改了下, 看看是否读文件有异常
import java.io.*;

public class addLine {

public static void main(String[] args) throws IOException {
String s;

FileReader fr;
BufferedReader br;

FileWriter fw;
BufferedWriter bw;

if (args.length != 2) {
System.out.println("Usage: addLine SourceFilename TargetFilename");
return;
}
fr = new FileReader(args[ 0]);
fw = new FileWriter(args[ 1]);
try {
br = new BufferedReader(fr);
bw = new BufferedWriter(fw);
while ((s = br.readLine()) != null) {
bw.write(s);
bw.newLine();
bw.newLine();
}
bw.flush();
} catch (IOException e) {
System.out.println(e.getMessage());
return;
} finally {
fr.close();
fw.close();
}
}
}