如何用Java实现FTP服务器

FTP(File Transfer Protocol 文件传输协议)是Internet 上用来传送文件的协议。在Internet上通过FTP 服务器可以进行文件的上传(Upload)或下载(Download)。FTP是实时联机服务，在使用它之前必须是具有该服务的一个用户(用户名和口令)，工作时客户端必须先登录到作为服务器一方的计算机上，用户登录后可以进行文件搜索和文件传送等有关操作，如改变当前工作目录、列文件目录、设置传输参数及传送文件等。使用FTP可以传送所有类型的文件，如文本文件、二进制可执行文件、图象文件、声音文件和数据压缩文件等。

FTP 命令

FTP 的主要操作都是基于各种命令基础之上的。常用的命令有：

设置传输模式，它包括ASCⅡ(文本) 和BINARY 二进制模式

目录操作，改变或显示远程计算机的当前目录(cd、dir/ls 命令)

连接操作，open命令用于建立同远程计算机的连接；close命令用于关闭连接

发送操作，put命令用于传送文件到远程计算机；mput 命令用于传送多个文件到远程计算机

获取操作，get命令用于接收一个文件；mget命令用于接收多个文件。

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import java.net.Socketimport org.apache.log4j.Logger/** * 角色——服务器A * @author Leon * */public class ServerA{ public static void main(String[] args){final String F_DIR = "c:/test"//根路径final int PORT = 22//监听端口号Logger.getRootLogger() Logger logger = Logger.getLogger("com") try{ ServerSocket s = new ServerSocket(PORT) logger.info("Connecting to server A...") logger.info("Connected Successful! Local Port:"+s.getLocalPort()+". Default Directory:'"+F_DIR+"'.") while( true ){//接受客户端请求Socket client = s.accept() //创建服务线程new ClientThread(client, F_DIR).start() }} catch(Exception e) { logger.error(e.getMessage()) for(StackTraceElement ste : e.getStackTrace()){logger.error(ste.toString()) }} }}import java.io.BufferedReaderimport java.io.Fileimport java.io.FileNotFoundExceptionimport java.io.IOExceptionimport java.io.InputStreamimport java.io.InputStreamReaderimport java.io.OutputStreamimport java.io.PrintWriterimport java.io.RandomAccessFileimport java.net.ConnectExceptionimport java.net.InetAddressimport java.net.ServerSocketimport java.net.Socketimport java.net.UnknownHostExceptionimport java.nio.charset.Charsetimport java.util.Randomimport org.apache.log4j.Logger/** * 客户端子线程类 * @author Leon * */public class ClientThread extends Thread { private Socket socketClient//客户端socket private Logger logger//日志对象 private String dir//绝对路径 private String pdir = "/"//相对路径 private final static Random generator = new Random()//随机数 public ClientThread(Socket client, String F_DIR){this.socketClient = client this.dir = F_DIR } @Override public void run() {Logger.getRootLogger() logger = Logger.getLogger("com") InputStream is = null OutputStream os = null try { is = socketClient.getInputStream() os = socketClient.getOutputStream() } catch (IOException e) { logger.error(e.getMessage()) for(StackTraceElement ste : e.getStackTrace()){logger.error(ste.toString()) }}BufferedReader br = new BufferedReader(new InputStreamReader(is,Charset.forName("UTF-8"))) PrintWriter pw = new PrintWriter(os) String clientIp = socketClient.getInetAddress().toString().substring(1)//记录客户端IPString username = "not logged in"//用户名String password = ""//口令String command = ""//命令boolean loginStuts = false//登录状态final String LOGIN_WARNING = "530 Please log in with USER and PASS first." String str = ""//命令内容字符串int port_high = 0 int port_low = 0 String retr_ip = ""//接收文件的IP地址Socket tempsocket = null //打印欢迎信息pw.println("220-FTP Server A version 1.0 written by Leon Guo") pw.flush() logger.info("("+username+") ("+clientIp+")>Connected, sending welcome message...") logger.info("("+username+") ("+clientIp+")>220-FTP Server A