在Java中，你可以使用Apache HttpClient庫來實現POST上傳附件。以下是一個示例代碼：

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.mime.MultipartEntityBuilder;
import org.apache.http.entity.mime.content.FileBody;
import org.apache.http.impl.client.HttpClients;

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

public class FileUploader {
    public static void main(String[] args) {
        String url = "http://example.com/upload"; // 上傳接口的URL
        
        File file = new File("path/to/file"); // 要上傳的文件路徑

        HttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost(url);

        MultipartEntityBuilder entityBuilder = MultipartEntityBuilder.create();
        entityBuilder.addPart("file", new FileBody(file)); // 設置文件參數

        HttpEntity entity = entityBuilder.build();
        httpPost.setEntity(entity);

        try {
            HttpResponse response = httpClient.execute(httpPost);
            // 處理上傳結果
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

在示例代碼中，首先創建一個HttpClient實例和一個HttpPost實例來執行POST請求。然后，使用MultipartEntityBuilder創建一個HttpEntity對象，并通過addPart方法添加要上傳的文件。最后，將HttpEntity對象設置到HttpPost中，并執行請求。

請注意替換url和file變量為你自己的實際值。