 @@ private String getDateAsStr(Date d) {
    */
   private Collection<Map<String, Object>> getModifiedConfFiles(
       List<Map<String, Object>> confFilesToDownload) {
-    if (confFilesToDownload == null || confFilesToDownload.isEmpty())
-      return Collections.emptyList();
+    if (confFilesToDownload == null || confFilesToDownload.isEmpty()) return List.of();
     // build a map with alias/name as the key
     Map<String, Map<String, Object>> nameVsFile = new HashMap<>();
     NamedList<String> names = new NamedList<>();
@@ -1555,7 +1554,7 @@ private Collection<Map<String, Object>> getModifiedConfFiles(
         nameVsFile.remove(name); // checksums are same so the file need not be downloaded
       }
     }
-    return nameVsFile.isEmpty() ? Collections.emptyList() : nameVsFile.values();
+    return nameVsFile.isEmpty() ? List.of() : nameVsFile.values();
   }
 
   static boolean delTree(Path dir) {
@@ -1600,25 +1599,25 @@ List<Map<String, Object>> getConfFilesToDownload() {
     // make a copy first because it can be null later
     List<Map<String, Object>> tmp = confFilesToDownload;
     // create a new instance. or else iterator may fail
-    return tmp == null ? Collections.emptyList() : new ArrayList<>(tmp);
+    return tmp == null ? List.of() : new ArrayList<>(tmp);
   }
 
   List<Map<String, Object>> getConfFilesDownloaded() {
     // make a copy first because it can be null later
     List<Map<String, Object>> tmp = confFilesDownloaded;
     // NOTE: it's safe to make a copy of a SynchronizedCollection(ArrayList)
-    return tmp == null ? Collections.emptyList() : new ArrayList<>(tmp);
+    return tmp == null ? List.of() : new ArrayList<>(tmp);
   }
 
   List<Map<String, Object>> getFilesToDownload() {
     // make a copy first because it can be null later
     List<Map<String, Object>> tmp = filesToDownload;
-    return tmp == null ? Collections.emptyList() : new ArrayList<>(tmp);
+    return tmp == null ? List.of() : new ArrayList<>(tmp);
   }
 
   List<Map<String, Object>> getFilesDownloaded() {
     List<Map<String, Object>> tmp = filesDownloaded;
-    return tmp == null ? Collections.emptyList() : new ArrayList<>(tmp);
+    return tmp == null ? List.of() : new ArrayList<>(tmp);
   }
 
   // TODO: currently does not reflect conf files

solr/core/src/java/org/apache/solr/handler/MoreLikeThisHandler.java

@@ -221,8 +221,7 @@ public void handleRequestBody(SolrQueryRequest req, SolrQueryResponse rsp) throw
         if (mltDocs.docSet == null) {
           rsp.add("facet_counts", null);
         } else {
-          final ResponseBuilder responseBuilder =
-              new ResponseBuilder(req, rsp, Collections.emptyList());
+          final ResponseBuilder responseBuilder = new ResponseBuilder(req, rsp, List.of());
           responseBuilder.setQuery(mlt.getRealMLTQuery());
           SimpleFacets f = new SimpleFacets(req, mltDocs.docSet, params, responseBuilder);
           FacetComponent.FacetContext.initContext(responseBuilder);

solr/core/src/java/org/apache/solr/handler/admin/CollectionsHandler.java

@@ -106,7 +106,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
@@ -1410,7 +1409,6 @@ static Map<String, Object> copy(
   /** Copy all params to the given map or if the given map is null create a new one */
   static Map<String, Object> copy(
       SolrParams source, Map<String, Object> sink, String... paramNames) {
-    return copy(
-        source, sink, paramNames == null ? Collections.emptyList() : Arrays.asList(paramNames));
+    return copy(source, sink, paramNames == null ? List.of() : Arrays.asList(paramNames));
   }
 }

solr/core/src/java/org/apache/solr/handler/admin/api/ReplicationAPIBase.java

@@ -28,7 +28,6 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 import java.util.regex.Pattern;
@@ -140,7 +139,7 @@ protected FileListResponse getFileList(long generation, ReplicationHandler repli
       if (generation == -1) {
         commit = delPol.getAndSaveLatestCommit();
         if (null == commit) {
-          filesResponse.fileList = Collections.emptyList();
+          filesResponse.fileList = List.of();
           return filesResponse;
         }
       } else {

solr/core/src/java/org/apache/solr/handler/component/CloudReplicaSource.java

@@ -160,7 +160,7 @@ private List<String> findReplicas(
     if (slice == null) {
       // Treat this the same as "all servers down" for a slice, and let things continue
       // if partial results are acceptable
-      return Collections.emptyList();
+      return List.of();
     } else {
       final Predicate<Replica> isShardLeader =
           new IsLeaderPredicate(