version 1.0 written by Leon Guo") boolean b = true while ( b ){ try {//获取用户输入的命令command = br.readLine() if(null == command) break } catch (IOException e) {pw.println("331 Failed to get command") pw.flush() logger.info("("+username+") ("+clientIp+")>331 Failed to get command") logger.error(e.getMessage()) for(StackTraceElement ste : e.getStackTrace()){ logger.error(ste.toString()) }b = false } /* * 访问控制命令 */ // USER命令 if(command.toUpperCase().startsWith("USER")){logger.info("(not logged in) ("+clientIp+")>"+command) username = command.substring(4).trim()if("".equals(username)){ pw.println("501 Syntax error") pw.flush() logger.info("(not logged in) ("+clientIp+")>501 Syntax error") username = "not logged in" }else{ pw.println("331 Password required for " + username) pw.flush() logger.info("(not logged in) ("+clientIp+")>331 Password required for " + username) }loginStuts = false } //end USER // PASS命令 else if(command.toUpperCase().startsWith("PASS")){logger.info("(not logged in) ("+clientIp+")>"+command) password = command.substring(4).trim()if(username.equals("root") &&password.equals("root")){ pw.println("230 Logged on") pw.flush() logger.info("("+username+") ("+clientIp+")>230 Logged on")// logger.info("客户端 "+clientIp+" 通过 "+username+"用户登录") loginStuts = true }else{ pw.println("530 Login or password incorrect!") pw.flush() logger.info("(not logged in) ("+clientIp+")>530 Login or password incorrect!") username = "not logged in" } } //end PASS // PWD命令 else if(command.toUpperCase().startsWith("PWD")){logger.info("("+username+") ("+clientIp+")>"+command) if(loginStuts){// logger.info("用户"+clientIp+"："+username+"执行PWD命令") pw.println("257 /""+pdir+"/" is current directory") pw.flush() logger.info("("+username+") ("+clientIp+")>257 /""+pdir+"/" is current directory") }else{ pw.println(LOGIN_WARNING) pw.flush() logger.info("("+username+") ("+clientIp+")>"+LOGIN_WARNING) } } //end PWD // CWD命令 else if(command.toUpperCase().startsWith("CWD")){logger.info("("+username+") ("+clientIp+")>"+command) if(loginStuts){ str = command.substring(3).trim() if("".equals(str)){pw.println("250 Broken client detected, missing argument to CWD. /""+pdir+"/" is current directory.") pw.flush() logger.info("("+username+") ("+clientIp+")>250 Broken client detected, missing argument to CWD. /""+pdir+"/" is current directory.") } else{//判断目录是否存在String tmpDir = dir + "/" + str File file = new File(tmpDir) if(file.exists()){//目录存在 dir = dir + "/" + str if("/".equals(pdir)){pdir = pdir + str } else{pdir = pdir + "/" + str }// logger.info("用户"+clientIp+"："+username+"执行CWD命令") pw.println("250 CWD successful. /""+pdir+"/" is current directory") pw.flush() logger.info("("+username+") ("+clientIp+")>250 CWD successful. /""+pdir+"/" is current directory") }else{//目录不存在 pw.println("550 CWD failed. /""+pdir+"/": directory not found.") pw.flush() logger.info("("+username+") ("+clientIp+")>550 CWD failed. /""+pdir+"/": directory not found.")

多次需要把文件上传到单独的服务器，而程序是在单独的服务器上部署的，在进行文件操作的时候就需要跨服务器进行操作包括：文件上传、文件下载、文件删除等。跨服务器文件操作一般是需要FTP协议和SFTP协议两种，现在就通过Java实现FTP协议的文件上传。要实现FTP操作文件需要引入jar包： commons-net-1.4.1.jar

参考资料来源：百度贴吧

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