Question

This is a logging function which logs error stream from the execution of an external program. Everything works fine. But I do not want to generate the log file when there is no data in error stream. Currently it is creating zero size file. Please help.

FileOutputStream fos = new FileOutputStream(logFile);
PrintWriter pw = new PrintWriter(fos);

Process proc = Runtime.getRuntime().exec(externalProgram);

InputStreamReader isr = new InputStreamReader(proc.getErrorStream());
BufferedReader br = new BufferedReader(isr);
String line=null;
while ( (line = br.readLine()) != null)
{
   if (pw != null){
      pw.println(line);
      pw.flush(); 
   }
}

Thank you.

Answer 1

Simply defer the creating of the FileOutputStream and PrintWriter until you need it:

PrintWriter pw = null;

Process proc = Runtime.getRuntime().exec(externalProgram);

InputStreamReader isr = new InputStreamReader(proc.getErrorStream());
BufferedReader br = new BufferedReader(isr);
String line;
while ( (line = br.readLine()) != null)
{
   if (pw == null)
   {
      pw = new PrintWriter(new FileOutputStream(logFile));
   }
   pw.println(line);
   pw.flush(); 
}

Personally I’m not a big fan of PrintWriter – the fact that it just swallows all exceptions concerns me. I’d also use OutputStreamWriter so that you can explicitly specify the encoding. Anyway, that’s aside from the real question here.