：Apache FTPClient操作“卡死”问题的分析和解决

    最近在和一个第三方的合作中不得已需要使用FTP文件接口。由于FTP Server由对方提供，而且双方背后各自的网络环境都很不单纯等等原因，造成测试环境无法模拟实际情况。测试环境中程序一切正常，但是在部署到生产环境之后发现FTP操作不规律性出现“卡死”现象：程序捕获不到任何异常一直卡着，导致轮巡无法正常工作（由于担心在轮巡时间间隔内处理不能完成，我没有采用类似quartz或者crontab的定时任务，而是采用while-true然后sleep的方式）。

    为了解决这个问题，我首先考虑的是对于FTPClient的使用上没有设置超时时间，于是设置了ConnectTimeout、DataTimeout、DefaultTimeout后在生产环境上继续观察，但是问题依旧没有解决。后来我有些怀疑FTPClient api本身是不是有什么问题，想实在不行自己实现一个超时机制吧，不过还是不甘心，还是想从FTPClient api本身去解决问题。又经过一翻研究之后发现：需要使用被动模式，以下摘抄别人的一段简单描述：
在项目中使用commons-net-3.0.1.jar实现FTP文件的下载，在windows xp上运行正常，但是放到linux上，却出现问题，程序运行到 FTPClient.listFiles()或者FTPClient.retrieveFile()方法时，就停止在那里，什么反应都没有，出现假死状态。google一把，发现很多人也出现了此类问题，最终在一个帖子里找到了解决办法。在调用这两个方法之前，调用FTPClient.enterLocalPassiveMode();这个方法的意思就是每次数据连接之前，ftp client告诉ftp server开通一个端口来传输数据。为什么要这样做呢，因为ftp server可能每次开启不同的端口来传输数据，但是在linux上，由于安全限制，可能某些端口没有开启，所以就出现阻塞。OK，问题解决。

    于是我回滚了之前的修改，改为被动模式（关于FTP主动/被动模式的解释，这里我不多说了，关注的朋友可以自己查阅）。但是问题依旧。于是能想到的就是最有的绝招：实在不行自己实现一个超时机制吧。经过一翻研究最简单的方式就是使用：Future解决：

public static void main(String[] args) throws InterruptedException, ExecutionException {
        final ExecutorService exec = Executors.newFixedThreadPool(1);

        Callable<String> call = new Callable<String>() {
            public String call() throws Exception {
                Thread.sleep(1000 * 5);
                return "线程执行完成.";
            }
        };

        try {
            Future<String> future = exec.submit(call);
            String obj = future.get(4 * 1000, TimeUnit.MILLISECONDS); // 任务处理超时时间设置
            System.out.println("任务成功返回:" + obj);
        } catch (TimeoutException ex) {
            System.out.println("处理超时啦....");
            ex.printStackTrace();
        } catch (Exception e) {
            System.out.println("处理失败.");
            e.printStackTrace();
        }
        // 关闭线程池
        exec.shutdown();
        
        System.out.println("完毕");
    }

当然了还有很多其他方式：
http://tech.sina.com.cn/s/2008-07-04/1051720260.shtml
http://itindex.net/blog/2010/08/11/1281486125717.html
http://darkmasky.iteye.com/blog/1115047
http://www.cnblogs.com/wasp520/archive/2012/07/06/2580101.html
http://coolxing.iteye.com/blog/1476289
http://www.cnblogs.com/chenying99/archive/2012/10/24/2737924.html

    虽然找到了终极的“必杀技”，但是此时我还是不甘心，还是想从FTPClient api本身去解决问题，但此时看来也别无它他法。只能试试：即设置被动模式又设置超时时间。经过实际测试，发现问题得以解决。下面把我的FTP工具类贴给大家分享，希望能帮到遇到同样问题的人。

import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPFile;
import org.apache.commons.net.ftp.FTPReply;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.List;

public class FtpUtil {
    public static final String ANONYMOUS_LOGIN = "anonymous";
    private FTPClient ftp;
    private boolean is_connected;

    public FtpUtil() {
        ftp = new FTPClient();
        is_connected = false;
    }
    
    public FtpUtil(int defaultTimeoutSecond, int connectTimeoutSecond, int dataTimeoutSecond){
        ftp = new FTPClient();
        is_connected = false;
        
        ftp.setDefaultTimeout(defaultTimeoutSecond * 1000);
        ftp.setConnectTimeout(connectTimeoutSecond * 1000);
        ftp.setDataTimeout(dataTimeoutSecond * 1000);
    }

    /**
     * Connects to FTP server.
     * 
     * @param host
     *            FTP server address or name
     * @param port
     *            FTP server port
     * @param user
     *            user name
     * @param password
     *            user password
     * @param isTextMode
     *            text / binary mode switch
     * @throws IOException
     *             on I/O errors
     */
    public void connect(String host, int port, String user, String password, boolean isTextMode) throws IOException {
        // Connect to server.
        try {
            ftp.connect(host, port);
        } catch (UnknownHostException ex) {
            throw new IOException("Can't find FTP server '" + host + "'");
        }

