java从system.in中读取写入txt中

我的程序如下（while循环一直跳不出去，求解）
import java.io.*;
import java.util.Scanner;

public class LineNumbering {
public static void main(String[] args) {
String a;
Scanner scanner = new Scanner(System.in);
a = scanner.nextLine();
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
FileWriter fw = null;

try{
File f = new File(a);
String str = null;
int n = 1;

fw = new FileWriter(f);

while ((str = br.readLine()) != null) {
fw.write(n + " " + str);
System.out.println(str);
n ++;
}

}
catch(Exception e){
e.printStackTrace();
}
finally
{
if(br!=null)
try{br.close();}catch (IOException e) {
e.printStackTrace();
}
if(fw!=null)
try{fw.close();}catch (IOException e) {
e.printStackTrace();
}
}

}

}

while (!"0".equals(str = br.readLine())) {
                fw.write(n + " " + str);
                System.out.println(str);
                n++;
            }
//空行并不是null啊，你要给一个明确的结束标志，我这种写法就是输入0就结束输入