String str="hello world!";
    FileWriter writer;
    try {
      writer = new FileWriter("E:/token.txt");
      writer.write(str);
      writer.flush();
      writer.close();
    } catch (IOException e) {
      e.printStackTrace();
    }

File txt=new File("E:/log1.txt");
       if(!txt.exists()){ 
         txt.createNewFile(); 
      } 
       byte bytes[]=new byte[512];  
       bytes=str.getBytes(); 
       int b=bytes.length;  //是字节的长度，不是字符串的长度
       FileOutputStream fos=new FileOutputStream(txt); 
       fos.write(bytes,0,b); 
       fos.write(bytes);
       fos.close();

FileOutputStream fos = new FileOutputStream("E:/log.txt",true);
//true表示在文件末尾追加 
fos.write(log.getBytes()); 
fos.close();