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StreamGobbler.java

package com.trilead.ssh2;

import java.io.IOException;
import java.io.InputStream;
import java.io.InterruptedIOException;

/**
 * A <code>StreamGobbler</code> is an InputStream that uses an internal worker
 * thread to constantly consume input from another InputStream. It uses a buffer
 * to store the consumed data. The buffer size is automatically adjusted, if needed.
 * <p>
 * This class is sometimes very convenient - if you wrap a session's STDOUT and STDERR
 * InputStreams with instances of this class, then you don't have to bother about
 * the shared window of STDOUT and STDERR in the low level SSH-2 protocol,
 * since all arriving data will be immediatelly consumed by the worker threads.
 * Also, as a side effect, the streams will be buffered (e.g., single byte
 * read() operations are faster).
 * <p>
 * Other SSH for Java libraries include this functionality by default in
 * their STDOUT and STDERR InputStream implementations, however, please be aware
 * that this approach has also a downside:
 * <p>
 * If you do not call the StreamGobbler's <code>read()</code> method often enough
 * and the peer is constantly sending huge amounts of data, then you will sooner or later
 * encounter a low memory situation due to the aggregated data (well, it also depends on the Java heap size).
 * Joe Average will like this class anyway - a paranoid programmer would never use such an approach.
 * <p>
 * The term "StreamGobbler" was taken from an article called "When Runtime.exec() won't",
 * see http://www.javaworld.com/javaworld/jw-12-2000/jw-1229-traps.html.
 * 
 * @author Christian Plattner, plattner@trilead.com
 * @version $Id: StreamGobbler.java,v 1.1 2007/10/15 12:49:56 cplattne Exp $
 */

00036 public class StreamGobbler extends InputStream
{
00038       class GobblerThread extends Thread
      {
            public void run()
            {
                  byte[] buff = new byte[8192];

                  while (true)
                  {
                        try
                        {
                              int avail = is.read(buff);

                              synchronized (synchronizer)
                              {
                                    if (avail <= 0)
                                    {
                                          isEOF = true;
                                          synchronizer.notifyAll();
                                          break;
                                    }
                                    
                                    int space_available = buffer.length - write_pos;
                                    
                                    if (space_available < avail)
                                    {
                                          /* compact/resize buffer */

                                          int unread_size = write_pos - read_pos;
                                          int need_space = unread_size + avail;

                                          byte[] new_buffer = buffer;

                                          if (need_space > buffer.length)
                                          {
                                                int inc = need_space / 3;
                                                inc = (inc < 256) ? 256 : inc;
                                                inc = (inc > 8192) ? 8192 : inc;
                                                new_buffer = new byte[need_space + inc];
                                          }
                                          
                                          if (unread_size > 0)
                                                System.arraycopy(buffer, read_pos, new_buffer, 0, unread_size);

                                          buffer = new_buffer;
                                          
                                          read_pos = 0;
                                          write_pos = unread_size;
                                    }
                                    
                                    System.arraycopy(buff, 0, buffer, write_pos, avail);
                                    write_pos += avail;

                                    synchronizer.notifyAll();
                              }     
                        }
                        catch (IOException e)
                        {
                              synchronized (synchronizer)
                              {
                                    exception = e;
                                    synchronizer.notifyAll();
                                    break;
                              }
                        }
                  }
            }
      }

      private InputStream is;
      private final GobblerThread t;

      private final Object synchronizer = new Object();

      private boolean isEOF = false;
      private boolean isClosed = false;
      private IOException exception = null;

      private byte[] buffer = new byte[2048];
      private int read_pos = 0;
      private int write_pos = 0;

      public StreamGobbler(InputStream is)
      {
            this.is = is;
            t = new GobblerThread();
            t.setDaemon(true);
            t.start();
      }

      public int read() throws IOException
      {
            synchronized (synchronizer)
            {
                  if (isClosed)
                        throw new IOException("This StreamGobbler is closed.");

                  while (read_pos == write_pos)
                  {
                        if (exception != null)
                              throw exception;

                        if (isEOF)
                              return -1;

                        try
                        {
                              synchronizer.wait();
                        }
                        catch (InterruptedException e)
                        {
                    throw new InterruptedIOException();
                        }
                  }

                  int b = buffer[read_pos++] & 0xff;

                  return b;
            }
      }

      public int available() throws IOException
      {
            synchronized (synchronizer)
            {
                  if (isClosed)
                        throw new IOException("This StreamGobbler is closed.");

                  return write_pos - read_pos;
            }
      }

      public int read(byte[] b) throws IOException
      {
            return read(b, 0, b.length);
      }

      public void close() throws IOException
      {
            synchronized (synchronizer)
            {
                  if (isClosed)
                        return;
                  isClosed = true;
                  isEOF = true;
                  synchronizer.notifyAll();
                  is.close();
            }
      }

      public int read(byte[] b, int off, int len) throws IOException
      {
            if (b == null)
                  throw new NullPointerException();

            if ((off < 0) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0) || (off > b.length))
                  throw new IndexOutOfBoundsException();

            if (len == 0)
                  return 0;

            synchronized (synchronizer)
            {
                  if (isClosed)
                        throw new IOException("This StreamGobbler is closed.");

                  while (read_pos == write_pos)
                  {
                        if (exception != null)
                              throw exception;

                        if (isEOF)
                              return -1;

                        try
                        {
                              synchronizer.wait();
                        }
                        catch (InterruptedException e)
                        {
                    throw new InterruptedIOException();
                        }
                  }

                  int avail = write_pos - read_pos;

                  avail = (avail > len) ? len : avail;

                  System.arraycopy(buffer, read_pos, b, off, avail);

                  read_pos += avail;

                  return avail;
            }
      }
}

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