Use Java SDK to perform streaming download of OSS files - Object Storage Service

When you handle large files or process data incrementally, you can use streaming download to read file content from OSS in chunks. This method avoids loading the entire file into memory at once, which improves the efficiency and performance of your program. This method is suitable for downloading files that exceed memory limits, processing data in real time to reduce memory usage, and retrieving data in steps over a network.

Precautions

In this topic, the public endpoint of the China (Hangzhou) region is used. To access OSS from other Alibaba Cloud services in the same region, use an internal endpoint. For details about supported regions and endpoints, see OSS regions and endpoints.
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 Configuration examples for common scenarios.
To perform a streaming download, you must have the oss:GetObject permission. For more information, see Attach a custom policy to a RAM user.

Sample code

The following sample code reads file content from OSS in chunks and stores the content in a byte array.

import com.aliyun.oss.ClientBuilderConfiguration;
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.common.comm.SignVersion;
import com.aliyun.oss.model.*;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;

public class Stream {
    public static void main(String[] args) throws Exception {
        // The endpoint of the China (Hangzhou) region is used as an example. Replace it with your actual endpoint. For more information about the endpoints of other regions, see Access domain names and data centers.
        String endpoint = "https://oss-cn-hangzhou.aliyuncs.com";
        // Obtain access credentials from environment variables. Before you run this code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are set.
        EnvironmentVariableCredentialsProvider credentialsProvider = CredentialsProviderFactory.newEnvironmentVariableCredentialsProvider();
        // Specify the bucket name. Example: examplebucket.
        String bucketName = "examplebucket";
        // Specify the full path of the object. Example: exampledir/exampleobject.txt. The full path cannot contain the bucket name.
        String objectName = "exampledir/exampleobject.txt";
        // Specify the region where the bucket is located. For example, if the bucket is in the China (Hangzhou) region, set the region to cn-hangzhou.
        String region = "cn-hangzhou";

        // Create an OSSClient instance.
        // When the OSSClient instance is no longer needed, call the shutdown method to release its resources.
        ClientBuilderConfiguration clientBuilderConfiguration = new ClientBuilderConfiguration();
        clientBuilderConfiguration.setSignatureVersion(SignVersion.V4);
        OSS ossClient = OSSClientBuilder.create()
                .endpoint(endpoint)
                .credentialsProvider(credentialsProvider)
                .clientConfiguration(clientBuilderConfiguration)
                .region(region)
                .build();

        try {
            // ossObject contains the bucket name, object name, object metadata, and an input stream.
            OSSObject ossObject = ossClient.getObject(bucketName, objectName);
            InputStream inputStream = ossObject.getObjectContent();
            ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
            // Read the file content into a byte array.
            byte[] readBuffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = inputStream.read(readBuffer)) != -1) {
                byteArrayOutputStream.write(readBuffer, 0, bytesRead);
            }
            // Get the final byte array.
            byte[] fileBytes = byteArrayOutputStream.toByteArray();
            // Print the length of the byte array.
            System.out.println("Downloaded file size: " + fileBytes.length + " bytes");
            // After the data is read, close the stream. Otherwise, connection leaks may occur, which can exhaust available connections and cause the program to fail.
            inputStream.close();
            byteArrayOutputStream.close();
            // After you are finished with the ossObject, you must close it. Otherwise, connection leaks may occur, which can exhaust available connections and cause the program to fail.
            ossObject.close();
        } 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();
            }
        }
    }
}

The following code downloads the streaming data of the exampleobject.txt file to examplefile.txt in the local D:\localpath directory.

import com.aliyun.oss.ClientBuilderConfiguration;
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.common.comm.SignVersion;
import com.aliyun.oss.model.OSSObject;

import java.io.*;

public class GetObjectStreamtoLocalFile {
    public static void main(String[] args) throws Exception {
        // The endpoint of the China (Hangzhou) region is used as an example. Replace it with your actual endpoint.
        String endpoint = "https://oss-cn-hangzhou.aliyuncs.com";
        // Obtain access credentials from environment variables.
        EnvironmentVariableCredentialsProvider credentialsProvider =
                CredentialsProviderFactory.newEnvironmentVariableCredentialsProvider();
        // Specify the bucket name. Example: examplebucket.
        String bucketName = "examplebucket";
        // Specify the full path of the object. Example: exampledir/exampleobject.txt.
        String objectName = "exampledir/exampleobject.txt";
        // Specify the region where the bucket is located. For example, if the bucket is in the China (Hangzhou) region, set the region to cn-hangzhou.
        String region = "cn-hangzhou";

        // Specify the local file path for saving.
        String localFilePath = "D:\\localpath\\examplefile.txt"; // Replace this with your local file path.

        // Create an OSSClient instance.
        // When the OSSClient instance is no longer needed, call the shutdown method to release its resources.
        ClientBuilderConfiguration clientBuilderConfiguration = new ClientBuilderConfiguration();
        clientBuilderConfiguration.setSignatureVersion(SignVersion.V4);
        OSS ossClient = OSSClientBuilder.create()
                .endpoint(endpoint)
                .credentialsProvider(credentialsProvider)
                .clientConfiguration(clientBuilderConfiguration)
                .region(region)
                .build();

        try {
            // Get the OSS object input stream.
            OSSObject ossObject = ossClient.getObject(bucketName, objectName);
            InputStream inputStream = ossObject.getObjectContent();

            // Create a local output stream.
            try (FileOutputStream fileOutputStream = new FileOutputStream(localFilePath)) {
                byte[] buffer = new byte[1024];
                int bytesRead;

                // Read in chunks and write to the local file.
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    fileOutputStream.write(buffer, 0, bytesRead);
                }

                System.out.println("Downloaded file saved to: " + localFilePath);
            }

            // Close the input stream.
            inputStream.close();
            // Close the OSSObject.
            ossObject.close();

        } 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();
            }
        }
    }
}

References

For the complete sample code for streaming download, see the GitHub example.
For more information about the API operation for streaming download, see getObject.