storablereadchannel.java

实现数据库的storage manager 功能

/*
 * Copyright (c) 2000-2004, Rickard C鰏ter, Martin Svensson
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without 
 * modification, are permitted provided that the following conditions are met:
 * 
 * Redistributions of source code must retain the above copyright notice, 
 * this list of conditions and the following disclaimer. 
 * Redistributions in binary form must reproduce the above copyright notice, 
 * this list of conditions and the following disclaimer in the documentation 
 * and/or other materials provided with the distribution. 
 * Neither the name of SICS nor the names of its contributors 
 * may be used to endorse or promote products derived from this software 
 * without specific prior written permission. 
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
 * POSSIBILITY OF SUCH DAMAGE.
 *
 *
 */

package com.mellowtech.disc;

import java.nio.*;
import java.io.*;
import java.nio.channels.*;

/**
 * Reads ByteStorables from a given byte channel.
 *
 * @author Martin Svensson
 * @version 1.0
 */
public class StorableReadChannel{
  
  private ByteStorable mTemplate;
  private ReadableByteChannel mRbc;
  private ByteBuffer mBuffer;
  private boolean endOfChannel = false;

  /**
   * Initialize to read ByteStorables from a given channel using
   * the specified template.
   *
   * @param rbc channel to read from
   * @param template a ByteStorable template
   */
  public StorableReadChannel(ReadableByteChannel rbc, ByteStorable template){
    mTemplate = template;
    mRbc = rbc;
    mBuffer = ByteBuffer.allocate(4096);
    mBuffer.flip();
  }

  /**
   * Checks if this channel is open.
   *
   * @return true if this channel is open
   * @see ReadableByteChannel#isOpen()
   */
  public boolean isOpen(){
    return mRbc.isOpen();
  }
  
  /**
   * Closes this channel (and thus the supplied backing channel)
   *
   * @exception IOException if an error occurs
   *
   */
  public void close() throws IOException{
    mRbc.close();
  }
  
  /**
   * Retrieve the next ByteStorable in the channel.
   *
   * @return the next object or null if there are no more to read.
   * @exception IOException if an error occurs
   */
  public ByteStorable next() throws IOException{
    int read;
    int slack = ByteStorable.slackOrSize(mBuffer, mTemplate);
    if(slack <= 0){
      slack = Math.abs(slack);
      if(slack > mBuffer.capacity()){
	ByteBuffer tmp = ByteBuffer.allocate(slack);
	tmp.put(mBuffer);
	mBuffer = tmp;
      }
      else
	ByteStorable.copyToBeginning(mBuffer, slack);
      
      if(mRbc.read(mBuffer) < 0){
	endOfChannel = true;
	return null;
      }
      mBuffer.flip();
    }
    return mTemplate.fromBytes(mBuffer);
  }

  /**
   * Checks if more ByteStorables can be read from the channel.
   *
   * @return true if no more ByteStorables can be read.
   */
  public boolean isEndOfChannel(){
    return endOfChannel;
  }
}