实现往一个文件以追加方式将数据写入到文件中 

File f = new File(fileName);


if(!f.exists()){

 f.createNewFile();

}

RandomAccessFile raf = new RandomAccessFile(f,"rw");

raf.seek(raf.length());

String msg = str + "\n";

raf.write(msg.getByte("utf-8"));

raf.close();