要恢复下载，您不仅需要发送 Range 请求头，还有 If-Range 请求标头，其中应包含唯一文件标识符或文件修改时间戳.

To resume a download, you need to send not only the Range request header, but also the If-Range request header which should contain either the unique file identifier or the file modification timestamp.

如果服务器返回 ETag 初始下载的响应头，那么您应该在后续恢复请求的 If-Range 头中使用它.或者如果它返回 Last-Modified 响应头，那么你应该在 If-Range 请求头中使用它.

If the server returns an ETag response header on the initial download, then you should use it in the If-Range header of the subsequent resume requests. Or if it returns a Last-Modified response header, then you should use it in the If-Range request header instead.

查看您的日志，服务器发送了一个 Last-Modified 响应标头.因此，您应该在恢复请求的 If-Range 标头中将其发送回.

Looking at your logs, the server has sent a Last-Modified response header. So you should send it back along in an If-Range header of the resume request.

// Initial download.
String lastModified = connection.getHeaderField("Last-Modified");

// ...

// Resume download.
connection.setRequestProperty("If-Range", lastModified); 

服务器将使用此信息来验证您请求的文件是否完全相同.

The server will use this information to verify if you're requesting exactly the same file.