When downloading objects, you can specify one or more constraints. If the specified conditions are met, the download will proceed; otherwise, an error will be returned and the download will not occur.

Specification of Constraints

The following are the specific conditions under which TOS support is provided.

Parameters

Description

Parameters	Description
If-Modified-Since	If the specified time is earlier than the object's last modification time, the download proceeds normally; otherwise, a 304 error (Not Modified) is returned.
If-Unmodified-Since	If the specified time is equal to or later than the object's last modification time, the download proceeds normally; otherwise, a 412 error (Precondition Failed) is returned.
If-Match	If the specified ETag matches the ETag of the object to be downloaded, the download proceeds normally; otherwise, a 412 error (Precondition Failed) is returned.
If-None-Match	If the specified ETag does not match the ETag of the object to be downloaded, the download proceeds normally; otherwise, a 304 error (Not Modified) is returned.

Sample Code

The following code demonstrates how to set constraints for object download.

import android.os.Bundle;​
import android.util.Log;​
​
import androidx.appcompat.app.AppCompatActivity;​
​
import com.volcengine.tos.TOSV2;​
import com.volcengine.tos.TOSV2ClientBuilder;​
import com.volcengine.tos.TosException;​
import com.volcengine.tos.model.object.GetObjectV2Input;​
import com.volcengine.tos.model.object.GetObjectV2Output;​
import com.volcengine.tos.model.object.ObjectMetaRequestOptions;​
​
import java.io.FileOutputStream;​
import java.io.IOException;​
import java.text.DateFormat;​
import java.text.ParseException;​
import java.text.SimpleDateFormat;​
import java.util.Date;​
​
public class GetObjectWithIfConditionExample extends AppCompatActivity {​
​
    @Override​
    protected void onCreate(Bundle savedInstanceState) {​
        String endpoint = "your endpoint";​
        String region = "your region";​
        String accessKey = "your access key";​
        String secretKey = "your secret key";​
        String securityToken = "your security token";​
​
        String bucketName = "your bucket name";​
        String objectKey = "your object key";​
        String filePath = "your file path";​
​
        super.onCreate(savedInstanceState);​
        setContentView(R.layout.activity_display_message);​
        TOSV2 tos = new TOSV2ClientBuilder().build(region, endpoint, accessKey, secretKey, securityToken);​
​
        Thread tosThread = new Thread(new Runnable() {​
            @Override​
            public void run() {​
                try{​
                    ​
                    DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");​
                    Date date = dateFormat.parse("2022-09-13 14:36:01");​
                    ObjectMetaRequestOptions options = new ObjectMetaRequestOptions().setIfModifiedSince(date);​
                    GetObjectV2Input input = new GetObjectV2Input().setBucket(bucketName).setKey(objectKey).setOptions(options);​
                    try(FileOutputStream fileOutputStream = new FileOutputStream(filePath);​
                        GetObjectV2Output output = tos.getObject(input)) {​
                        byte[] buffer = new byte[1024];​
                        int length;​
                        while ((length = output.getContent().read(buffer)) != -1) {​
                            fileOutputStream.write(buffer, 0, length);​
                        }​
                        Log.i("getObject", "getObject succeed, object's metadata is " + output.getGetObjectBasicOutput());​
                    } catch (IOException e) {​
                        Log.e("IOException", "write data to file failed");​
                        e.printStackTrace();​
                    }​
                } catch (TosException e) {​
                    ​
                    if (e.getStatusCode() == 304) {​
                        Log.e("TosException", "server return 304 not modified");​
                    } else if (e.getStatusCode() == 412) {​
                        ​
                        Log.e("TosException", "server return 412 precondition failed.");​
                    } else if (e.getStatusCode() == 404) {​
                       ​
                        Log.e("TosException", "the object you want to download is not found");​
                    } else {​
                        Log.e("TosException", "getObject failed");​
                    }​
                    e.printStackTrace();​
                } catch (ParseException e) {​
                    Log.e("ParseException", "getObject failed");​
                    e.printStackTrace();​
                }​
            }​
        });​
​
        tosThread.start();​
    }​
}​

Relevant Documentation

For the API documentation on downloading objects, see GetObject.

Last updated: 2025.09.25 14:23:26

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