how to use sun.net.ftp.FtpClient within jdk1.7 on Android platform

Question

I'm new here, and forgive me of my poor English. I want to ask a question about Android and ftp interface.

I use sun.net.ftp.FtpClient correctly in java. But when I copy the code to Android project, it doesn't work. However, the compiler doesn't report error. The error occurs at run-time.

In FTPUtils.java file, I use the following 3 packages:

import sun.net.TelnetOutputStream;  
import sun.net.ftp.FtpClient;  
import sun.net.ftp.FtpProtocolException; 

when I call:

FTPUtils ftpUtils = new FTPUtils();

it crashed.

Here are the stacktrace:

01-25 15:47:03.020: I/dalvikvm(16225): Could not find method sun.net.ftp.FtpClient.close, referenced from method com.cse.yunmixiang.yunapi.FTPUtils.closeConnect
01-25 15:47:03.020: W/dalvikvm(16225): VFY: unable to resolve virtual method 17959: Lsun/net/ftp/FtpClient;.close ()V
01-25 15:47:03.020: D/dalvikvm(16225): VFY: replacing opcode 0x6e at 0x0002
01-25 15:47:03.020: I/dalvikvm(16225): Could not find method sun.net.ftp.FtpClient.create, referenced from method com.cse.yunmixiang.yunapi.FTPUtils.connectServer
01-25 15:47:03.020: W/dalvikvm(16225): VFY: unable to resolve static method 17960: Lsun/net/ftp/FtpClient;.create (Ljava/lang/String;)Lsun/net/ftp/FtpClient;
01-25 15:47:03.020: D/dalvikvm(16225): VFY: replacing opcode 0x71 at 0x0002
01-25 15:47:03.030: I/dalvikvm(16225): Could not find method sun.net.ftp.FtpClient.getFileStream, referenced from method com.example.FTPUtils.getFileSize
...... similier errors ....

There are many functions in stacktrace:

FTPClient.close(); FtpClient.create(); FtpClient.getFileStream(); TelnetOutputStream.close();  Ftp.deleteFile();

I don't know why and how to solve it. Thank you for advance!

Answer 1

It's because Android SDK is not a 1:1 copy of Oracle JDK.

Packages in sun.* are proprietary and they only exist in Oracle's implementation of JVM, they're not a part of the spec and may get dropped from it any minute. Oracle's comment on this

To get FTP on Android you'll have to use a 3rd party lib.

Most common choice is Apache commons-net (no pun intended), another good option is ftp4j

Answer 2

You shouldn't use it on any platform. There is a specific warning in the Javadoc against using classes from the sun.* package. It's there to support FTP URLs used via the URL/URLConnection classes, and that is the only way you should use it. Or use the Apache or other FTP client.