Object Storage Service:Conditional download (Android SDK)

Prerequisites

Before you begin, ensure that you have:

• An OSSClient instance initialized. For setup options, including custom domain names and Security Token Service (STS), see Initialization

Download conditions

Each condition is set as an HTTP request header. OSS evaluates the header against the stored object metadata before streaming the object body.

An object's ETag is a string that identifies a specific version of the object. Last-Modified is the timestamp of the object's most recent modification. Use ETag-based conditions (If-Match, If-None-Match) for version checks and timestamp-based conditions (If-Modified-Since, If-Unmodified-Since) for cache control.

If-Modified-Since and If-Unmodified-Since can be used together. If-Match and If-None-Match can also be used together. To get an object's ETag, call ossClient.getObjectMeta.

Download an object conditionally

The following example uses If-Modified-Since to download exampledir/exampleobject.txt only if it has been modified after the specified date. The other three headers are shown commented out — uncomment any of them to apply that condition instead.

// Bucket name and full object path (the path must not include the bucket name)
String bucketName = "examplebucket";
String objectKey = "exampledir/exampleobject.txt";

// Build the conditional request headers
Map<String, String> headers = new HashMap<>();

// Download only if the object was modified after this date; otherwise returns 304 Not Modified
headers.put(OSSHeaders.GET_OBJECT_IF_MODIFIED_SINCE, "Fri, 13 Nov 2015 14:47:53 GMT");

// Download only if the object was NOT modified after this date; otherwise returns 412 Precondition Failed
// headers.put(OSSHeaders.GET_OBJECT_IF_UNMODIFIED_SINCE, "Fri, 13 Nov 2015 14:47:53 GMT");

// Download only if the object's ETag matches; otherwise returns 412 Precondition Failed
// headers.put(OSSHeaders.GET_OBJECT_IF_MATCH, "5B3C1A2E0563E1B002CC607C*****");

// Download only if the object's ETag does not match; otherwise returns 304 Not Modified
// headers.put(OSSHeaders.GET_OBJECT_IF_NONE_MATCH, "5B3C1A2E0563E1B002CC607C*****");

GetObjectRequest get = new GetObjectRequest(bucketName, objectKey);
get.setRequestHeaders(headers);

OSSAsyncTask task = oss.asyncGetObject(get, new OSSCompletedCallback<GetObjectRequest, GetObjectResult>() {
    @Override
    public void onSuccess(GetObjectRequest request, GetObjectResult result) {
        InputStream inputStream = result.getObjectContent();
        byte[] buffer = new byte[2048];
        int len;
        try {
            while ((len = inputStream.read(buffer)) != -1) {
                // Process the downloaded data
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onFailure(GetObjectRequest request, ClientException clientExcepion, ServiceException serviceException) {
        if (clientExcepion != null) {
            // Client-side error, such as a network failure
            clientExcepion.printStackTrace();
        }
        if (serviceException != null) {
            // Server-side error returned by OSS
            Log.e("ErrorCode", serviceException.getErrorCode());
            Log.e("RequestId", serviceException.getRequestId());
            Log.e("HostId", serviceException.getHostId());
            Log.e("RawMessage", serviceException.getRawMessage());
        }
    }
});

What's next

• Full test code: OSSGetObjectTest.java on GitHub

• Underlying API: GetObject

• OSSClient setup: Initialization