Download an object by using a presigned URL (C#) - Object Storage Service

By default, objects in an Object Storage Service (OSS) bucket are private and can be accessed only by the object owner. This topic describes how to use the OSS C# SDK to generate a presigned URL for a GET request. The URL contains an expiration time to grant temporary download access to other users. The URL can be accessed multiple times before it expires. After the URL expires, you must generate a new one.

Precautions

The sample code in this topic uses the China (Hangzhou) region as an example. The region ID is cn-hangzhou. By default, the public endpoint is used. If you want to access OSS from other Alibaba Cloud services in the same region, use an internal endpoint. For more information about the regions and endpoints that OSS supports, see Regions and endpoints.
You do not need specific permissions to generate a presigned URL. However, to allow others to use the presigned URL to download an object, you must have the oss:GetObject permission. For more information, see Common examples of RAM policies.

Process

The following flowchart shows how to download an object by using a presigned URL.

Sample code

The object owner generates a presigned URL for a GET request.

using OSS = AlibabaCloud.OSS.V2;  // Create an alias for Alibaba Cloud OSS SDK to simplify subsequent use.

var region = "cn-hangzhou";  // Required. Specify the region where the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the region to cn-hangzhou.
var endpoint = null as string;  // Optional. Specify the endpoint used to access OSS. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com.
var bucket = "your bucket name";  // Required. The bucket name.
var key = "your object name";  // Required. The name of the object for which you want to generate a presigned URL.

// Load the default configurations of the OSS SDK. The configurations automatically read credential information (such as an AccessKey) from environment variables.
var cfg = OSS.Configuration.LoadDefault();
// Explicitly set the use of environment variables to obtain credentials for identity verification (format: OSS_ACCESS_KEY_ID, OSS_ACCESS_KEY_SECRET).
cfg.CredentialsProvider = new OSS.Credentials.EnvironmentVariableCredentialsProvider();
// Set the region of the bucket in the configuration.
cfg.Region = region;   
// If an endpoint is specified, it overwrites the default endpoint. 
if(endpoint != null) 
{
    cfg.Endpoint = endpoint;
} 

// Create an OSS client instance using the configuration information.
using var client = new OSS.Client(cfg); 

// Call the Presign method to generate a presigned URL.
var result = client.Presign(new OSS.Models.GetObjectRequest()
{
    Bucket = bucket,
    Key = key,
});

using var hc = new HttpClient();  // Use HttpClient to send an HTTP request to obtain the object content.
var httpResult = await hc.GetAsync(result.Url);  // Send a GET request to the presigned URL.
var stream = await httpResult.Content.ReadAsStreamAsync();  // Read the response content as a stream. 
using var reader = new StreamReader(stream);  // Use StreamReader to convert the stream to a string.
var got = await reader.ReadToEndAsync();

// Print a message indicating that the operation is complete.
Console.WriteLine("GetObject done");
Console.WriteLine($"StatusCode: {httpResult.StatusCode}");  //The HTTP status code of the response.

Other users download the object using the presigned URL for the GET request.

curl

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******************************************************"

Java

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) {
        // Specify the presigned URL that allows HTTP GET requests. 
        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******************************************************";
        // Specify the path in which the downloaded object is stored, including the object name and extension. 
        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");

        // Specify the response code.
        int responseCode = httpConn.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            // Configure the input stream.
            InputStream inputStream = new BufferedInputStream(httpConn.getInputStream());
            // Configure the output stream.
            FileOutputStream outputStream = new FileOutputStream(savePath);

            byte[] buffer=new byte[4096]; // Specify the size of the buffer.
            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();
    }
}

Node.js

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);
});

Python

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)

Go

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)
    }
}

JavaScript

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"; // Specify the name of the downloaded object.

fetch(fileURL)
    .then(response => {
        if (!response.ok) {
            throw new Error(`Server replied HTTP code: ${response.status}`);
        }
        return response.blob(); // Change the type of the response to blob.
    })
    .then(blob => {
        const link = document.createElement('a');
        link.href = window.URL.createObjectURL(blob);
        link.download=savePath; // Specify the name of the downloaded object.
        document.body.appendChild(link); // This step ensures that the presigned URL exists in the document.
        link.click(); // Click the presigned URL to simulate the object download.
        link.remove(); // Remove the presigned URL after the object is downloaded.
        console.log("Download completed!");
    })
    .catch(error => {
        console.error("Error during download:", error);
    });

Android-Java

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"; // Specify the path in which you want to store the downloaded object.
        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();
        }
    }
}

Objective-C

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Specify the presigned URL and the path in which you want to store the object.
        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"; // Replace your_username with your username.
        
        // Create a URL object.
        NSURL *url = [NSURL URLWithString:fileURL];
        
        // Create an object download task.
        NSURLSessionDataTask *task = [[NSURLSession sharedSession] dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            // Handle errors.
            if (error) {
                NSLog(@"Error during download: %@", error.localizedDescription);
                return;
            }
            
            // Check the data in the object.
            if (!data) {
                NSLog(@"No data received.");
                return;
            }
            
            // Save the object.
            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);
            }
        }];
        
        // Start the object download task.
        [task resume];
        
        // Continue to run the main thread to complete the asynchronous request.
        [[NSRunLoop currentRunLoop] run];
    }
    return 0;
}

References

For the complete sample code that is used to download an object using a presigned URL, see PresignGetObject.cs.