Download objects as files (OSS SDK for Go 1.0)

When you download a single object from a bucket, you can specify conditions based on the last modified time or the ETag of the object. The object is downloaded only if these conditions are met. Otherwise, an error is returned and the download operation is not triggered. This reduces unnecessary network transmission and resource consumption and improves the download efficiency.

The following table describes the available conditions that you can specify.

Condition	Description	Method
IfModifiedSince	If the specified time is earlier than the time when an object was last modified, the object can be downloaded. Otherwise, 304 Not Modified is returned.	oss.IfModifiedSince
IfUnmodifiedSince	If the specified time is later than or equal to the time when an object was last modified, the object can be downloaded. Otherwise, 412 Precondition Failed is returned.	oss.IfUnmodifiedSince
IfMatch	If the specified ETag matches that of an object, the object can be downloaded. Otherwise, 412 Precondition Failed is returned.	oss.IfMatch
IfNoneMatch	If the specified ETag does not match that of an object, the object can be downloaded. Otherwise, 304 Not Modified is returned.	oss.IfNoneMatch

The following sample code provides an example on how to perform conditional download:

package main

import (
    "log"
    "time"
    
    "github.com/aliyun/aliyun-oss-go-sdk/oss"
)

func main() {
    // Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. 
    provider, err := oss.NewEnvironmentVariableCredentialsProvider()
    if err != nil {
        log.Fatalf("Failed to create credentials provider: %v", err)
    }

    // Create an OSSClient instance. 
    // Specify the endpoint of the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com. Specify your actual endpoint. 
    // Specify the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the region to cn-hangzhou. Specify the actual region.
    clientOptions := []oss.ClientOption{oss.SetCredentialsProvider(&provider)}
    clientOptions = append(clientOptions, oss.Region("yourRegion"))
    // Specify the version of the signature algorithm.
    clientOptions = append(clientOptions, oss.AuthVersion(oss.AuthV4))
    client, err := oss.New("yourEndpoint", "", "", clientOptions...)
    if err != nil {
	    log.Fatalf("Failed to create OSS client: %v", err)
    }

    // Specify the name of the bucket. 
    bucketName := "yourBucketName" // Replace yourBucketName with the actual bucket name.
    bucket, err := client.Bucket(bucketName)
    if err != nil {
        log.Fatalf("Failed to get bucket: %v", err)
    }

    // For example, an object was last modified at 18:43:02, November 21, 2023. If the specified time is earlier than the last modified time, the object meets the If-Modified-Since condition and the object is downloaded. 
    date := time.Date(2023, time.November, 21, 18, 43, 2, 0, time.UTC)

    // Specify the full path of the object. Do not include the bucket name in the full path. 
    objectName := "yourObjectName" // Replace yourObjectName with the actual object name.
    localFilePath := "LocalFile" // Specify the full path of the local file in which you want to store the downloaded object.

    // The object is not downloaded if it does not meet the specified conditions. 
    err = bucket.GetObjectToFile(objectName, localFilePath, oss.IfUnmodifiedSince(date))
    if err == nil {
        log.Fatal("Expected an error when the condition is not met, but got nil")
    }

    // The object is downloaded if it meets the specified conditions. 
    err = bucket.GetObjectToFile(objectName, localFilePath, oss.IfModifiedSince(date))
    if err != nil {
        log.Fatalf("Failed to download file: %v", err)
    }

    log.Println("File has been downloaded successfully.")
}

When you download an object, you can use the progress bar to view the download progress in real time, which helps you check whether the download task is stuck after you wait for the object download for a long time.

The following sample code provides an example on how to display the progress of an object named exampleobject.txt that is being downloaded from a bucket named examplebucket:

package main

import (
	"log"
	"sync/atomic"

	"github.com/aliyun/aliyun-oss-go-sdk/oss"
)

// Specify the progress bar listener. 
type OssProgressListener struct {
	lastProgress int64
}

// Specify the function that is used to handle progress change events. 
func (listener *OssProgressListener) ProgressChanged(event *oss.ProgressEvent) {
	switch event.EventType {
	case oss.TransferStartedEvent:
		log.Printf("Transfer Started, ConsumedBytes: %d, TotalBytes %d.\n",
			event.ConsumedBytes, event.TotalBytes)
	case oss.TransferDataEvent:
		if event.TotalBytes != 0 {
			progress := int64(event.ConsumedBytes * 100 / event.TotalBytes)
			if progress > atomic.LoadInt64(&listener.lastProgress) {
				atomic.StoreInt64(&listener.lastProgress, progress)
				log.Printf("\rTransfer Data, ConsumedBytes: %d, TotalBytes %d, %d%%.",
					event.ConsumedBytes, event.TotalBytes, progress)
			}
		}
	case oss.TransferCompletedEvent:
		log.Printf("\nTransfer Completed, ConsumedBytes: %d, TotalBytes %d.\n",
			event.ConsumedBytes, event.TotalBytes)
	case oss.TransferFailedEvent:
		log.Printf("\nTransfer Failed, ConsumedBytes: %d, TotalBytes %d.\n",
			event.ConsumedBytes, event.TotalBytes)
	default:
	}
}

func main() {
	// Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. 
	provider, err := oss.NewEnvironmentVariableCredentialsProvider()
	if err != nil {
		log.Fatalf("Error: %v", err)
	}

	// Create an OSSClient instance. 
	// Specify the endpoint of the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com. Specify your actual endpoint.
	// Specify the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the region to cn-hangzhou. Specify the actual region.
	clientOptions := []oss.ClientOption{oss.SetCredentialsProvider(&provider)}
	clientOptions = append(clientOptions, oss.Region("yourRegion"))
	// Specify the version of the signature algorithm.
	clientOptions = append(clientOptions, oss.AuthVersion(oss.AuthV4))
	client, err := oss.New("yourEndpoint", "", "", clientOptions...)
	if err != nil {
		log.Fatalf("Error: %v", err)
	}

	// Specify the name of the bucket. 
	bucketName := "examplebucket"
	// Specify the full path of the object. Do not include the bucket name in the full path. 
	objectName := "exampleobject.txt"
	// Specify the full path of the local file in which you want to store the downloaded object. If a file that has the same name already exists in the path, the downloaded object overwrites the file. If no file that has the same name exists in the path, the downloaded object is saved in the path. 
	// If you do not specify a local path for the downloaded object, the downloaded object is saved to the path of the project to which the sample program belongs. 
	localFile := "D:\\localpath\\examplefile.txt"

	// Query the name of the bucket. 
	bucket, err := client.Bucket(bucketName)
	if err != nil {
		log.Fatalf("Error: %v", err)
	}

	// Display the progress of the download task. 
	err = bucket.GetObjectToFile(objectName, localFile, oss.Progress(&OssProgressListener{}))
	if err != nil {
		log.Fatalf("Error: %v", err)
	}
	log.Println("Transfer Completed.")
}