common-net（http://commons.apache.org/proper/commons-net/）是一个比较实用的开源组件，可以基于common-net实现许多比较实用的功能，比如说ssh、ftp、smtp 等等，先来说说比较简单的ftp功能的实现。

    基于common-net实现ftp的功能，需要先获取FTPClient对象，利用FTPClient对象的public boolean retrieveFile(String remote, OutputStream local)方法实现文件下载功能，利用public boolean storeFile(String remote, InputStream local)方法实现文件上传功能。

    下面一个简单的demo：

import java.io.BufferedInputStream;  import java.io.BufferedOutputStream;  import java.io.File;  import java.io.FileInputStream;  import java.io.FileOutputStream;  import java.io.IOException;  import java.io.InputStream;  import java.io.OutputStream;  import java.net.SocketException;  import org.apache.commons.net.ftp.FTPClient;      public class FtpTest {      public FtpTest(String userName,String password,String ip){    this.userName = userName;    this.password = password;    this.ip = ip;   }      /**    * 上传文件    *     * @param File f 要上传的文件    * @param String uploadDir 上传文件的路径    *     * @return boolean b 上传结果    * */   public boolean putFile(File f,String uploadDir) {    InputStream instream = null;    boolean result = false;    try{     try{      ftpClient.changeWorkingDirectory(uploadDir);      instream = new BufferedInputStream(new FileInputStream(f));      result = ftpClient.storeFile(f.getName(), instream);     }finally{      if(instream!=null){       instream.close();      }     }    }catch(IOException e){     e.printStackTrace();    }        return result;   }      /**    * 从ftp服务器下载文件    *     * @param File f 要获取的ftp服务器上的文件    * @param String localPath 本地存放的路径    *     * @return boolean 文件下载是否成功    * */   public boolean getFile(File f  , String localPath){    OutputStream outStream = null;    boolean result = false;    try{     try{      outStream = new BufferedOutputStream(new FileOutputStream(new File(localPath)));      String path = f.getPath();      path = path.replaceAll("\\\\", "/");      String filepath = path.substring(0, path.lastIndexOf("/")+1)+"";      String fileName = path.substring(path.lastIndexOf("/")+1)+"";      boolean b = ftpClient.changeWorkingDirectory(filepath);      if(b){       result = ftpClient.retrieveFile(fileName, outStream);      }     }finally{      if(outStream != null){       outStream.close();      }     }    }catch(IOException e){     e.printStackTrace();    }    return result;   }   /**    * 获取ftp链接    *     * @return ftpClient    * */   public FTPClient getFTPClient(){        try {     ftpClient = new FTPClient();       ftpClient.connect(ip);     ftpClient.login(userName, password);    } catch (SocketException e) {     e.printStackTrace();    } catch (IOException e) {     e.printStackTrace();    }    return ftpClient;   }      /**    *  关闭ftpClient链接    *      *  @param FTPClient 要关闭的ftpClient对象    *      * */   public void closeFTPClient(FTPClient ftpClient){    try {     try{      ftpClient.logout();     }finally{      ftpClient.disconnect();     }    } catch (IOException e) {     e.printStackTrace();    }   }      /*ftp用户名*/   private String userName;   /*ftp密码*/   private String password;   /*ftp服务器ip*/   private String ip;   private FTPClient ftpClient;  }