Added SingleOrderedMap.of(...) for immutable empty or single pair (#2932

)

SimpleFacets:  use this, avoiding needless new NamedList creation

solr/core/src/java/org/apache/solr/request/SimpleFacets.java

@@ -871,12 +871,12 @@ public void execute(Runnable r) {
    * @see #getFieldMissingCount
    * @see #getFacetTermEnumCounts
    */
-  public NamedList<Object> getFacetFieldCounts() throws IOException, SyntaxError {
+  public SimpleOrderedMap<Object> getFacetFieldCounts() throws IOException, SyntaxError {
 
-    NamedList<Object> res = new SimpleOrderedMap<>();
     String[] facetFs = global.getParams(FacetParams.FACET_FIELD);
+
     if (null == facetFs) {
-      return res;
+      return  SimpleOrderedMap.of();
     }
 
     // Passing a negative number for FACET_THREADS implies an unlimited number of threads is
@@ -891,6 +891,7 @@ public NamedList<Object> getFacetFieldCounts() throws IOException, SyntaxError {
       fdebugParent.putInfoItem("maxThreads", maxThreads);
     }
 
+    SimpleOrderedMap<Object> res;
     try {
       // Loop over fields; submit to executor, keeping the future
       for (String f : facetFs) {
@@ -914,10 +915,8 @@ public NamedList<Object> getFacetFieldCounts() throws IOException, SyntaxError {
                   result.add(key, getTermCounts(facetValue, parsed));
                 }
                 return result;
-              } catch (SolrException se) {
+              } catch (SolrException | ExitableDirectoryReader.ExitingReaderException se) {
                 throw se;
-              } catch (ExitableDirectoryReader.ExitingReaderException timeout) {
-                throw timeout;
               } catch (Exception e) {
                 throw new SolrException(
                     ErrorCode.SERVER_ERROR, "Exception during facet.field: " + facetValue, e);
@@ -932,6 +931,7 @@ public NamedList<Object> getFacetFieldCounts() throws IOException, SyntaxError {
         futures.add(runnableFuture);
       } // facetFs loop
 
+      res = new SimpleOrderedMap<>();
       // Loop over futures to get the values. The order is the same as facetFs but shouldn't matter.
       for (Future<NamedList<?>> future : futures) {
         res.addAll(future.get());

solr/solrj/src/java/org/apache/solr/common/util/SimpleOrderedMap.java

@@ -34,6 +34,8 @@
  * serialized. It aims to minimize overhead and to be efficient at adding new elements.
  */
 public class SimpleOrderedMap<T> extends NamedList<T> {
+
+  private static final SimpleOrderedMap<Object> EMPTY = new SimpleOrderedMap<>(List.of());
   /** Creates an empty instance */
   public SimpleOrderedMap() {
     super();
@@ -67,4 +69,23 @@ public SimpleOrderedMap<T> clone() {
     newList.addAll(nvPairs);
     return new SimpleOrderedMap<>(newList);
   }
+
+  /**
+   * Returns a shared, empty, and immutable instance of SimpleOrderedMap.
+   * @return Empty SimpleOrderedMap (immutable) 
+   */
+  public static SimpleOrderedMap<Object> of() {
+    return EMPTY;
+  }
+
+  /**
+   * Returns an immutable instance of SimpleOrderedMap with a single key-value pair.
+   * @return SimpleOrderedMap containing one key-value pair
+   */
+
+  public static <T> SimpleOrderedMap<T> of(String name, T val)
+  {
+    return new SimpleOrderedMap<>(List.of(name, val));   
+  }
+
 }