DeferredFileOutputStream

/*
 * $Header: /home/cvs/jakarta-commons/fileupload/src/java/org/apache/commons/fileupload/DeferredFileOutputStream.java,v 1.2 2003/05/31 22:31:08 martinc Exp $
 * $Revision: 1.2 $
 * $Date: 2003/05/31 22:31:08 $
 *
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package com.streambus.setting.remoteserver;


import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;


/**
 * <p>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.</p>
 *
 * @author <a href="mailto:[e-mail]">Martin Cooper</a>
 *
 * @version $Id: DeferredFileOutputStream.java,v 1.2 2003/05/31 22:31:08 martinc 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 <code>memoryOutputStream</code> or
     * <code>diskOutputStream</code>.
     */
    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 <code>true</code> if the data is available in memory;
     *         <code>false</code> 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 <code>null</code>.
     *
     * @return The data for this output stream, or <code>null</code> 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 <code>File</code>, assuming
     * that the data was written to disk. If the data was retained in memory,
     * this method returns <code>null</code>.
     *
     * @return The file for this output stream, or <code>null</code> if no such
     *         file exists.
     */
    public File getFile()
    {
        return outputFile;
    }
}