java ftp 断点续传

Java中实现FTP断点续传可以使用Apache Commons Net库中的FTPClient类。以下是一个简单的示例代码：

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

import java.io.*;

public class FTPResumeExample {
    private static final String server = "ftp.example.com";
    private static final int port = 21;
    private static final String username = "username";
    private static final String password = "password";
    private static final String remoteFilePath = "/path/to/file";
    private static final String localFilePath = "/path/to/save/file";

    public static void main(String[] args) {
        FTPClient ftpClient = new FTPClient();
        try {
            ftpClient.connect(server, port);
            int replyCode = ftpClient.getReplyCode();
            if (!FTPReply.isPositiveCompletion(replyCode)) {
                System.out.println("FTP server refused connection.");
                return;
            }

            boolean success = ftpClient.login(username, password);
            if (!success) {
                System.out.println("Could not login to the FTP server.");
                return;
            }

            // 检查本地文件是否存在，如果存在，则断点续传
            File localFile = new File(localFilePath);
            long resumePosition = 0;
            if (localFile.exists()) {
                resumePosition = localFile.length();
                ftpClient.setRestartOffset(resumePosition);
            }

            // 下载文件
            OutputStream outputStream = new FileOutputStream(localFile, true);
            InputStream inputStream = ftpClient.retrieveFileStream(remoteFilePath);
            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }
            outputStream.close();
            inputStream.close();

            ftpClient.completePendingCommand();
            ftpClient.logout();
            ftpClient.disconnect();

            System.out.println("File downloaded successfully.");
        } catch (IOException e) {
            System.out.println("Error occurred: " + e.getMessage());
        }
    }
}

在上述示例代码中，我们使用`ftpClient.retrieveFileStream(remoteFilePath)`方法获取远程文件的输入流，并将其写入本地文件的输出流中。如果本地文件已存在，则通过`ftpClient.setRestartOffset(resumePosition)`方法设置断点续传的位置。

请根据实际情况修改示例代码中的服务器地址、端口、用户名、密码、远程文件路径和本地文件路径。另外，请确保项目中包含Apache Commons Net库的依赖。