feat(java): expose ArrowArrayStream export on LanceScanner

java/src/main/java/org/lance/ipc/LanceScanner.java

@@ -146,6 +146,44 @@ public ArrowReader scanBatches() {
     }
   }
 
+  /**
+   * Populate a caller-provided {@link ArrowArrayStream} with this scan's results, using the C Data
+   * Interface release callback to return ownership.
+   *
+   * <p>Unlike {@link #scanBatches()}, no Java Arrow {@link ArrowReader} is created: the caller
+   * supplies a stream that they allocated (typically from their own {@link
+   * org.apache.arrow.memory.BufferAllocator}), and Lance writes the C struct directly into it. This
+   * lets a downstream consumer drive the read loop with their own Arrow runtime, which is required
+   * when the caller and Lance are loaded by different classloaders / different Arrow versions.
+   *
+   * <p>The caller owns the stream and is responsible for closing it. The release callback installed
+   * on the C struct routes back through Lance's native side.
+   *
+   * <p>Example:
+   *
+   * <pre>{@code
+   * try (ArrowArrayStream stream = ArrowArrayStream.allocateNew(callerAllocator)) {
+   *   scanner.exportArrowStream(stream);
+   *   try (ArrowReader reader = Data.importArrayStream(callerAllocator, stream)) {
+   *     while (reader.loadNextBatch()) {
+   *       VectorSchemaRoot batch = reader.getVectorSchemaRoot();
+   *       // ...
+   *     }
+   *   }
+   * }
+   * }</pre>
+   *
+   * @param stream the caller-allocated stream to populate
+   * @throws IOException if the native scan fails to start
+   */
+  public void exportArrowStream(ArrowArrayStream stream) throws IOException {
+    Preconditions.checkNotNull(stream);
+    try (LockManager.ReadLock readLock = lockManager.acquireReadLock()) {
+      Preconditions.checkArgument(nativeScannerHandle != 0, "Scanner is closed");
+      openStream(stream.memoryAddress());
+    }
+  }
+
   private native void openStream(long streamAddress) throws IOException;
 
   @Override

java/src/test/java/org/lance/ScannerTest.java

@@ -22,6 +22,8 @@
 import org.lance.ipc.ScanOptions;
 import org.lance.ipc.ScanStats;
 
+import org.apache.arrow.c.ArrowArrayStream;
+import org.apache.arrow.c.Data;
 import org.apache.arrow.dataset.scanner.Scanner;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.memory.RootAllocator;
@@ -158,6 +160,51 @@ void testDatasetScannerSchema(@TempDir Path tempDir) throws Exception {
     }
   }
 
+  @Test
+  void testDatasetScannerExportArrowStream(@TempDir Path tempDir) throws Exception {
+    String datasetPath = tempDir.resolve("dataset_scanner_export_stream").toString();
+    try (BufferAllocator allocator = new RootAllocator()) {
+      TestUtils.SimpleTestDataset testDataset =
+          new TestUtils.SimpleTestDataset(allocator, datasetPath);
+      testDataset.createEmptyDataset().close();
+      int totalRows = 40;
+      int batchRows = 20;
+      try (Dataset dataset = testDataset.write(1, totalRows)) {
+        try (LanceScanner scanner =
+            dataset.newScan(
+                new ScanOptions.Builder()
+                    .batchSize(batchRows)
+                    .columns(Arrays.asList("id"))
+                    .build())) {
+          // Caller allocates the C stream from their own allocator; the scanner only fills the
+          // C struct. This is the path callers loaded by a different classloader use to avoid
+          // sharing Java Arrow vector classes with Lance.
+          try (ArrowArrayStream stream = ArrowArrayStream.allocateNew(allocator)) {
+            scanner.exportArrowStream(stream);
+            try (ArrowReader reader = Data.importArrayStream(allocator, stream)) {
+              VectorSchemaRoot root = reader.getVectorSchemaRoot();
+              int index = 0;
+              while (reader.loadNextBatch()) {
+                List<FieldVector> fieldVectors = root.getFieldVectors();
+                assertEquals(1, fieldVectors.size());
+                FieldVector fieldVector = fieldVectors.get(0);
+                assertEquals(
+                    ArrowType.ArrowTypeID.Int, fieldVector.getField().getType().getTypeID());
+                assertEquals(batchRows, fieldVector.getValueCount());
+                IntVector vector = (IntVector) fieldVector;
+                for (int i = 0; i < batchRows; i++) {
+                  assertEquals(index, vector.get(i));
+                  index++;
+                }
+              }
+              assertEquals(totalRows, index);
+            }
+          }
+        }
+      }
+    }
+  }
+
   @Test
   void testDatasetScannerCountRows(@TempDir Path tempDir) throws Exception {
     String datasetPath = tempDir.resolve("dataset_scanner_count").toString();