.NET使用预签名URL下载_对象存储(OSS)-阿里云帮助中心

默认情况下，OSS Bucket中的文件是私有的，仅文件拥有者可访问。您可以使用OSS .NET SDK生成带有过期时间的GET方法预签名URL，以允许他人临时下载文件。在有效期内可多次访问，超期后需重新生成。

注意事项

本文以华东1（杭州）外网Endpoint为例。如果您希望通过与OSS同地域的其他阿里云产品访问OSS，请使用内网Endpoint。关于OSS支持的Region与Endpoint的对应关系，请参见OSS地域和访问域名。
本文以从环境变量读取访问凭证为例。如何配置访问凭证，请参见配置访问凭证。
生成GET方法的预签名URL时，您必须具有oss:GetObject权限。具体操作，请参见为RAM用户授权自定义的权限策略。
说明
生成预签名URL过程中，SDK利用本地存储的密钥信息，根据特定算法计算出签名（signature），然后将其附加到URL上，以确保URL的有效性和安全性。这一系列计算和构造URL的操作都是在客户端完成，不涉及网络请求到服务端。因此，生成预签名URL时不需要授予调用者特定权限。但是，为避免第三方用户无法对预签名URL授权的资源执行相关操作，需要确保调用生成预签名URL接口的身份主体被授予对应的权限。
本文以V4预签名URL为例，有效期最大为7天。更多信息，请参见签名版本4（推荐）。
通过以下示例生成的预签名URL中如果包含特殊符号+，可能出现无法正常访问该预签名URL的现象。如需正常访问该预签名URL，请将预签名URL中的+替换为%2B。
如果需要生成HTTPS协议的预签名URL，请将Endpoint中的通信协议设置为HTTPS。

使用过程

使用预签名URL下载文件的过程如下：

生成预签名URL并通过预签名URL下载文件

生成用于下载的预签名URL

using Aliyun.OSS;
using Aliyun.OSS.Common;
// 填写Bucket所在地域对应的Endpoint。以华东1（杭州）为例，Endpoint填写为https://oss-cn-hangzhou.aliyuncs.com。
var endpoint = "https://oss-cn-hangzhou.aliyuncs.com";
// 从环境变量中获取访问凭证。运行本代码示例之前，请确保已设置环境变量OSS_ACCESS_KEY_ID和OSS_ACCESS_KEY_SECRET。
var accessKeyId = Environment.GetEnvironmentVariable("OSS_ACCESS_KEY_ID");
var accessKeySecret = Environment.GetEnvironmentVariable("OSS_ACCESS_KEY_SECRET");
// 填写Bucket名称，例如examplebucket。
var bucketName = "examplebucket";
// 填写Object完整路径，完整路径中不包含Bucket名称，例如exampledir/exampleobject.txt。
var objectName = "exampledir/exampleobject.txt";
// 填写Object下载到本地文件的完整路径。例如D:\\localpath\\examplefile.txt。如果指定的本地文件存在会覆盖，不存在则新建。
var downloadFilename = "D:\\localpath\\examplefile.txt";
// 填写Bucket所在地域对应的Region。以华东1（杭州）为例，Region填写为cn-hangzhou。
const string region = "cn-hangzhou";

// 创建ClientConfiguration实例，按照您的需要修改默认参数。
var conf = new ClientConfiguration();

// 设置v4签名。
conf.SignatureVersion = SignatureVersion.V4;

// 创建OssClient实例。
var client = new OssClient(endpoint, accessKeyId, accessKeySecret, conf);
conf.SetRegion(region);
try
{
    var metadata = client.GetObjectMetadata(bucketName, objectName);
    var etag = metadata.ETag;
    // 生成预签名URL。
    var req = new GeneratePresignedUriRequest(bucketName, objectName, SignHttpMethod.Get)
      {
        // 设置预签名URL过期时间，默认值为3600秒。
        Expiration = DateTime.Now.AddHours(1),
    };
    var uri = client.GeneratePresignedUri(req);
}
catch (OssException ex)
{
    Console.WriteLine("Failed with error code: {0}; Error info: {1}. \nRequestID:{2}\tHostID:{3}",
        ex.ErrorCode, ex.Message, ex.RequestId, ex.HostId);
}
catch (Exception ex)
{
    Console.WriteLine("Failed with error info: {0}", ex.Message);
}

其他人使用GET方法的预签名URL下载文件。

curl

Java

Node.js

Python

JavaScript

Android-Java

Objective-C

curl -SO "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************"

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class Demo {
    public static void main(String[] args) {
        // 替换为生成的GET方法的预签名URL。
        String fileURL = "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************";
        // 填写文件保存的目标路径，包括文件名和扩展名。
        String savePath = "C:/downloads/myfile.txt";

        try {
            downloadFile(fileURL, savePath);
            System.out.println("Download completed!");
        } catch (IOException e) {
            System.err.println("Error during download: " + e.getMessage());
        }
    }

    private static void downloadFile(String fileURL, String savePath) throws IOException {
        URL url = new URL(fileURL);
        HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setRequestMethod("GET");

