datainputbuffer.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.*;


/** A reusable {@link DataInput} implementation that reads from an in-memory
 * buffer.
 *
 * <p>This saves memory over creating a new DataInputStream and
 * ByteArrayInputStream each time data is read.
 *
 * <p>Typical usage is something like the following:<pre>
 *
 * DataInputBuffer buffer = new DataInputBuffer();
 * while (... loop condition ...) {
 *   byte[] data = ... get data ...;
 *   int dataLength = ... get data length ...;
 *   buffer.reset(data, dataLength);
 *   ... read buffer using DataInput methods ...
 * }
 * </pre>
 *  
 * @author Doug Cutting
 */
public class DataInputBuffer extends DataInputStream {

  private static class Buffer extends ByteArrayInputStream {
    public Buffer() {
      super(new byte[] {});
    }

    public void reset(byte[] input, int start, int length) {
      this.buf = input;
      this.count = start+length;
      this.mark = start;
      this.pos = start;
    }

    public int getPosition() { return pos; }
    public int getLength() { return count; }
  }

  private Buffer buffer;
  
  /** Constructs a new empty buffer. */
  public DataInputBuffer() {
    this(new Buffer());
  }

  private DataInputBuffer(Buffer buffer) {
    super(buffer);
    this.buffer = buffer;
  }

  /** Resets the data that the buffer reads. */
  public void reset(byte[] input, int length) {
    buffer.reset(input, 0, length);
  }

  /** Resets the data that the buffer reads. */
  public void reset(byte[] input, int start, int length) {
    buffer.reset(input, start, length);
  }

  /** Returns the current position in the input. */
  public int getPosition() { return buffer.getPosition(); }

  /** Returns the length of the input. */
  public int getLength() { return buffer.getLength(); }

}