fix: clean up [iceberg] integration APIs

common/src/main/java/org/apache/comet/parquet/BatchReader.java

@@ -99,20 +99,20 @@ public class BatchReader extends RecordReader<Void, ColumnarBatch> implements Cl
   private StructType partitionSchema;
   private InternalRow partitionValues;
   private PartitionedFile file;
-  private final Map<String, SQLMetric> metrics;
+  protected Map<String, SQLMetric> metrics;
 
   private long rowsRead;
-  private StructType sparkSchema;
+  protected StructType sparkSchema;
   private MessageType requestedSchema;
-  private CometVector[] vectors;
-  private AbstractColumnReader[] columnReaders;
+  protected CometVector[] vectors;
+  protected AbstractColumnReader[] columnReaders;
   private CometSchemaImporter importer;
-  private ColumnarBatch currentBatch;
+  protected ColumnarBatch currentBatch;
   private Future<Option<Throwable>> prefetchTask;
   private LinkedBlockingQueue<Pair<PageReadStore, Long>> prefetchQueue;
   private FileReader fileReader;
   private boolean[] missingColumns;
-  private boolean isInitialized;
+  protected boolean isInitialized;
   private ParquetMetadata footer;
 
   /** The total number of rows across all row groups of the input split. */
@@ -143,7 +143,9 @@ public class BatchReader extends RecordReader<Void, ColumnarBatch> implements Cl
   private boolean useLegacyDateTimestamp;
 
   /** The TaskContext object for executing this task. */
-  private final TaskContext taskContext;
+  private TaskContext taskContext;
+
+  public BatchReader() {}
 
   // Only for testing
   public BatchReader(String file, int capacity) {
@@ -183,6 +185,9 @@ public BatchReader(
     this.taskContext = TaskContext$.MODULE$.get();
   }
 
+  /**
+   * @deprecated since 0.9.1, will be removed in 0.10.0.
+   */
   public BatchReader(AbstractColumnReader[] columnReaders) {
     // Todo: set useDecimal128 and useLazyMaterialization
     int numColumns = columnReaders.length;
@@ -377,10 +382,16 @@ public void init() throws URISyntaxException, IOException {
     }
   }
 
+  /**
+   * @deprecated since 0.9.1, will be removed in 0.10.0.
+   */
   public void setSparkSchema(StructType schema) {
     this.sparkSchema = schema;
   }
 
+  /**
+   * @deprecated since 0.9.1, will be removed in 0.10.0.
+   */
   public AbstractColumnReader[] getColumnReaders() {
     return columnReaders;
   }

common/src/main/java/org/apache/comet/parquet/IcebergCometBatchReader.java

@@ -0,0 +1,42 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.comet.parquet;
+
+import java.util.HashMap;
+
+import org.apache.spark.sql.types.StructType;
+import org.apache.spark.sql.vectorized.ColumnarBatch;
+
+import org.apache.comet.vector.CometVector;
+
+public class IcebergCometBatchReader extends BatchReader {
+  public IcebergCometBatchReader(int numColumns, StructType schema) {
+    this.columnReaders = new AbstractColumnReader[numColumns];
+    this.vectors = new CometVector[numColumns];
+    this.currentBatch = new ColumnarBatch(vectors);
+    this.metrics = new HashMap<>();
+    this.sparkSchema = schema;
+  }
+
+  public void init(AbstractColumnReader[] columnReaders) {
+    this.columnReaders = columnReaders;
+    this.isInitialized = true;
+  }
+}

common/src/main/java/org/apache/comet/parquet/Utils.java

@@ -37,13 +37,26 @@ public static ColumnReader getColumnReader(
       CometSchemaImporter importer,
       int batchSize,
       boolean useDecimal128,
-      boolean useLazyMaterialization) {
+      boolean useLazyMaterialization,
+      boolean useLegacyTimestamp) {
 
     ColumnDescriptor descriptor = buildColumnDescriptor(columnSpec);
     return getColumnReader(
-        type, descriptor, importer, batchSize, useDecimal128, useLazyMaterialization, true);
+        type,
+        descriptor,
+        importer,
+        batchSize,
+        useDecimal128,
+        useLazyMaterialization,
+        useLegacyTimestamp);
   }
 
+  /**
+   * This method is called from Apache Iceberg.
+   *
+   * @deprecated since 0.9.1, will be removed in 0.10.0; use getColumnReader with ParquetColumnSpec
+   *     instead.
+   */
   public static ColumnReader getColumnReader(
       DataType type,
       ColumnDescriptor descriptor,