java ftp 多线程下载文件

以下是Java实现FTP多线程下载文件的示例代码：

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

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class FTPMultiThreadDownload {
    private static final String FTP_SERVER = "ftp.example.com";
    private static final int FTP_PORT = 21;
    private static final String FTP_USERNAME = "username";
    private static final String FTP_PASSWORD = "password";
    private static final String REMOTE_FILE_PATH = "/path/to/remote/file";
    private static final String LOCAL_FILE_PATH = "/path/to/local/file";

    public static void main(String[] args) {
        FTPClient ftpClient = new FTPClient();
        try {
            ftpClient.connect(FTP_SERVER, FTP_PORT);
            ftpClient.login(FTP_USERNAME, FTP_PASSWORD);
            ftpClient.enterLocalPassiveMode();
            ftpClient.setFileType(FTP.BINARY_FILE_TYPE);

            int threadCount = 5; // 设置线程数量
            ExecutorService executorService = Executors.newFixedThreadPool(threadCount);

            long fileSize = ftpClient.listFiles(REMOTE_FILE_PATH)[0].getSize();
            long partSize = fileSize / threadCount;

            for (int i = 0; i < threadCount; i++) {
                long startPos = i * partSize;
                long endPos = (i + 1) * partSize - 1;
                if (i == threadCount - 1) {
                    endPos = fileSize - 1;
                }

                executorService.execute(new DownloadThread(ftpClient, REMOTE_FILE_PATH, LOCAL_FILE_PATH, startPos, endPos));
            }

            executorService.shutdown();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ftpClient.isConnected()) {
                    ftpClient.logout();
                    ftpClient.disconnect();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private static class DownloadThread implements Runnable {
        private FTPClient ftpClient;
        private String remoteFilePath;
        private String localFilePath;
        private long startPos;
        private long endPos;

        public DownloadThread(FTPClient ftpClient, String remoteFilePath, String localFilePath, long startPos, long endPos) {
            this.ftpClient = ftpClient;
            this.remoteFilePath = remoteFilePath;
            this.localFilePath = localFilePath;
            this.startPos = startPos;
            this.endPos = endPos;
        }

        @Override
        public void run() {
            OutputStream outputStream = null;
            try {
                outputStream = new FileOutputStream(localFilePath, true);
                ftpClient.setRestartOffset(startPos);
                ftpClient.retrieveFile(remoteFilePath, outputStream);
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    if (outputStream != null) {
                        outputStream.close();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

请注意，上述代码使用了Apache Commons Net库来处理FTP操作。在代码中，我们首先建立与FTP服务器的连接，然后设置登录凭据和文件类型。接下来，我们确定要下载的文件的大小，并根据线程数量将文件分成多个部分。然后，我们使用线程池来创建并执行多个下载线程，每个线程负责下载文件的一个部分。每个线程都会将其下载的部分写入本地文件。最后，我们关闭连接和线程池。