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Download a blob with Java

  • Article

This article shows how to download a blob using the Azure Storage client library for Java. You can download blob data to various destinations, including a local file path, stream, or text string. You can also open a blob stream and read from it.

Prerequisites

  • This article assumes you already have a project set up to work with the Azure Blob Storage client library for Java. To learn about setting up your project, including package installation, adding import directives, and creating an authorized client object, see Get Started with Azure Storage and Java.
  • The authorization mechanism must have permissions to perform a download operation. To learn more, see the authorization guidance for the following REST API operation:

Download a blob

You can use any of the following methods to download a blob:

Download to a file path

The following example downloads a blob to a local file path:

public void downloadBlobToFile(BlobClient blobClient) {
    blobClient.downloadToFile("filepath/local-file.png");
}

Download to a stream

The following example downloads a blob to an OutputStream object:

public void downloadBlobToStream(BlobClient blobClient) {
    try (ByteArrayOutputStream outputStream = new ByteArrayOutputStream()) {
        blobClient.downloadStream(outputStream);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Download to a string

The following example assumes that the blob is a text file, and downloads the blob to a String object:

public void downloadBlobToText(BlobClient blobClient) {
    String content = blobClient.downloadContent().toString();
    System.out.printf("Blob contents: %s%n", content);
}

Download from a stream

The following example downloads a blob by opening a BlobInputStream and reading from the stream:

public void readBlobFromStream(BlobClient blobClient) {
    // Opening a blob input stream allows you to read from a blob through a normal
    // stream interface

    try (BlobInputStream blobStream = blobClient.openInputStream()) {
        blobStream.read();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Download a block blob with configuration options

You can define client library configuration options when downloading a blob. These options can be tuned to improve performance and enhance reliability. The following code examples show how to use BlobDownloadToFileOptions to define configuration options when calling a download method.

Specify data transfer options on download

You can configure values in ParallelTransferOptions to improve performance for data transfer operations. The following values can be tuned for downloads based on the needs of your app:

  • blockSize: The maximum block size to transfer for each request. You can set this value by using the setBlockSizeLong method.
  • maxConcurrency: The maximum number of parallel requests issued at any given time as a part of a single parallel transfer. You can set this value by using the setMaxConcurrency method.

Add the following import directive to your file to use ParallelTransferOptions for a download:

import com.azure.storage.common.*;

The following code example shows how to set values for ParallelTransferOptions and include the options as part of a BlobDownloadToFileOptions instance. The values provided in this sample aren't intended to be a recommendation. To properly tune these values, you need to consider the specific needs of your app.

public void downloadBlobWithTransferOptions(BlobClient blobClient) {
    ParallelTransferOptions parallelTransferOptions = new ParallelTransferOptions()
            .setBlockSizeLong((long) (4 * 1024 * 1024)) // 4 MiB block size
            .setMaxConcurrency(2);

    BlobDownloadToFileOptions options = new BlobDownloadToFileOptions("<localFilePath>");
    options.setParallelTransferOptions(parallelTransferOptions);

    blobClient.downloadToFileWithResponse(options, null, null);
}

To learn more about tuning data transfer options, see Performance tuning for uploads and downloads with Java.

Resources

To learn more about how to download blobs using the Azure Blob Storage client library for Java, see the following resources.

REST API operations

The Azure SDK for Java contains libraries that build on top of the Azure REST API, allowing you to interact with REST API operations through familiar Java paradigms. The client library methods for downloading blobs use the following REST API operation:

Code samples

Client library resources