        // Check rsponse after connection attempt.
        int reply = ftp.getReplyCode();
        if (!FTPReply.isPositiveCompletion(reply)) {
            disconnect();
            throw new IOException("Can't connect to server '" + host + "'");
        }

        if (user == "") {
            user = ANONYMOUS_LOGIN;
        }

        // Login.
        if (!ftp.login(user, password)) {
            is_connected = false;
            disconnect();
            throw new IOException("Can't login to server '" + host + "'");
        } else {
            is_connected = true;
        }

        // Set data transfer mode.
        if (isTextMode) {
            ftp.setFileType(FTP.ASCII_FILE_TYPE);
        } else {
            ftp.setFileType(FTP.BINARY_FILE_TYPE);
        }
    }

    /**
     * Uploads the file to the FTP server.
     * 
     * @param ftpFileName
     *            server file name (with absolute path)
     * @param localFile
     *            local file to upload
     * @throws IOException
     *             on I/O errors
     */
    public void upload(String ftpFileName, File localFile) throws IOException {
        // File check.
        if (!localFile.exists()) {
            throw new IOException("Can't upload '" + localFile.getAbsolutePath() + "'. This file doesn't exist.");
        }

        // Upload.
        InputStream in = null;
        try {

            // Use passive mode to pass firewalls.
            ftp.enterLocalPassiveMode();

            in = new BufferedInputStream(new FileInputStream(localFile));
            if (!ftp.storeFile(ftpFileName, in)) {
                throw new IOException("Can't upload file '" + ftpFileName + "' to FTP server. Check FTP permissions and path.");
            }

        } finally {
            try {
                in.close();
            } catch (IOException ex) {
            }
        }
    }

    /**
     * Downloads the file from the FTP server.
     * 
     * @param ftpFileName
     *            server file name (with absolute path)
     * @param localFile
     *            local file to download into
     * @throws IOException
     *             on I/O errors
     */
    public void download(String ftpFileName, File localFile) throws IOException {
        // Download.
        OutputStream out = null;
        try {
            // Use passive mode to pass firewalls.
            ftp.enterLocalPassiveMode();

            // Get file info.
            FTPFile[] fileInfoArray = ftp.listFiles(ftpFileName);
            if (fileInfoArray == null) {
                throw new FileNotFoundException("File " + ftpFileName + " was not found on FTP server.");
            }

            // Check file size.
            FTPFile fileInfo = fileInfoArray[0];
            long size = fileInfo.getSize();
            if (size > Integer.MAX_VALUE) {
                throw new IOException("File " + ftpFileName + " is too large.");
            }

            // Download file.
            out = new BufferedOutputStream(new FileOutputStream(localFile));
            if (!ftp.retrieveFile(ftpFileName, out)) {
                throw new IOException("Error loading file " + ftpFileName + " from FTP server. Check FTP permissions and path.");
            }

            out.flush();
        } finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException ex) {
                }
            }
        }
    }

    /**
     * Removes the file from the FTP server.
     * 
     * @param ftpFileName
     *            server file name (with absolute path)
     * @throws IOException
     *             on I/O errors
     */
    public void remove(String ftpFileName) throws IOException {
        if (!ftp.deleteFile(ftpFileName)) {
            throw new IOException("Can't remove file '" + ftpFileName + "' from FTP server.");
        }
    }

    /**
     * Lists the files in the given FTP directory.
     * 
     * @param filePath
     *            absolute path on the server
     * @return files relative names list
     * @throws IOException
     *             on I/O errors
     */
    public List<String> list(String filePath) throws IOException {
        List<String> fileList = new ArrayList<String>();
        
        // Use passive mode to pass firewalls.
        ftp.enterLocalPassiveMode();
        
        FTPFile[] ftpFiles = ftp.listFiles(filePath);
        int size = (ftpFiles == null) ? 0 : ftpFiles.length;
        for (int i = 0; i < size; i++) {
            FTPFile ftpFile = ftpFiles[i];
            if (ftpFile.isFile()) {
                fileList.add(ftpFile.getName());
            }
        }
        
        return fileList;
    }

    /**
     * Sends an FTP Server site specific command
     * 
     * @param args
     *            site command arguments
     * @throws IOException
     *             on I/O errors
     */
    public void sendSiteCommand(String args) throws IOException {
        if (ftp.isConnected()) {
            try {
                ftp.sendSiteCommand(args);
            } catch (IOException ex) {
            }
        }
    }

    /**
     * Disconnects from the FTP server
     * 
     * @throws IOException
     *             on I/O errors
     */
    public void disconnect() throws IOException {

        if (ftp.isConnected()) {
            try {
                ftp.logout();
                ftp.disconnect();
                is_connected = false;
            } catch (IOException ex) {
            }
        }
    }

