文件下载

您可以通过指定桶名和对象名调用getObject接口进行文件下载。AWS S3 Java SDK提供了如下下载方式：

• 流式下载
• 下载到本地文件
• Range下载
• 指定If-Modified-Since进行下载

也是可以直接使用加密client进行替换，除了StrictAuthenticatedEncryption不支持range

流式下载

S3Object nosObject = AmazonS3.getObject("your-bucketname","your-objectname");
//可以通过NOSObject对象的getObjectMetadata方法获取对象的ContentType等元数据信息
String contentType = nosObject.getObjectMetadata().getContentType();
//流式获取文件内容
InputStream in = nosObject.getObjectContent();
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
while (true) {
  String line;
  try {
    line = reader.readLine();
    if (line == null) break;
    System.out.println("\n" + line);
  } catch (IOException e) {
    e.printStackTrace();
  }
}
try {
  reader.close();
} catch (IOException e) {
  e.printStackTrace();
}

下载到本地文件

你可以通过调用AWS S3 Java SDK的getObject接口直接将NOS中的对象下载到本地文件

String destinationFile = "your-local-filepath";
GetObjectRequest getObjectRequest = new GetObjectRequest("your-bucketname","your-objectname");
ObjectMetadata objectMetadata = s3Client.getObject(getObjectRequest, new File(destinationFile));

Range下载

AWS S3 Java SDK支持范围(Range)下载，即下载指定对象的指定范围的数据。

GetObjectRequest getObjectRequest = new GetObjectRequest("your-bucketname","your-objectname");
getObjectRequest.setRange(0, 100);
S3Object nosObject = s3Client.getObject(getObjectRequest);
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
while (true) {
  String line;
  try {
    line = reader.readLine();
    if (line == null) break;
      System.out.println("\n" + line);
  } catch (IOException e) {
    e.printStackTrace();
  }
}
try {
  reader.close();
} catch (IOException e) {
  e.printStackTrace();
}

指定If-Modified-Since进行下载

下载文件时，您可以指定If-Modified-Since参数，满足文件的最后修改时间小于等于If-Modified-Since参数指定的时间，则不进行下载，否则正常下载文件。

//假设需要下载的文件的最后修改时间为: Date lastModified;
//lastModified小于等于指定If-Modified-Since参数
GetObjectRequest getObjectRequest = new GetObjectRequest("your-bucketname","your-objectname");
Date afterTime = new Date(lastModified.getTime() + 1000);
getObjectRequest.setModifiedSinceConstraint(afterTime);
//此时nosObject等于null
S3Object s3Object = client.getObject(getObjectRequest);
Date beforeTime = new Date(lastModified.getTime() -1000);
getObjectRequest.setModifiedSinceConstraint(beforeTime);
//此时s3Object不等于null，可以正常获取文件内容
s3Object = client.getObject(getObjectRequest);
BufferedReader reader = new BufferedReader(new InputStreamReader(s3Object.getObjectContent()));
while (true) {
  String line;
  try {
    line = reader.readLine();
    if (line == null) break;
      System.out.println("\n" + line);
  } catch (IOException e) {
    e.printStackTrace();
  }
}
try {
  reader.close();
} catch (IOException e) {
  e.printStackTrace();
}