        // 检查响应代码
        int responseCode = httpConn.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            // 输入流
            InputStream inputStream = new BufferedInputStream(httpConn.getInputStream());
            // 输出流
            FileOutputStream outputStream = new FileOutputStream(savePath);

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

            outputStream.close();
            inputStream.close();
        } else {
            System.out.println("No file to download. Server replied HTTP code: " + responseCode);
        }
        httpConn.disconnect();
    }
}

const https = require('https');
const fs = require('fs');

const fileURL = "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************";
const savePath = "C:/downloads/myfile.txt";

https.get(fileURL, (response) => {
    if (response.statusCode === 200) {
        const fileStream = fs.createWriteStream(savePath);
        response.pipe(fileStream);
        
        fileStream.on('finish', () => {
            fileStream.close();
            console.log("Download completed!");
        });
    } else {
        console.error(`Download failed. Server responded with code: ${response.statusCode}`);
    }
}).on('error', (err) => {
    console.error("Error during download:", err.message);
});

import requests

file_url = "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************"
save_path = "C:/downloads/myfile.txt"

try:
    response = requests.get(file_url, stream=True)
    if response.status_code == 200:
        with open(save_path, 'wb') as f:
            for chunk in response.iter_content(4096):
                f.write(chunk)
        print("Download completed!")
    else:
        print(f"No file to download. Server replied HTTP code: {response.status_code}")
except Exception as e:
    print("Error during download:", e)

package main

import (
    "io"
    "net/http"
    "os"
)

func main() {
    fileURL := "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************"
    savePath := "C:/downloads/myfile.txt"

    response, err := http.Get(fileURL)
    if err != nil {
        panic(err)
    }
    defer response.Body.Close()

    if response.StatusCode == http.StatusOK {
        outFile, err := os.Create(savePath)
        if err != nil {
            panic(err)
        }
        defer outFile.Close()

        _, err = io.Copy(outFile, response.Body)
        if err != nil {
            panic(err)
        }
        println("Download completed!")
    } else {
        println("No file to download. Server replied HTTP code:", response.StatusCode)
    }
}

const fileURL = "https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************";
const savePath = "C:/downloads/myfile.txt"; // 文件将在下载时使用的文件名

fetch(fileURL)
    .then(response => {
        if (!response.ok) {
            throw new Error(`Server replied HTTP code: ${response.status}`);
        }
        return response.blob(); // 将响应转换为 blob
    })
    .then(blob => {
        const link = document.createElement('a');
        link.href = window.URL.createObjectURL(blob);
        link.download = savePath; // 设置下载文件的名字
        document.body.appendChild(link); // 此步骤确保链接存在于文档中
        link.click(); // 模拟点击下载链接
        link.remove(); // 完成后移除链接
        console.log("Download completed!");
    })
    .catch(error => {
        console.error("Error during download:", error);
    });

import android.os.AsyncTask;
import android.os.Environment;
import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadTask extends AsyncTask<String, String, String> {
    @Override
    protected String doInBackground(String... params) {
        String fileURL = params[0];
        String savePath = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS) + "/myfile.txt"; // 修改后的保存路径
        try {
            URL url = new URL(fileURL);
            HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
            httpConn.setRequestMethod("GET");
            int responseCode = httpConn.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_OK) {
                InputStream inputStream = new BufferedInputStream(httpConn.getInputStream());
                FileOutputStream outputStream = new FileOutputStream(savePath);
                byte[] buffer = new byte[4096];
                int bytesRead;
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, bytesRead);
                }
                outputStream.close();
                inputStream.close();
                return "Download completed!";
            } else {
                return "No file to download. Server replied HTTP code: " + responseCode;
            }
        } catch (Exception e) {
            return "Error during download: " + e.getMessage();
        }
    }
}

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 定义文件 URL 和保存路径（修改为有效的路径）
        NSString *fileURL = @"https://examplebucket.oss-cn-hangzhou.aliyuncs.com/exampleobject.txt?x-oss-date=20241112T092756Z&x-oss-expires=3599&x-oss-signature-version=OSS4-HMAC-SHA256&x-oss-credential=LTAI****************/20241112/cn-hangzhou/oss/aliyun_v4_request&x-oss-signature=ed5a******************************************************";
        NSString *savePath = @"/Users/your_username/Desktop/myfile.txt"; // 请替换为您的用户名
        
        // 创建 URL 对象
        NSURL *url = [NSURL URLWithString:fileURL];
        
        // 创建下载任务
        NSURLSessionDataTask *task = [[NSURLSession sharedSession] dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            // 错误处理
            if (error) {
                NSLog(@"Error during download: %@", error.localizedDescription);
                return;
            }
            
            // 检查数据
            if (!data) {
                NSLog(@"No data received.");
                return;
            }
            
            // 保存文件
            NSError *writeError = nil;
            BOOL success = [data writeToURL:[NSURL fileURLWithPath:savePath] options:NSDataWritingAtomic error:&writeError];
            if (success) {
                NSLog(@"Download completed!");
            } else {
                NSLog(@"Error saving file: %@", writeError.localizedDescription);
            }
        }];
        
        // 启动任务
        [task resume];
        
        // 让主线程继续运行以便异步请求能够完成
        [[NSRunLoop currentRunLoop] run];
    }
    return 0;
}