java.lang.NullPointerException：println需要一條FTP下載的消息

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我正在從FTP服務器上下載文件，但是我得到了一個NullPointerException。我究竟做錯了什麼？java.lang.NullPointerException：println需要一條FTP下載的消息

public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 

    ftpConnect("domain","username", "password", portnum); 

    try 
    { 
     String s = Environment.getExternalStorageDirectory().toString(); 
     File f = new File(s+"/???/item"); 
     f.mkdirs(); 
     ftpDownload("ftp://[email protected]?????.com/rooms.txt", f); 
    } 
    catch(Exception e) 
    { 
     Toast toast = Toast.makeText(getApplicationContext(), "Download error: "+e.getMessage(), Toast.LENGTH_SHORT); 
     toast.show(); 
    } 

    ftpDisconnect(); 
} 

public boolean ftpDownload(String srcFilePath, File desFilePath) 
{ 
    boolean status = false; 
    try { 
     FileOutputStream desFileStream = new FileOutputStream(desFilePath); 

     status = mFTPClient.retrieveFile(srcFilePath, desFileStream); 
     desFileStream.close(); 
     Toast toast = Toast.makeText(getApplicationContext(), "Downloaded", Toast.LENGTH_SHORT); 
     toast.show(); 

     return status; 
     } catch (Exception e) { 
     Toast toast = Toast.makeText(getApplicationContext(), "Download error"+e.getMessage(), Toast.LENGTH_SHORT); 
     toast.show(); 
    } 

    return status; 
} 

public boolean ftpConnect(String host, String username, 
     String password, int port) 
{ 
    try{ 
     mFTPClient = new FTPClient(); 
     // connecting to the host 

      mFTPClient.connect(host, port); 


     // now check the reply code, if positive mean connection success 
     if (FTPReply.isPositiveCompletion(mFTPClient.getReplyCode())) { 
      // login using username & password 
      boolean status = mFTPClient.login(username, password); 

      /* Set File Transfer Mode 
      * 
      * To avoid corruption issue you must specified a correct 
      * transfer mode, such as ASCII_FILE_TYPE, BINARY_FILE_TYPE, 
      * EBCDIC_FILE_TYPE .etc. Here, I use BINARY_FILE_TYPE 
      * for transferring text, image, and compressed files. 
      */ 

      mFTPClient.setFileType(FTP.BINARY_FILE_TYPE); 
      mFTPClient.enterLocalPassiveMode(); 

      return status; 
     } 
    } catch(Exception e) { 
     Log.e("ERROR", e.getMessage()); 
    } 
    return false; 
} 
}

來源

2013-04-23 user1900738

我們需要一個堆棧跟蹤...告訴我們哪一行出錯了。 – 2013-04-23 13:39:48

'我在做什麼錯誤？'...第一：你在主線程上做網絡相關的東西... – Selvin 2013-04-23 13:43:02

如果傳遞的消息爲空，Log.e會拋出異常。確保'e.getMessage（）'不爲空 – 2013-04-23 13:43:50

嘗試使用這樣的：

Log.e("ERROR", String.valueOf(e.getMessage()));

這樣一來，萬一消息爲空，你會得到一個「錯誤」的輸出，「空」。

此外，你應該明智地傾聽那些建議你在主線程上做與網絡相關的東西的人。

來源

2013-04-23 13:51:26 npace

java.lang.NullPointerException：println需要一條FTP下載的消息

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