storablewritechannel.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.*;

/**
* Writes ByteStorables to a given output channel.
*
* @author Martin Svensson
* @version 1.0
*/
public class StorableWriteChannel{
  
  private WritableByteChannel mWbc;
  private ByteBuffer mBuffer;



  /**
   * Initialize to write ByteStorables to a given channel. The
   * StorableWriteChannel allocates a ByteBuffer (4096) to
   * used for faster writing.
   *
   * @param wbc channel to write to
   * @see ByteBuffer
   */
  public StorableWriteChannel(WritableByteChannel wbc){
    mWbc = wbc;
    mBuffer = ByteBuffer.allocate(4096);
  }

  /**
   * Flushes and closes this channel.
   *
   * @exception IOException if an error occurs
   */
  public void close() throws IOException{
    flush();
    mWbc.close();
  }
  
  /**
   * Write a ByteStorable to the stream.
   *
   * @param tmp a <code>ByteStorable</code> value
   * @exception IOException if an error occurs
   */
  public void write(ByteStorable tmp) throws IOException{
    if(mBuffer.remaining() < tmp.byteSize()){
      mBuffer.flip();
      mWbc.write(mBuffer);
      if(mBuffer.capacity() < tmp.byteSize())
	mBuffer = ByteBuffer.allocate(tmp.byteSize());
      mBuffer.clear();
    }
    tmp.toBytes(mBuffer);
  }
  
  /**
   * Flushes any buffered content that has not yet been written.
   *
   * @exception IOException if an error occurs
   */
  public void flush() throws IOException{
    mBuffer.flip();
    mWbc.write(mBuffer);
    mBuffer.clear();
  }
}