SOLR-17477: adding changes and one more demo test

solr/CHANGES.txt

@@ -112,6 +112,8 @@ Improvements
 * SOLR-17397: SkipExistingDocumentsProcessor now functions correctly with child documents.  (Tim Owens via Eric Pugh)
 * SOLR-17180: Deprecate snapshotscli.sh in favour of bin/solr snapshot sub commands.  Now able to manage Snapshots from the CLI.  HDFS module specific snapshot script now ships as part of that module in the modules/hdfs/bin directory. (Eric Pugh)  
 
+* SOLR-17477: Let to extend FacetModule aka JSON DSL Facet (Tim Owen, Christine Poerschke via Mikhail Khludnev)
+
 Optimizations
 ---------------------
 * SOLR-14985: Solrj CloudSolrClient with Solr URLs had serious performance regressions (since the

solr/core/src/test/org/apache/solr/search/function/CustomParseHandlerTest.java

@@ -0,0 +1,47 @@
+package org.apache.solr.search.function;
+
+import org.apache.solr.SolrTestCaseJ4;
+import org.apache.solr.handler.component.ResponseBuilder;
+import org.apache.solr.request.SolrQueryRequest;
+import org.apache.solr.response.SolrQueryResponse;
+import org.apache.solr.search.SyntaxError;
+import org.apache.solr.search.facet.FacetField;
+import org.apache.solr.search.facet.FacetParser;
+import org.apache.solr.search.facet.FacetRequest;
+import org.junit.BeforeClass;
+
+import java.util.Collections;
+import java.util.Map;
+
+
+public class CustomParseHandlerTest extends SolrTestCaseJ4 {
+
+    public static final FacetField FACET_FIELD_STUB = new FacetField();
+
+    static class CustomParseHandler implements FacetParser.ParseHandler {
+        @Override
+        public Object doParse(FacetParser<?> parent, String key, Object args) {
+            assertEquals("arg", args);
+            return FACET_FIELD_STUB;
+        }
+    }
+
+    @BeforeClass
+    public static void beforeClass() throws Exception {
+        initCore("solrconfig.xml", "schema.xml");
+    }
+
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        FacetParser.registerParseHandler("custom", new CustomParseHandler());
+    }
+
+    public void testCustomParseHandler() {
+        SolrQueryRequest req = req();
+        ResponseBuilder rsp = new ResponseBuilder(req, new SolrQueryResponse(), Collections.emptyList());
+
+        FacetRequest facetRequest = FacetRequest.parse(rsp.req, Map.of("bogus",Map.of("custom","arg")));
+        assertEquals(FACET_FIELD_STUB, facetRequest.getSubFacets().get("bogus"));
+    }
+}