Provide a debug trace of the stuff thats being written out into the DataOutputStream : FilterOutputStream « File « Java Tutorial

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/*
 * The contents of this file are subject to the terms 
 * 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 
 * own identifying information: Portions Copyright [yyyy] 
 * [name of copyright owner]
 */

/*
 * @(#)TraceOutputStream.java 1.5 05/08/29
 *
 * Copyright 1997-2005 Sun Microsystems, Inc. All Rights Reserved.
 */

// revised from sun mail util

import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * This class is a subclass of DataOutputStream that copies the
 * data being written into the DataOutputStream into another output
 * stream. This class is used here to provide a debug trace of the
 * stuff thats being written out into the DataOutputStream.
 *
 * @author John Mani
 */

public class TraceOutputStream extends FilterOutputStream {
    private boolean trace = false;
    private boolean quote = false;
    private OutputStream traceOut;

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

    /**
     * Set 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;
    }

    /**
     * Writes the specified <code>byte</code> to this output stream.
     * Writes out the byte into the trace stream if the trace mode
     * is <code>true</code>
     */
    public void write(int b) throws IOException {
  if (trace) {
      if (quote)
    writeByte(b);
      else
    traceOut.write(b);
  }
  out.write(b);
    }
      
    /**
     * Writes <code>b.length</code> bytes to this output stream.
     * Writes out the bytes into the trace stream if the trace
     * mode is <code>true</code>
     */
    public void write(byte b[], int off, int len) throws IOException {
  if (trace) {
      if (quote) {
    for (int i = 0; i < len; i++)
        writeByte(b[off + i]);
      } else
    traceOut.write(b, off, len);
  }
  out.write(b, off, len);
    }

    /**
     * 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);
  }
    }
}