LUCENE-7983: IndexWriter.IndexReaderWarmer is now a functional interface instead of an abstract class with a single method.

lucene/CHANGES.txt

@@ -106,6 +106,10 @@ Other
 * LUCENE-7978: Add some more documentation about setting up build
   environment.  (Anton R. Yuste via Uwe Schindler)
 
+* LUCENE-7983: IndexWriter.IndexReaderWarmer is now a functional interface
+  instead of an abstract class with a single method (Dawid Weiss)
+
+
 ======================= Lucene 7.0.1 =======================
 (No Changes)
 

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -4804,19 +4804,16 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable {
    *
    * @lucene.experimental
    *
-   * <p><b>NOTE</b>: warm is called before any deletes have
-   * been carried over to the merged segment. */
-  public static abstract class IndexReaderWarmer {
-
-    /** Sole constructor. (For invocation by subclass 
-     *  constructors, typically implicit.) */
-    protected IndexReaderWarmer() {
-    }
-
-    /** Invoked on the {@link LeafReader} for the newly
+   * <p><b>NOTE</b>: {@link #warm(LeafReader)} is called before any
+   * deletes have been carried over to the merged segment. */
+  @FunctionalInterface
+  public static interface IndexReaderWarmer {
+    /**
+     * Invoked on the {@link LeafReader} for the newly
      * merged segment, before that segment is made visible
-     *  to near-real-time readers. */
-    public abstract void warm(LeafReader reader) throws IOException;
+     * to near-real-time readers.
+     */
+    void warm(LeafReader reader) throws IOException;
   }
 
   void tragicEvent(Throwable tragedy, String location) throws IOException {

lucene/core/src/java/org/apache/lucene/index/SimpleMergedSegmentWarmer.java

@@ -26,7 +26,7 @@ import org.apache.lucene.util.InfoStream;
  * A very simple merged segment warmer that just ensures 
  * data structures are initialized.
  */
-public class SimpleMergedSegmentWarmer extends IndexReaderWarmer {
+public class SimpleMergedSegmentWarmer implements IndexReaderWarmer {
   private final InfoStream infoStream;
 
   /**

lucene/core/src/test/org/apache/lucene/index/TestDemoParallelLeafReader.java

@@ -129,16 +129,13 @@ public class TestDemoParallelLeafReader extends LuceneTestCase {
       }
       w = new IndexWriter(indexDir, iwc);
 
-      w.getConfig().setMergedSegmentWarmer(new IndexWriter.IndexReaderWarmer() {
-          @Override
-          public void warm(LeafReader reader) throws IOException {
+      w.getConfig().setMergedSegmentWarmer((reader) -> {
         // This will build the parallel index for the merged segment before the merge becomes visible, so reopen delay is only due to
         // newly flushed segments:
         if (DEBUG) System.out.println(Thread.currentThread().getName() +": TEST: now warm " + reader);
         // TODO: it's not great that we pass false here; it means we close the reader & reopen again for NRT reader; still we did "warm" by
         // building the parallel index, if necessary
         getParallelLeafReader(reader, false, getCurrentSchemaGen());
-          }
       });
 
       // start with empty commit:

lucene/core/src/test/org/apache/lucene/index/TestIndexWriterReader.java

@@ -612,24 +612,15 @@ public class TestIndexWriterReader extends LuceneTestCase {
     }
   }
 
-  private static class MyWarmer extends IndexWriter.IndexReaderWarmer {
-    int warmCount;
-    @Override
-    public void warm(LeafReader reader) throws IOException {
-      warmCount++;
-    }
-  }
-
   public void testMergeWarmer() throws Exception {
-
     Directory dir1 = getAssertNoDeletesDirectory(newDirectory());
     // Enroll warmer
-    MyWarmer warmer = new MyWarmer();
+    AtomicInteger warmCount = new AtomicInteger();
     IndexWriter writer = new IndexWriter(
         dir1,
         newIndexWriterConfig(new MockAnalyzer(random()))
           .setMaxBufferedDocs(2)
-          .setMergedSegmentWarmer(warmer)
+          .setMergedSegmentWarmer((leafReader) -> warmCount.incrementAndGet())
           .setMergeScheduler(new ConcurrentMergeScheduler())
           .setMergePolicy(newLogMergePolicy())
     );
@@ -648,12 +639,12 @@ public class TestIndexWriterReader extends LuceneTestCase {
     }
     ((ConcurrentMergeScheduler) writer.getConfig().getMergeScheduler()).sync();
 
-    assertTrue(warmer.warmCount > 0);
-    final int count = warmer.warmCount;
+    assertTrue(warmCount.get() > 0);
+    final int count = warmCount.get();
 
     writer.addDocument(DocHelper.createDocument(17, "test", 4));
     writer.forceMerge(1);
-    assertTrue(warmer.warmCount > count);
+    assertTrue(warmCount.get() > count);
     
     writer.close();
     r1.close();
@@ -969,19 +960,16 @@ public class TestIndexWriterReader extends LuceneTestCase {
     final AtomicBoolean didWarm = new AtomicBoolean();
     IndexWriter w = new IndexWriter(
         dir,
-        newIndexWriterConfig(new MockAnalyzer(random())).
-            setMaxBufferedDocs(2).
-            setReaderPooling(true).
-            setMergedSegmentWarmer(new IndexWriter.IndexReaderWarmer() {
-              @Override
-              public void warm(LeafReader r) throws IOException {
+        newIndexWriterConfig(new MockAnalyzer(random()))
+           .setMaxBufferedDocs(2)
+           .setReaderPooling(true)
+           .setMergedSegmentWarmer((r) -> {
               IndexSearcher s = newSearcher(r);
               TopDocs hits = s.search(new TermQuery(new Term("foo", "bar")), 10);
               assertEquals(20, hits.totalHits);
               didWarm.set(true);
-              }
-            }).
-            setMergePolicy(newLogMergePolicy(10))
+           })
+           .setMergePolicy(newLogMergePolicy(10))
     );
 
     Document doc = new Document();

lucene/replicator/src/java/org/apache/lucene/replicator/nrt/PreCopyMergedSegmentWarmer.java

@@ -34,7 +34,7 @@ import org.apache.lucene.index.SegmentReader;
 
 // TODO: or ... replica node can do merging locally?  tricky to keep things in sync, when one node merges more slowly than others...
 
-class PreCopyMergedSegmentWarmer extends IndexReaderWarmer {
+class PreCopyMergedSegmentWarmer implements IndexReaderWarmer {
 
   private final PrimaryNode primary;
 

lucene/test-framework/src/java/org/apache/lucene/index/ThreadedIndexingAndSearchingTestCase.java

@@ -16,7 +16,6 @@
  */
 package org.apache.lucene.index;
 
-import java.io.IOException;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -468,9 +467,7 @@ public abstract class ThreadedIndexingAndSearchingTestCase extends LuceneTestCas
       mp.setNoCFSRatio(Math.max(0.25d, mp.getNoCFSRatio()));
     }
 
-    conf.setMergedSegmentWarmer(new IndexWriter.IndexReaderWarmer() {
-      @Override
-      public void warm(LeafReader reader) throws IOException {
+    conf.setMergedSegmentWarmer((reader) -> {
       if (VERBOSE) {
         System.out.println("TEST: now warm merged reader=" + reader);
       }
@@ -492,7 +489,6 @@ public abstract class ThreadedIndexingAndSearchingTestCase extends LuceneTestCas
       if (VERBOSE) {
         System.out.println("TEST: warm visited " + sum + " fields");
       }
-      }
     });
 
     if (VERBOSE) {