Object Storage Service:Download objects as files (OSS SDK for Go 1.0)

Ao baixar um único objeto de um bucket, especifique condições com base na hora da última modificação ou no ETag do objeto. O sistema baixa o objeto apenas se essas condições forem atendidas. Caso contrário, o sistema retorna um erro e não aciona a operação de download. Isso reduz a transmissão de rede e o consumo desnecessário de recursos, além de melhorar a eficiência do download.

A tabela a seguir descreve as condições disponíveis.

Condição	Descrição	Método
IfModifiedSince	Se a hora especificada for anterior à hora da última modificação de um objeto, o objeto é baixado. Caso contrário, o sistema retorna 304 Not Modified.	oss.IfModifiedSince
IfUnmodifiedSince	Se a hora especificada for posterior ou igual à hora da última modificação de um objeto, o objeto é baixado. Caso contrário, o sistema retorna 412 Precondition Failed.	oss.IfUnmodifiedSince
IfMatch	Se o ETag especificado corresponder ao de um objeto, o objeto é baixado. Caso contrário, o sistema retorna 412 Precondition Failed.	oss.IfMatch
IfNoneMatch	Se o ETag especificado não corresponder ao de um objeto, o objeto é baixado. Caso contrário, o sistema retorna 304 Not Modified.	oss.IfNoneMatch

O código de exemplo a seguir demonstra como baixar um objeto condicionalmente:

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.")
}

Ao baixar um objeto, use a barra de progresso para visualizar o progresso do download em tempo real. Isso permite verificar se a tarefa de download travou durante uma longa espera.

O código de exemplo a seguir demonstra como exibir o progresso durante o download do objeto exampleobject.txt de um bucket chamado 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.")
}