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SubEtha SMTP is an easy-to-use server-side SMTP library for Java.
There is a newer version: 3.1.7
/*
 * $Id: DeferredFileOutputStream.java 337 2009-06-29 19:20:58Z latchkey $
 * $URL: https://subethasmtp.googlecode.com/svn/trunk/src/org/subethamail/smtp/io/DeferredFileOutputStream.java $
 */
package org.subethamail.smtp.io;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * This works like a ByteArrayOutputStream until a certain size is
 * reached, then creates a temp file and acts like a buffered
 * FileOutputStream.  The data can be retreived afterwards by
 * calling getInputStream().
 *
 * When this object is closed, the temporary file is deleted.  You
 * can no longer call getInputStream().
 *
 * @author Jeff Schnitzer
 */
public class DeferredFileOutputStream extends ThresholdingOutputStream
{
	/**
	 * Initial size of the byte array buffer.  Better to make this
	 * large to start with so that we can avoid reallocs; mail
	 * messages are rarely tiny.
	 */
	static final int INITIAL_BUF_SIZE = 8192;

	/** */
	public static final String TMPFILE_PREFIX = "subetha";
	public static final String TMPFILE_SUFFIX = ".msg";

	/** If we switch to file output, this is the file. */
	File outFile;

	/** If we switch to file output, this is the stream. */
	FileOutputStream outFileStream;

	/** When the output stream is closed, this becomes true */
	boolean closed;

	boolean thresholdReached = false;

	/**
	 * @param transitionSize is the number of bytes at which to convert
	 *  from a byte array to a real file.
	 */
	public DeferredFileOutputStream(int transitionSize)
	{
		super(new BetterByteArrayOutputStream(INITIAL_BUF_SIZE), transitionSize);
	}

	/*
	 * (non-Javadoc)
	 * @see org.subethamail.common.io.ThresholdingOutputStream#thresholdReached(int, int)
	 */
	@Override
	protected void thresholdReached(int current, int predicted) throws IOException
	{
		// Open a temp file, write the byte array version, and swap the
		// output stream to the file version.

		this.outFile = File.createTempFile(TMPFILE_PREFIX, TMPFILE_SUFFIX);
		this.outFileStream = new FileOutputStream(this.outFile);

		((ByteArrayOutputStream)this.output).writeTo(this.outFileStream);
		this.output = new BufferedOutputStream(this.outFileStream);
	}

	/**
	 * Closes the output stream and creates an InputStream on the same data.
	 *
	 * @return either a BetterByteArrayOutputStream or buffered FileInputStream,
	 *  depending on what state we are in.
	 */
	public InputStream getInputStream() throws IOException
	{
		if (this.output instanceof BetterByteArrayOutputStream)
		{
			return ((BetterByteArrayOutputStream)this.output).getInputStream();
		}
		else
		{
			if (!this.closed)
			{
				this.output.flush();
				this.output.close();
				this.closed = true;
			}

			return new BufferedInputStream(new FileInputStream(this.outFile));
		}
	}

	/* (non-Javadoc)
	 * @see org.subethamail.common.io.ThresholdingOutputStream#close()
	 */
	@Override
	public void close() throws IOException
	{
		if (!this.closed)
		{
			this.output.flush();
			this.output.close();
			this.closed = true;
		}

		if (this.outFile != null)
			this.outFile.delete();
	}

}