hbase-common/src/main/java/org/apache/hadoop/hbase/CellScanner.java

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