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FTP Java Client

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使用Apache Commons Net，从FTP服务器上下载文件。

需要使用的依赖：

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<dependency>
    <groupId>commons-net</groupId>
    <artifactId>commons-net</artifactId>
    <version>3.6</version>
</dependency>

有几点需要注意的：

设置编码需要在connect之前。
设置ftp模式需要在connect之后。

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package gy.finolo;

import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPFile;
import org.apache.commons.net.ftp.FTPReply;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class FTPUtils {

    private static Logger log = LoggerFactory.getLogger(FTPUtils.class);

    public static FTPClient loginFTP(String hostname, int port, String username, String password) throws IOException {

        FTPClient ftpClient = new FTPClient();
        // 中文文件读取支持, 必须在connect之前设置
        ftpClient.setControlEncoding("UTF-8");

        ftpClient.connect(hostname, port);
        // 被动模式设置, 必须在connect之后
        ftpClient.enterLocalPassiveMode();

        ftpClient.login(username, password);
        // 设置文件类型为二进制, 若不设置, 压缩文件可能失败
//        ftpClient.setFileType(FTPClient.BINARY_FILE_TYPE);

        if (FTPReply.isPositiveCompletion(ftpClient.getReplyCode())) {
            log.info("ftp connection succeed");
        } else {
            String message = "ftp server connection error: " + ftpClient.getReplyString();
            log.error(message);
            throw new IOException(message);
        }

        return ftpClient;
    }

    public static void downloadFromFTP(FTPClient ftpClient, String ftpFile, String outDir) throws IOException {
        // 在这个方法里把该转的路径都转为Path
        boolean isDirectory = ftpClient.changeWorkingDirectory(ftpFile);
        Path outDirPath = Paths.get(outDir);
        if (isDirectory) {
            downloadFTPDirectory2Directory(ftpClient, ftpFile, outDirPath);
        } else {
            downloadFTPFile2Directory(ftpClient, ftpFile, outDirPath);
        }
    }

    // downlaod directory from ftp to directory (outDir)
    private static void downloadFTPDirectory2Directory(FTPClient ftpClient, String ftpFileDir, Path outDirPath) throws IOException {
        // 保证输出文件夹要存在
        if (!Files.exists(outDirPath)) {
            Files.createDirectories(outDirPath);
        }

        FTPFile[] ftpFiles = ftpClient.listFiles(ftpFileDir);
        for (FTPFile file: ftpFiles) {
            if (file.isFile()) {
                // ftp file full path
                Path ftpFileFullPath = Paths.get(ftpFileDir).resolve(file.getName());
                downloadFTPFile2Directory(ftpClient, ftpFileFullPath.toString(), outDirPath);
                // download files under the directory to the outDir
            } else {
                String dirName = file.getName();
                Path subOutDirPath = outDirPath.resolve(dirName);
                Path subFtpDirPath = Paths.get(ftpFileDir).resolve(dirName);
                downloadFTPDirectory2Directory(ftpClient, subFtpDirPath.toString(), subOutDirPath);
            }
        }
    }

    // download ftp file (ftpFile) to directory (outDir)
    private static void downloadFTPFile2Directory(FTPClient ftpClient, String ftpFile, Path outDirPath) throws IOException {
        // 保证输出文件夹要存在
        if (!Files.exists(outDirPath)) {
            Files.createDirectories(outDirPath);
        }
        // ftpFile is full path
        try (InputStream is = ftpClient.retrieveFileStream(ftpFile)) {
            if (is != null) {
                Path ftpFileNamePath = Paths.get(ftpFile).getFileName();
                Path outPath = outDirPath.resolve(ftpFileNamePath);
                
                int bufSize = 8 * 1024;
                byte[] buffer = new byte[bufSize];
                int len;
                                    
                try (OutputStream os = Files.newOutputStream(outPath, StandardOpenOption.TRUNCATE_EXISTING,
                        StandardOpenOption.WRITE, StandardOpenOption.CREATE)) {
                    
                    while ((len = is.read(buffer)) > 0) {
                        os.write(buffer, 0, len);
                    }
                }
            }
        }
        ftpClient.completePendingCommand();
    }

    public static void main(String[] args) throws IOException {
        String hostname = "192.168.1.2";
        int port = 21;
        String username = "ftpuser";
        String password = "ftppass";
        
        FTPClient ftpClient;
        try {
            ftpClient = FTPUtils.loginFTP(hostname, port, username, password);
            downloadFromFTP(ftpClient, ".", "./dir");
            // log out and disconnect from the server
            ftpClient.logout();
            ftpClient.disconnect();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}
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