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
19 package org.apache.hadoop.hbase;
20
21 import java.io.IOException;
22
23 import org.apache.hadoop.hbase.classification.InterfaceAudience;
24 import org.apache.hadoop.hbase.classification.InterfaceStability;
25 import org.apache.hadoop.hbase.Cell;
26
27 /**
28 * An interface for iterating through a sequence of cells. Similar to Java's Iterator, but without
29 * the hasNext() or remove() methods. The hasNext() method is problematic because it may require
30 * actually loading the next object, which in turn requires storing the previous object somewhere.
31 *
32 * <p>The core data block decoder should be as fast as possible, so we push the complexity and
33 * performance expense of concurrently tracking multiple cells to layers above the CellScanner.
34 * <p>
35 * The {@link #current()} method will return a reference to a Cell implementation. This reference
36 * may or may not point to a reusable cell implementation, so users of the CellScanner should not,
37 * for example, accumulate a List of Cells. All of the references may point to the same object,
38 * which would be the latest state of the underlying Cell. In short, the Cell is mutable.
39 * </p>
40 * Typical usage:
41 *
42 * <pre>
43 * while (scanner.advance()) {
44 * Cell cell = scanner.current();
45 * // do something
46 * }
47 * </pre>
48 * <p>Often used reading {@link org.apache.hadoop.hbase.Cell}s written by
49 * {@link org.apache.hadoop.hbase.io.CellOutputStream}.
50 */
51 @InterfaceAudience.Private
52 @InterfaceStability.Unstable
53 public interface CellScanner {
54 /**
55 * @return the current Cell which may be mutable
56 */
57 Cell current();
58
59 /**
60 * Advance the scanner 1 cell.
61 * @return true if the next cell is found and {@link #current()} will return a valid Cell
62 * @throws IOException
63 */
64 boolean advance() throws IOException;
65 }