[core] Only issue a deprecation warning, if the referenced ruleset is not empty

and all rules have been deprecated.
Fixes #782

pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java

@@ -520,7 +520,7 @@ public class RuleSetFactory {
         }
 
         boolean rulesetDeprecated = false;
-        if (potentialRules.size() == countDeprecated) {
+        if (!potentialRules.isEmpty() && potentialRules.size() == countDeprecated) {
             // all rules in the ruleset have been deprecated - the ruleset itself is considered to be deprecated
             rulesetDeprecated = true;
             LOG.warning("The RuleSet " + ref + " has been deprecated.");

pmd-core/src/test/java/net/sourceforge/pmd/RuleSetFactoryTest.java

@@ -14,10 +14,15 @@ import static org.junit.Assert.assertTrue;
 import java.io.ByteArrayInputStream;
 import java.io.InputStream;
 import java.io.UnsupportedEncodingException;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.logging.Handler;
+import java.util.logging.LogRecord;
+import java.util.logging.Logger;
 
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
@@ -575,6 +580,59 @@ public class RuleSetFactoryTest {
         assertEquals(0, ruleset.getRules().size());
     }
 
+    private static class InMemoryLogHandler extends Handler {
+        private List<LogRecord> records = Collections.synchronizedList(new ArrayList<LogRecord>());
+
+        @Override
+        public void close() throws SecurityException {
+        }
+
+        @Override
+        public void flush() {
+            records.clear();
+        }
+
+        @Override
+        public void publish(LogRecord record) {
+            records.add(record);
+        }
+
+        public List<LogRecord> getLogRecords() {
+            return new ArrayList<>(records);
+        }
+    }
+
+    /**
+     * See https://github.com/pmd/pmd/issues/782
+     * Empty ruleset should be interpreted as deprecated.
+     *
+     * @throws Exception
+     *             any error
+     */
+    @Test
+    public void testEmptyRuleSetReferencedShouldNotBeDeprecated() throws Exception {
+        InMemoryLogHandler logHandler = new InMemoryLogHandler();
+        Logger logger = Logger.getLogger(RuleSetFactory.class.getName());
+        try {
+            logger.addHandler(logHandler);
+
+            RuleSetReferenceId ref = createRuleSetReferenceId("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" + "\n"
+                    + "<ruleset name=\"Custom ruleset\" xmlns=\"http://pmd.sourceforge.net/ruleset/2.0.0\"\n"
+                    + "    xmlns:xsi=\"http:www.w3.org/2001/XMLSchema-instance\"\n"
+                    + "    xsi:schemaLocation=\"http://pmd.sourceforge.net/ruleset/2.0.0 http://pmd.sourceforge.net/ruleset_2_0_0.xsd\">\n"
+                    + "    <description>Ruleset which references a empty ruleset</description>\n" + "\n"
+                    + "    <rule ref=\"rulesets/dummy/empty-ruleset.xml\" />\n"
+                    + "</ruleset>\n");
+            RuleSetFactory ruleSetFactory = new RuleSetFactory(new ResourceLoader(), RulePriority.LOW, true, true);
+            RuleSet ruleset = ruleSetFactory.createRuleSet(ref);
+            assertEquals(0, ruleset.getRules().size());
+
+            assertEquals(0, logHandler.getLogRecords().size());
+        } finally {
+            logger.removeHandler(logHandler);
+        }
+    }
+
     /**
      * See https://sourceforge.net/p/pmd/bugs/1231/
      *

pmd-core/src/test/resources/rulesets/dummy/empty-ruleset.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<ruleset name="Test Ruleset which is empty" xmlns="http://pmd.sourceforge.net/ruleset/2.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 http://pmd.sourceforge.net/ruleset_2_0_0.xsd">
+
+    <description>
+  Empty Ruleset which contains no rules.
+  Although it is empty, it should not be marked as deprecated, just because it has no rules (yet).
+  </description>
+
+<!-- Deliberately empty, no rules. -->
+
+</ruleset>