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This file is included in the "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

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The source code

 * Copyright 2001-2004 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
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * See the License for the specific language governing permissions and
 * limitations under the License.




An output stream which will retain data in memory until a specified * threshold is reached, and only then commit it to disk. If the stream is * closed before the threshold is reached, the data will not be written to * disk at all.

* * @author Martin Cooper * * @version $Id:,v 1.2 2004/02/23 04:40:29 bayard Exp $ */ public class DeferredFileOutputStream extends ThresholdingOutputStream { // ----------------------------------------------------------- Data members /** * The output stream to which data will be written prior to the theshold * being reached. */ private ByteArrayOutputStream memoryOutputStream; /** * The output stream to which data will be written after the theshold is * reached. */ private FileOutputStream diskOutputStream; /** * The output stream to which data will be written at any given time. This * will always be one of memoryOutputStream or * diskOutputStream. */ private OutputStream currentOutputStream; /** * The file to which output will be directed if the threshold is exceeded. */ private File outputFile; // ----------------------------------------------------------- Constructors /** * Constructs an instance of this class which will trigger an event at the * specified threshold, and save data to a file beyond that point. * * @param threshold The number of bytes at which to trigger an event. * @param outputFile The file to which data is saved beyond the threshold. */ public DeferredFileOutputStream(int threshold, File outputFile) { super(threshold); this.outputFile = outputFile; memoryOutputStream = new ByteArrayOutputStream(threshold); currentOutputStream = memoryOutputStream; } // --------------------------------------- ThresholdingOutputStream methods /** * Returns the current output stream. This may be memory based or disk * based, depending on the current state with respect to the threshold. * * @return The underlying output stream. * * @exception IOException if an error occurs. */ protected OutputStream getStream() throws IOException { return currentOutputStream; } /** * Switches the underlying output stream from a memory based stream to one * that is backed by disk. This is the point at which we realise that too * much data is being written to keep in memory, so we elect to switch to * disk-based storage. * * @exception IOException if an error occurs. */ protected void thresholdReached() throws IOException { byte[] data = memoryOutputStream.toByteArray(); FileOutputStream fos = new FileOutputStream(outputFile); fos.write(data); diskOutputStream = fos; currentOutputStream = fos; memoryOutputStream = null; } // --------------------------------------------------------- Public methods /** * Determines whether or not the data for this output stream has been * retained in memory. * * @return true if the data is available in memory; * false otherwise. */ public boolean isInMemory() { return (!isThresholdExceeded()); } /** * Returns the data for this output stream as an array of bytes, assuming * that the data has been retained in memory. If the data was written to * disk, this method returns null. * * @return The data for this output stream, or null if no such * data is available. */ public byte[] getData() { if (memoryOutputStream != null) { return memoryOutputStream.toByteArray(); } return null; } /** * Returns the data for this output stream as a File, assuming * that the data was written to disk. If the data was retained in memory, * this method returns null. * * @return The file for this output stream, or null if no such * file exists. */ public File getFile() { return outputFile; } }
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