在 Java 中可以使用 Apache HttpClient 或者 HttpURLConnection 來實現文件上傳到指定服務器，以下是兩種方法的示例：

使用 Apache HttpClient:

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.impl.client.HttpClients;

import java.io.File;
import java.io.IOException;

public class FileUploader {

    public static void main(String[] args) {
        HttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost("http://example.com/upload");

        File file = new File("path/to/file.txt");

        HttpEntity entity = MultipartEntityBuilder.create()
                .addBinaryBody("file", file, ContentType.DEFAULT_BINARY, file.getName())
                .build();

        httpPost.setEntity(entity);

        try {
            HttpResponse response = httpClient.execute(httpPost);
            System.out.println(response.getStatusLine());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

使用 HttpURLConnection:

import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

public class FileUploader {

    public static void main(String[] args) {
        String url = "http://example.com/upload";
        String fileName = "path/to/file.txt";

        try {
            URL serverUrl = new URL(url);
            HttpURLConnection connection = (HttpURLConnection) serverUrl.openConnection();
            connection.setRequestMethod("POST");
            connection.setDoOutput(true);
            connection.setRequestProperty("Content-Type", "multipart/form-data");

            File file = new File(fileName);
            FileInputStream fileInputStream = new FileInputStream(file);
            DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream());

            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = fileInputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }

            fileInputStream.close();
            outputStream.flush();
            outputStream.close();

            int responseCode = connection.getResponseCode();
            System.out.println("Server response code: " + responseCode);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

以上示例代碼演示了如何使用 Apache HttpClient 和 HttpURLConnection 來上傳文件到指定服務器。在實際應用中，需要根據服務器的接口要求和認證方式進行適當的配置。