You may fail to download a large object if the network is unstable or other exceptions occur. In some cases, you may fail to download the object even after multiple attempts. To resolve this issue, Object Storage Service (OSS) provides the resumable download feature. In resumable download, OSS splits an object into multiple parts and downloads each part separately. After all parts are downloaded, OSS combines the parts into a complete object.
Usage notes
In this topic, the public endpoint of the China (Hangzhou) region is used. If you want to access OSS from other Alibaba Cloud services in the same region as OSS, use an internal endpoint. For more information about OSS regions and endpoints, see Regions, endpoints and open ports.
In this topic, access credentials are obtained from environment variables. For more information about how to configure access credentials, see Configure access credentials.
In this topic, an OSSClient instance is created by using an OSS endpoint. If you want to create an OSSClient instance by using custom domain names or Security Token Service (STS), see Create an OSSClient instance.
To use resumable download, you must have the
oss:GetObjectpermission. For more information, see Attach a custom policy to a RAM user.
Examples
The following sample code provides an example on how to perform resumable download:
import com.aliyun.oss.OSS;
import com.aliyun.oss.common.auth.*;
import com.aliyun.oss.OSSClientBuilder;
import com.aliyun.oss.OSSException;
import com.aliyun.oss.model.*;
public class Demo {
public static void main(String[] args) throws Exception {
// In this example, the endpoint of the China (Hangzhou) region is used. Specify your actual endpoint.
String endpoint = "https://oss-cn-hangzhou.aliyuncs.com";
// 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.
EnvironmentVariableCredentialsProvider credentialsProvider = CredentialsProviderFactory.newEnvironmentVariableCredentialsProvider();
// Specify the name of the bucket. Example: examplebucket.
String bucketName = "examplebucket";
// Specify the full path of the object. Example: exampledir/exampleobject.txt. Do not include the bucket name in the full path.
String objectName = "exampledir/exampleobject.txt";
// 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.
String region = "cn-hangzhou";
// Create an OSSClient instance.
ClientBuilderConfiguration clientBuilderConfiguration = new ClientBuilderConfiguration();
clientBuilderConfiguration.setSignatureVersion(SignVersion.V4);
OSS ossClient = OSSClientBuilder.create()
.endpoint(endpoint)
.credentialsProvider(credentialsProvider)
.clientConfiguration(clientBuilderConfiguration)
.region(region)
.build();
try {
// Perform resumable download in which 10 parts can be concurrently downloaded.
DownloadFileRequest downloadFileRequest = new DownloadFileRequest(bucketName, objectName);
// Specify the full path to which you want to download the object. Example: D:\\localpath\\examplefile.txt.
downloadFileRequest.setDownloadFile("D:\\localpath\\examplefile.txt");
// Specify the part size. Unit: bytes. Valid values: 100 KB to 5 GB. The default part size is 100 KB.
downloadFileRequest.setPartSize(1 * 1024 * 1024);
// Specify the number of concurrent threads for the resumable download task. Default value: 1.
downloadFileRequest.setTaskNum(10);
// Specify whether to enable resumable download. By default, resumable download is disabled.
downloadFileRequest.setEnableCheckpoint(true);
// Specify the full path of the checkpoint file. Example: D:\\localpath\\examplefile.txt.dcp.
// The checkpoint file is generated when the download is interrupted. If you want to resume the download task, you must specify the full path of the checkpoint file. After the object is downloaded, the checkpoint file is deleted.
//downloadFileRequest.setCheckpointFile("D:\\localpath\\examplefile.txt.dcp");
// Download the object.
DownloadFileResult downloadRes = ossClient.downloadFile(downloadFileRequest);
// After the object is downloaded, the object metadata is returned.
ObjectMetadata objectMetadata = downloadRes.getObjectMetadata();
System.out.println(objectMetadata.getETag());
System.out.println(objectMetadata.getLastModified());
System.out.println(objectMetadata.getUserMetadata().get("meta"));
} catch (OSSException oe) {
System.out.println("Caught an OSSException, which means your request made it to OSS, "
+ "but was rejected with an error response for some reason.");
System.out.println("Error Message:" + oe.getErrorMessage());
System.out.println("Error Code:" + oe.getErrorCode());
System.out.println("Request ID:" + oe.getRequestId());
System.out.println("Host ID:" + oe.getHostId());
} catch (Throwable ce) {
System.out.println("Caught an ClientException, which means the client encountered "
+ "a serious internal problem while trying to communicate with OSS, "
+ "such as not being able to access the network.");
System.out.println("Error Message:" + ce.getMessage());
} finally {
if (ossClient != null) {
ossClient.shutdown();
}
}
}
}