多线程实现ftp 文件下载并且打印文件下载进度Java

好的，下面是一个简单的Java代码示例，用于实现FTP文件下载并打印下载进度：

import java.io.*;
import org.apache.commons.net.ftp.*;
import java.util.concurrent.*;

public class FTPDownload {
    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 FTP_DIRECTORY = "/path/to/file";
    private static final String REMOTE_FILENAME = "file.txt";
    private static final String LOCAL_FILENAME = "local_file.txt";

    public static void main(String[] args) {
        FTPClient ftp = new FTPClient();
        try {
            ftp.connect(FTP_SERVER, FTP_PORT);
            ftp.login(FTP_USERNAME, FTP_PASSWORD);
            ftp.changeWorkingDirectory(FTP_DIRECTORY);

            long remoteFileSize = ftp.listFiles(REMOTE_FILENAME)[0].getSize();

            FileOutputStream fos = new FileOutputStream(new File(LOCAL_FILENAME));
            ProgressMonitorInputStream pmis = new ProgressMonitorInputStream(null, "Downloading " + REMOTE_FILENAME, ftp.retrieveFileStream(REMOTE_FILENAME));
            ProgressMonitor progressMonitor = pmis.getProgressMonitor();
            progressMonitor.setMaximum((int) remoteFileSize);
            progressMonitor.setMillisToPopup(1000);
            progressMonitor.setMillisToDecideToPopup(1000);

            ExecutorService executor = Executors.newSingleThreadExecutor();
            executor.submit(() -> {
                byte[] buffer = new byte[1024];
                int bytesRead = 0;
                while ((bytesRead = pmis.read(buffer)) != -1) {
                    fos.write(buffer, 0, bytesRead);
                    progressMonitor.setProgress(progressMonitor.getProgress() + bytesRead);
                }
                fos.close();
            });
            executor.shutdown();
            executor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);

            ftp.logout();
            ftp.disconnect();
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在上述代码中，我们首先定义了要连接的FTP服务器的地址、端口号、用户名、密码和文件路径等信息。

然后，我们使用Apache Commons Net库中的FTPClient类来建立FTP连接，并获取要下载的文件的大小。

接下来，我们创建一个ProgressMonitorInputStream对象来实现进度条的显示，并使用Executor框架来启动下载线程。

最后，我们下载文件并在屏幕上打印下载进度。

需要注意的是，在Java中，线程操作要格外小心，需要确保线程安全和正确关闭。