You need to tell it to allow a privileged action. Using the AccessController. Try this:
public void start() {
AccessController.doPrivileged(new PrivilegedAction<Object>(){
@Override
public Object run() {
try{
BufferedReader br=new BufferedReader(new FileReader(new File("database.txt")));
BufferedWriter br2=new BufferedWriter(new FileWriter(new File("database1.txt")));
String st;
while((st=br.readLine())!=null){
String []str=st.split(";");
br2.write(st+"\n");
}
}
catch(IOException e){}
//catch your exceptions
}
}
});
}