1 /**
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing, software
13 * distributed under the License is distributed on an "AS IS" BASIS,
14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 * See the License for the specific language governing permissions and
16 * limitations under the License.
17 */
18 package org.apache.hadoop.hbase;
19
20 import java.io.IOException;
21
22 import org.apache.hadoop.hbase.classification.InterfaceAudience;
23 import org.apache.hadoop.hbase.classification.InterfaceStability;
24
25 /**
26 * An interface for iterating through a sequence of cells. Similar to Java's Iterator, but without
27 * the hasNext() or remove() methods. The hasNext() method is problematic because it may require
28 * actually loading the next object, which in turn requires storing the previous object somewhere.
29 *
30 * <p>The core data block decoder should be as fast as possible, so we push the complexity and
31 * performance expense of concurrently tracking multiple cells to layers above the CellScanner.
32 * <p>
33 * The {@link #current()} method will return a reference to a Cell implementation. This reference
34 * may or may not point to a reusable cell implementation, so users of the CellScanner should not,
35 * for example, accumulate a List of Cells. All of the references may point to the same object,
36 * which would be the latest state of the underlying Cell. In short, the Cell is mutable.
37 * </p>
38 * Typical usage:
39 *
40 * <pre>
41 * while (scanner.advance()) {
42 * Cell cell = scanner.current();
43 * // do something
44 * }
45 * </pre>
46 * <p>Often used reading {@link org.apache.hadoop.hbase.Cell}s written by
47 * {@link org.apache.hadoop.hbase.io.CellOutputStream}.
48 */
49 @InterfaceAudience.Private
50 @InterfaceStability.Unstable
51 public interface CellScanner {
52 /**
53 * @return the current Cell which may be mutable
54 */
55 Cell current();
56
57 /**
58 * Advance the scanner 1 cell.
59 * @return true if the next cell is found and {@link #current()} will return a valid Cell
60 * @throws IOException if advancing the scanner fails
61 */
62 boolean advance() throws IOException;
63 }