Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1478278)
+++ lucene/CHANGES.txt	(working copy)
@@ -78,6 +78,9 @@
 
 * LUCENE-4974: CommitIndexTask was broken if no params were set. (Shai Erera)
 
+* LUCENE-4972: DirectoryTaxonomyWriter created empty commits even if no changes 
+  were made. (Shai Erera)
+  
 Optimizations
 
 * LUCENE-4938: Don't use an unnecessarily large priority queue in IndexSearcher
Index: lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(revision 1478278)
+++ lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(working copy)
@@ -103,6 +103,7 @@
   
   // Records the taxonomy index epoch, updated on replaceTaxonomy as well.
   private long indexEpoch;
+  private boolean isDirty = false;
   
   private char delimiter = Consts.DEFAULT_DELIMITER;
   private SinglePositionTokenStream parentStream = new SinglePositionTokenStream(Consts.PAYLOAD_PARENT);
@@ -354,14 +355,14 @@
   @Override
   public synchronized void close() throws IOException {
     if (!isClosed) {
-      indexWriter.setCommitData(combinedCommitData(indexWriter.getCommitData()));
-      indexWriter.commit();
+      commit();
       doClose();
     }
   }
   
   private void doClose() throws IOException {
     indexWriter.close();
+    isDirty = false;
     isClosed = true;
     closeResources();
   }
@@ -549,6 +550,7 @@
     // before the parents array (LUCENE-4596)
     addToCache(categoryPath, id);
 
+    isDirty = true;
     return id;
   }
 
@@ -616,8 +618,11 @@
   @Override
   public synchronized void commit() throws IOException {
     ensureOpen();
-    indexWriter.setCommitData(combinedCommitData(indexWriter.getCommitData()));
-    indexWriter.commit();
+    if (isDirty) { // LUCENE-4972: if we always call setCommitData, we create empty commits
+      indexWriter.setCommitData(combinedCommitData(indexWriter.getCommitData()));
+      indexWriter.commit();
+      isDirty = false;
+    }
   }
 
   /** Combine original user data with the taxonomy epoch. */
@@ -633,6 +638,7 @@
   @Override
   public void setCommitData(Map<String,String> commitUserData) {
     indexWriter.setCommitData(combinedCommitData(commitUserData));
+    isDirty = true;
   }
   
   @Override
@@ -647,8 +653,10 @@
   @Override
   public synchronized void prepareCommit() throws IOException {
     ensureOpen();
-    indexWriter.setCommitData(combinedCommitData(indexWriter.getCommitData()));
-    indexWriter.prepareCommit();
+    if (isDirty) {
+      indexWriter.setCommitData(combinedCommitData(indexWriter.getCommitData()));
+      indexWriter.prepareCommit();
+    }
   }
   
   @Override
@@ -973,6 +981,7 @@
     
     // update indexEpoch as a taxonomy replace is just like it has be recreated
     ++indexEpoch;
+    isDirty = true;
   }
 
   /** Returns the {@link Directory} of this taxonomy writer. */
Index: lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java
===================================================================
--- lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java	(revision 1478278)
+++ lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java	(working copy)
@@ -359,5 +359,58 @@
     taxoWriter.close();
     dir.close();
   }
+
+  @Test
+  public void testCommitNoEmptyCommits() throws Exception {
+    // LUCENE-4972: DTW used to create empty commits even if no changes were made
+    Directory dir = newDirectory();
+    DirectoryTaxonomyWriter taxoWriter = new DirectoryTaxonomyWriter(dir);
+    taxoWriter.addCategory(new CategoryPath("a"));
+    taxoWriter.commit();
+    
+    long gen1 = SegmentInfos.getLastCommitGeneration(dir);
+    taxoWriter.commit();
+    long gen2 = SegmentInfos.getLastCommitGeneration(dir);
+    assertEquals("empty commit should not have changed the index", gen1, gen2);
+    
+    taxoWriter.close();
+    dir.close();
+  }
   
+  @Test
+  public void testCloseNoEmptyCommits() throws Exception {
+    // LUCENE-4972: DTW used to create empty commits even if no changes were made
+    Directory dir = newDirectory();
+    DirectoryTaxonomyWriter taxoWriter = new DirectoryTaxonomyWriter(dir);
+    taxoWriter.addCategory(new CategoryPath("a"));
+    taxoWriter.commit();
+    
+    long gen1 = SegmentInfos.getLastCommitGeneration(dir);
+    taxoWriter.close();
+    long gen2 = SegmentInfos.getLastCommitGeneration(dir);
+    assertEquals("empty commit should not have changed the index", gen1, gen2);
+    
+    taxoWriter.close();
+    dir.close();
+  }
+  
+  @Test
+  public void testPrepareCommitNoEmptyCommits() throws Exception {
+    // LUCENE-4972: DTW used to create empty commits even if no changes were made
+    Directory dir = newDirectory();
+    DirectoryTaxonomyWriter taxoWriter = new DirectoryTaxonomyWriter(dir);
+    taxoWriter.addCategory(new CategoryPath("a"));
+    taxoWriter.prepareCommit();
+    taxoWriter.commit();
+    
+    long gen1 = SegmentInfos.getLastCommitGeneration(dir);
+    taxoWriter.prepareCommit();
+    taxoWriter.commit();
+    long gen2 = SegmentInfos.getLastCommitGeneration(dir);
+    assertEquals("empty commit should not have changed the index", gen1, gen2);
+    
+    taxoWriter.close();
+    dir.close();
+  }
+  
 }