prime numbers

Class to write prime numbers into a file.
Suggestions how to optimize the code are welcome.

package at.ande.primenumbers;

import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.LineNumberReader;

public class primenumbers {

 public static void main(String[] args) throws IOException {
  int i = 3;
  boolean newMark = false;
  File output;
  if (!((output = new File("output.txt")).exists())) {
   output.createNewFile();
  }
  LineNumberReader lineReader = new LineNumberReader(new FileReader(output));
  int lineNumbers = 0;
  String sLast = null;
  while ((sLast = lineReader.readLine()) != null) {
   lineNumbers++;
   if (lineReader.getLineNumber() == 1) {
    lineReader.mark(65365);
   }
   i = Integer.parseInt(sLast);
  }
  FileWriter fw = new FileWriter("output.txt", true);
  if (lineNumbers == 0) {
   fw.write("3\n");
   fw.flush();
   lineReader.mark(65535);
  }
  while (true) {
   try {
    lineReader.reset();
   } catch (Exception e) {
    lineReader = new LineNumberReader(new FileReader(output));
    newMark = true;
   }
   i = i + 2;
   while (true) {
    String sPrime = lineReader.readLine();
    if (newMark && lineReader.getLineNumber() == 1) {
     lineReader.mark(65365);
     newMark = false;
    }
    int prime = 0;
    if (sPrime != null) {
     prime = Integer.parseInt(sPrime);
    }
    if (sPrime == null || i < (prime * prime)) {
     fw.write(i + "\n");
     fw.flush();
     break;
    }
    if (i % prime == 0) {
     break;
    }
   }
  }
 }
}

update2: catchblock for .insert()