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From: bugzilla@apache.org
To: log4j-dev@logging.apache.org
Subject: DO NOT REPLY [Bug 44032]  - NullPointerException in
 RollingFileAppender
In-Reply-To: <bug-44032-31091@http.issues.apache.org/bugzilla/>
Message-Id: <20080115150358.C864A714204@brutus.apache.org>
Date: Tue, 15 Jan 2008 07:03:58 -0800 (PST)

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http://issues.apache.org/bugzilla/show_bug.cgi?id=44032


------- Additional Comments From peter@baxter-it.com  2008-01-15 07:03 -------
We had a similar problem.

The problem is that we use an AsynchAppender and a SocketAppender for logging.
Because of how AsynchAppender works, it is possible that 
ThrowableInformation.getThrowableStrRep() runs on one LoggingEvent at the same time.

  public
  String[] getThrowableStrRep() {
    if(rep != null) {
      return (String[]) rep.clone();
    } else {
      StringWriter sw = new StringWriter();
      PrintWriter pw = new PrintWriter(sw);
      throwable.printStackTrace(pw);
      pw.flush();
      LineNumberReader reader = new LineNumberReader(
              new StringReader(sw.toString()));
      ArrayList lines = new ArrayList();
      try {
        String line = reader.readLine();
        while(line != null) {
          lines.add(line);
          line = reader.readLine();
        }
      } catch(IOException ex) {
          lines.add(ex.toString());
      }
      rep = new String[lines.size()];
      lines.toArray(rep);
    }
    return rep;
  }

The problem is with ThrowableInformation.rep. 
If the first concurrent thread gets only to rep = new String[lines.size()];
and then the next concurrent thread gets to if(rep != null),
then only rep.clone(); is returned with an empty array.

Then later concurrent thread #1 will fill up the array but that's already late
for the another concurrent thread.

I think in this case the solution would be either to 

a) synchronize on ThrowableInformation.getThrowableStrRep() or 
b) modify
      rep = new String[lines.size()];
      lines.toArray(rep);

in method with

      String[] tempRep = new String[lines.size()];
      lines.toArray(tempRep);
      rep = tempRep;


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