A trace of the data that is being retrieved from an input stream : FilterInputStream « File « Java Tutorial

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 * of the Common Development and Distribution License 
 * (the "License").  You may not use this file except 
 * in compliance with the License.
 * 
 * You can obtain a copy of the license at 
 * glassfish/bootstrap/legal/CDDLv1.0.txt or 
 * https://glassfish.dev.java.net/public/CDDLv1.0.html. 
 * See the License for the specific language governing 
 * permissions and limitations under the License.
 * 
 * When distributing Covered Code, include this CDDL 
 * HEADER in each file and include the License file at 
 * glassfish/bootstrap/legal/CDDLv1.0.txt.  If applicable, 
 * add the following below this CDDL HEADER, with the 
 * fields enclosed by brackets "[]" replaced with your 
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/*
 * @(#)TraceInputStream.java  1.7 05/08/29
 *
 * Copyright 1997-2005 Sun Microsystems, Inc. All Rights Reserved.
 */

//Revised from sun mail util

import java.io.*;

/**
 * This class is a FilterInputStream that writes the bytes
 * being read from the given input stream into the given output
 * stream. This class is typically used to provide a trace of
 * the data that is being retrieved from an input stream.
 *
 * @author John Mani
 */

public class TraceInputStream extends FilterInputStream {
    private boolean trace = false;
    private boolean quote = false;
    private OutputStream traceOut;

    /**
     * Creates an input stream filter built on top of the specified
     * input stream.
     *   
     * @param   in   the underlying input stream.
     * @param   out  the trace stream
     */
    public TraceInputStream(InputStream in, OutputStream traceOut) {
  super(in);
  this.traceOut = traceOut;
    }

    /**
     * Set trace mode.
     * @param trace the trace mode
     */
    public void setTrace(boolean trace) {
  this.trace = trace;
    }

    /**
     * Set quote mode.
     * @param quote the quote mode
     */
    public void setQuote(boolean quote) {
  this.quote = quote;
    }

    /**
     * Reads the next byte of data from this input stream. Returns
     * <code>-1</code> if no data is available. Writes out the read
     * byte into the trace stream, if trace mode is <code>true</code>
     */
    public int read() throws IOException {
  int b = in.read();
  if (trace && b != -1) {
      if (quote)
    writeByte(b);
      else
    traceOut.write(b);
  }
  return b;
    }

    /**
     * Reads up to <code>len</code> bytes of data from this input stream
     * into an array of bytes. Returns <code>-1</code> if no more data
     * is available. Writes out the read bytes into the trace stream, if 
     * trace mode is <code>true</code>
     */
    public int read(byte b[], int off, int len) throws IOException {
  int count = in.read(b, off, len);
  if (trace && count != -1) {
      if (quote) {
    for (int i = 0; i < count; i++)
        writeByte(b[off + i]);
      } else
    traceOut.write(b, off, count);
  }
  return count;
    }

    /**
     * Write a byte in a way that every byte value is printable ASCII.
     */
    private final void writeByte(int b) throws IOException {
  b &= 0xff;
  if (b > 0x7f) {
      traceOut.write('M');
      traceOut.write('-');
      b &= 0x7f;
  }
  if (b == '\r') {
      traceOut.write('\\');
      traceOut.write('r');
  } else if (b == '\n') {
      traceOut.write('\\');
      traceOut.write('n');
      traceOut.write('\n');
  } else if (b == '\t') {
      traceOut.write('\\');
      traceOut.write('t');
  } else if (b < ' ') {
      traceOut.write('^');
      traceOut.write('@' + b);
  } else {
      traceOut.write(b);
  }
    }
}