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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,
13   *  software distributed under the License is distributed on an
14   *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15   *  KIND, either express or implied.  See the License for the
16   *  specific language governing permissions and limitations
17   *  under the License.
18   *
19   */
20  package org.apache.directory.mavibot.btree;
21  
22  
23  import java.io.IOException;
24  import java.lang.ref.SoftReference;
25  
26  import org.apache.directory.mavibot.btree.exception.EndOfFileExceededException;
27  
28  
29  /**
30   * A Value holder. As we may not store all the values in memory (except for an in-memory
31   * BTree), we will use a SoftReference to keep a reference to a Value, and if it's null,
32   * then we will load the Value from the underlying physical support, using the offset. 
33   * 
34   * @param <E> The type for the stored element (either a value or a page)
35   * @param <K> The type of the BTree key
36   * @param <V> The type of the BTree value
37   *
38   * @author <a href="mailto:labs@labs.apache.org">Mavibot labs Project</a>
39   */
40  public class ReferenceHolder<E, K, V> implements ElementHolder<E, K, V>
41  {
42      /** The BTree */
43      private BTree<K, V> btree;
44  
45      /** The offset of the first {@link PageIO} storing the page on disk */
46      private long offset;
47  
48      /** The offset of the last {@link PageIO} storing the page on disk */
49      private long lastOffset;
50  
51      /** The reference to the element instance, or null if it's not present */
52      private SoftReference<E> reference;
53  
54  
55      /**
56       * Create a new holder storing an offset and a SoftReference containing the element.
57       * 
58       * @param offset The offset in disk for this value
59       * @param element The element to store into a SoftReference
60       */
61      public ReferenceHolder( BTree<K, V> btree, E element, long offset, long lastOffset )
62      {
63          this.btree = btree;
64          this.offset = offset;
65          this.lastOffset = lastOffset;
66          this.reference = new SoftReference<E>( element );
67      }
68  
69  
70      /**
71       * {@inheritDoc}
72       * @throws IOException 
73       * @throws EndOfFileExceededException 
74       */
75      @Override
76      public E getValue( BTree<K, V> btree ) throws EndOfFileExceededException, IOException
77      {
78          E element = reference.get();
79  
80          if ( element == null )
81          {
82              // We have to fetch the element from disk, using the offset now
83              element = fetchElement( btree );
84              reference = new SoftReference( element );
85          }
86  
87          return element;
88      }
89  
90  
91      /**
92       * Retrieve the value from the disk, using the BTree and offset
93       * @return The deserialized element (
94       * @throws IOException 
95       * @throws EndOfFileExceededException 
96       */
97      private E fetchElement( BTree<K, V> btree ) throws EndOfFileExceededException, IOException
98      {
99          E element = ( E ) btree.getRecordManager().deserialize( btree, offset );
100 
101         return element;
102     }
103 
104 
105     /**
106      * @return The offset of the first {@link PageIO} storing the data on disk
107      */
108     /* No qualifier */long getOffset()
109     {
110         return offset;
111     }
112 
113 
114     /**
115      * @return The offset of the last {@link PageIO} storing the data on disk
116      */
117     /* No qualifier */long getLastOffset()
118     {
119         return lastOffset;
120     }
121 
122 
123     /**
124      * @see Object#toString()
125      */
126     public String toString()
127     {
128         StringBuilder sb = new StringBuilder();
129 
130         E element = reference.get();
131 
132         if ( element != null )
133         {
134             sb.append( btree.getName() ).append( "[" ).append( offset ).append( "]:" ).append( element );
135         }
136         else
137         {
138             sb.append( btree.getName() ).append( "[" ).append( offset ).append( "]" );
139         }
140 
141         return sb.toString();
142     }
143 }