    /**
     * Makes the full name of the file on the FTP server by joining its path and
     * the local file name.
     * 
     * @param ftpPath
     *            file path on the server
     * @param localFile
     *            local file
     * @return full name of the file on the FTP server
     */
    public String makeFTPFileName(String ftpPath, File localFile) {
        if (ftpPath == "") {
            return localFile.getName();
        } else {
            String path = ftpPath.trim();
            if (path.charAt(path.length() - 1) != '/') {
                path = path + "/";
            }

            return path + localFile.getName();
        }
    }

    /**
     * Test coonection to ftp server
     * 
     * @return true, if connected
     */
    public boolean isConnected() {
        return is_connected;
    }

    /**
     * Get current directory on ftp server
     * 
     * @return current directory
     */
    public String getWorkingDirectory() {
        if (!is_connected) {
            return "";
        }

        try {
            return ftp.printWorkingDirectory();
        } catch (IOException e) {
        }

        return "";
    }

    /**
     * Set working directory on ftp server
     * 
     * @param dir
     *            new working directory
     * @return true, if working directory changed
     */
    public boolean setWorkingDirectory(String dir) {
        if (!is_connected) {
            return false;
        }

        try {
            return ftp.changeWorkingDirectory(dir);
        } catch (IOException e) {
        }

        return false;
    }

    /**
     * Change working directory on ftp server to parent directory
     * 
     * @return true, if working directory changed
     */
    public boolean setParentDirectory() {
        if (!is_connected) {
            return false;
        }

        try {
            return ftp.changeToParentDirectory();
        } catch (IOException e) {
        }

        return false;
    }

    /**
     * Get parent directory name on ftp server
     * 
     * @return parent directory
     */
    public String getParentDirectory() {
        if (!is_connected) {
            return "";
        }

        String w = getWorkingDirectory();
        setParentDirectory();
        String p = getWorkingDirectory();
        setWorkingDirectory(w);

        return p;
    }

    /**
     * Get directory contents on ftp server
     * 
     * @param filePath
     *            directory
     * @return list of FTPFileInfo structures
     * @throws IOException
     */
    public List<FfpFileInfo> listFiles(String filePath) throws IOException {
        List<FfpFileInfo> fileList = new ArrayList<FfpFileInfo>();
        
        // Use passive mode to pass firewalls.
        ftp.enterLocalPassiveMode();
        FTPFile[] ftpFiles = ftp.listFiles(filePath);
        int size = (ftpFiles == null) ? 0 : ftpFiles.length;
        for (int i = 0; i < size; i++) {
            FTPFile ftpFile = ftpFiles[i];
            FfpFileInfo fi = new FfpFileInfo();
            fi.setName(ftpFile.getName());
            fi.setSize(ftpFile.getSize());
            fi.setTimestamp(ftpFile.getTimestamp());
            fi.setType(ftpFile.isDirectory());
            fileList.add(fi);
        }

        return fileList;
    }

    /**
     * Get file from ftp server into given output stream
     * 
     * @param ftpFileName
     *            file name on ftp server
     * @param out
     *            OutputStream
     * @throws IOException
     */
    public void getFile(String ftpFileName, OutputStream out) throws IOException {
        try {
            // Use passive mode to pass firewalls.
            ftp.enterLocalPassiveMode();
            
            // Get file info.
            FTPFile[] fileInfoArray = ftp.listFiles(ftpFileName);
            if (fileInfoArray == null) {
                throw new FileNotFoundException("File '" + ftpFileName + "' was not found on FTP server.");
            }

            // Check file size.
            FTPFile fileInfo = fileInfoArray[0];
            long size = fileInfo.getSize();
            if (size > Integer.MAX_VALUE) {
                throw new IOException("File '" + ftpFileName + "' is too large.");
            }

            // Download file.
            if (!ftp.retrieveFile(ftpFileName, out)) {
                throw new IOException("Error loading file '" + ftpFileName + "' from FTP server. Check FTP permissions and path.");
            }

            out.flush();

        } finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException ex) {
                }
            }
        }
    }

    /**
     * Put file on ftp server from given input stream
     * 
     * @param ftpFileName
     *            file name on ftp server
     * @param in
     *            InputStream
     * @throws IOException
     */
    public void putFile(String ftpFileName, InputStream in) throws IOException {
        try {
            // Use passive mode to pass firewalls.
            ftp.enterLocalPassiveMode();
            
            if (!ftp.storeFile(ftpFileName, in)) {
                throw new IOException("Can't upload file '" + ftpFileName + "' to FTP server. Check FTP permissions and path.");
            }
        } finally {
            try {
                in.close();
            } catch (IOException ex) {
            }
        }
    }
}