VersionedWritable


1   /* Copyright (c) 2003 The Nutch Organization.  All rights reserved.   */
2   /* Use subject to the conditions in http://www.nutch.org/LICENSE.txt. */
3   
4   package net.nutch.io;
5   
6   import java.io.DataOutput  ;
7   import java.io.DataInput  ;
8   import java.io.IOException  ;
9   
10  /** A base class for Writables that provides version checking.
11   *
12   * <p>This is useful when a class may evolve, so that instances written by the
13   * old version of the class may still be processed by the new version.  To
14   * handle this situation, {@link #readFields(DataInput)}
15   * implementations should catch {@link VersionMismatchException}.
16   *
17   * @author Doug Cutting
18   */
19  public abstract class VersionedWritable implements Writable {
20  
21    /** Return the version number of the current implementation. */
22    public abstract byte getVersion();
23      
24    // javadoc from Writable
25    public void write(DataOutput   out) throws IOException   {
26      out.writeByte(getVersion());                  // store version
27    }
28  
29    // javadoc from Writable
30    public void readFields(DataInput   in) throws IOException   {
31      byte version = in.readByte();                 // read version
32      if (version != getVersion())
33        throw new VersionMismatchException(getVersion(),version);
34    }
35  
36      
37  }
38

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