当我尝试从FTP下载文件时引发NullPointerException

Question

我试图从FTP顺序下载两个文件。首先下载文件没有任何问题。但是当InputStream用ftpClient.retrieveFileStream()初始化时，第二次抛出NullPointerException。我试图改变序列，但它没有效果。文件存在并在FTP上可用。

CODE

FTPClient ftpClient; 

    @Override 
    protected void onPreExecute() { 
     String server = "xxxxxxxxxxxxxxxxxxxxx"; 
     int port = 21; 
     String user = "xxxxxx"; 
     String pass = "xxxxxx"; 
     long fileSize = 0; 

     try { 
      ftpClient = new FTPClient(); 
      ftpClient.connect(server, port); 
      ftpClient.login(user, pass); 
      ftpClient.enterLocalPassiveMode(); 
      fileSize += getFileSize(ftpClient, CARDS_DB); 
      fileSize += getFileSize(ftpClient, IMG_DB); 
      showDialog(PROGRESS_DLG_ID); 
      downloadProgress.setMax((int) fileSize); 
     } catch (Exception e) { 
      Log.e("ERROR", e.toString()); 
     } 
    } 

    @Override 
    protected Boolean doInBackground(String... strings) { 

     final String PATH = getExternalCacheDir().toString() + "/"; 
     int total = 0; 

     try { 
      downloadFile(ftpClient, PATH, CARDS_DB, total); 
      downloadFile(ftpClient, PATH, IMG_DB, total); 
     } catch (Exception e) { 
      Log.e("EXCEPTION", e.toString()); 
      return false; 
     } finally { 
      try { 
       if (ftpClient.isConnected()) { 
        ftpClient.logout(); 
        ftpClient.disconnect(); 
       } 
      } catch (IOException ex) { 
       ex.printStackTrace(); 
      } 
     } 

     return true; 
    } 

    private void downloadFile(FTPClient ftpClient, String PATH, String fileName, int total) { 
     try { 
      File downloadFile = new File(PATH + fileName); 
      OutputStream outputStream = new BufferedOutputStream(new FileOutputStream(downloadFile)); 
      InputStream inputStream; 
      inputStream = ftpClient.retrieveFileStream(fileName); //here i have exception at second method invocation. 

      byte[] bytesArray = new byte[8192]; 
      int bytesRead; 
      while ((bytesRead = inputStream.read(bytesArray)) != -1) { 
       total += bytesRead; 
       outputStream.write(bytesArray, 0, bytesRead); 
       publishProgress(total); 
      } 
      outputStream.close(); 
      inputStream.close(); 

     } catch (Exception ex) { 
      Log.d("ERROR", "Error: " + ex.getMessage()); 
      ex.printStackTrace(); 
     } 
    } 

logcat的：

10-13 14:51:29.519 W/System.err(9554): java.lang.NullPointerException 
10-13 14:51:29.529 W/System.err(9554): at com.FHS.ActivityMain$DBLoad.downloadFile(ActivityMain.java:170) 
10-13 14:51:29.529 W/System.err(9554): at com.FHS.ActivityMain$DBLoad.doInBackground(ActivityMain.java:141) 
10-13 14:51:29.529 W/System.err(9554): at com.FHS.ActivityMain$DBLoad.doInBackground(ActivityMain.java:102) 
10-13 14:51:29.529 W/System.err(9554): at android.os.AsyncTask$2.call(AsyncTask.java:185) 
10-13 14:51:29.529 W/System.err(9554): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306) 
10-13 14:51:29.529 W/System.err(9554): at java.util.concurrent.FutureTask.run(FutureTask.java:138) 
10-13 14:51:29.529 W/System.err(9554): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088) 
10-13 14:51:29.529 W/System.err(9554): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581) 
10-13 14:51:29.529 W/System.err(9554): at java.lang.Thread.run(Thread.java:1019) 

解决 问题解决了。我只需要在每次调用方法downloadFile（）时初始化FTPClient并在方法内部打开/关闭连接。

Answer 1

我认为这个问题是这行代码：

final String PATH = getExternalCacheDir().toString() + "/"; 

，因为该方法getExternalCacheDir()将，并仅返回你的缓存目录，如果缓存不是空。如果没有缓存，则不会有目录。

您可以改为使用getExternalFilesDir()。

参考：

• getExternalCacheDir()
• getExternalFilesDir()

编辑：请修改代码到初始化ftpClient到：

int reply; 
ftp.connect(server,port); 
reply = ftp.getReplyCode(); 

，并检查所述回复码。

Answer 2

我认为你的解决方案是掩盖你的问题，这是你没有在关闭你的流之前在downloadFile（）中调用ftpClient.completePendingCommand（）。