Access the file through a class that contains a synchronized method to write to the file. Only one thread at a time will be able to execute the method.
I think that Singleton pattern would fit for your problem:
package com.test.singleton;
public class Singleton {
private static final Singleton inst= new Singleton();
private Singleton() {
super();
}
public synchronized void writeToFile(String str) {
// Do whatever
}
public Singleton getInstance() {
return inst;
}
}
Every time you need to write to your file, you only would have to call:
Singleton.getInstance().writeToFile("Hello!!");