versionedwritable.java

Hadoop是一个用于运行应用程序在大型集群的廉价硬件设备上的框架。Hadoop为应用程序透明的提供了一组稳定/可靠的接口和数据运动。在 Hadoop中实现了Google的MapReduce算法

/**
 * Copyright 2005 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.io;

import java.io.DataOutput;
import java.io.DataInput;
import java.io.IOException;

/** A base class for Writables that provides version checking.
 *
 * <p>This is useful when a class may evolve, so that instances written by the
 * old version of the class may still be processed by the new version.  To
 * handle this situation, {@link #readFields(DataInput)}
 * implementations should catch {@link VersionMismatchException}.
 *
 * @author Doug Cutting
 */
public abstract class VersionedWritable implements Writable {

  /** Return the version number of the current implementation. */
  public abstract byte getVersion();
    
  // javadoc from Writable
  public void write(DataOutput out) throws IOException {
    out.writeByte(getVersion());                  // store version
  }

  // javadoc from Writable
  public void readFields(DataInput in) throws IOException {
    byte version = in.readByte();                 // read version
    if (version != getVersion())
      throw new VersionMismatchException(getVersion(),version);
  }

    
}