New to Java

I am having trouble writing to a text file in my application. I have a class to write to the file:

public class writeText {
 public FileOutputStream out; 
 public PrintStream p; 

 public writeText() {
 }
 
 public void open(HttpServletRequest request) {
  HttpSession session = request.getSession(true);
  String filename="file.txt";
  try {
   out = new FileOutputStream(filename);
   p = new PrintStream(out);
  } catch (Exception e) { System.err.println("Error writing to file "+filename); }
 }
 
 public void print(String message) {
  p.println(message);
 }

 public static void main(String[] args) {
 }
}

My question is, how can I call 'print' from multiple classes?

I have created an instance of writeText in <Class1> and can open/write to the file, but if I try to write to it from <class2> I get a null pointer (as I haven't created an instance in this class). I know I could open it in each class and append to the file, but in some instances I cannot pass the request at the time I need it open it.

My aim is to be able to open the file as you start the application, then append to it from all the classes I have during the session. Am I going in the wrong direction with the code I have, and if so, can you please point me in the right direction.

Thanks.