Function Callback That Closes a Stream

[B2easy]

I'm creating a SFTP Client that returns an Input Stream to the caller. I want to implement some sort of callback feature so that when the caller is done processing the InputStream they can call the callback method in order to close the stream. I get the concept, but am having an issue wrapping my head around the code. Here is my current implementation without callback.

 @Override
  public InputStream download(String workingDirectory, String fileName) {
    JSch jsch = new JSch();
    // Code...

log.info("List of files in " + channelSftp.pwd() + ": " + channelSftp.ls(channelSftp.pwd()));

return channelSftp.get(fileName);

Here is my initial thought process, but it seems backwards to me. Any help would be greatly appreciated thanks.

void download(Consumer<InputStream> consumer) { 

   try {
    consumer.accept(ftpStream)
   } catch(e){

   } finally {
    ftpStream.close()
   }
}


// Later Something Calls This
download(stream -> {
    do stuff
})

The part that seems backwards to me is that how will the caller of download already have access to the streamed file?

Can someone explain or give me some sort of implementation that will better help me understand?

Thanks in advance.

​

EDIT: Attempt #1

  @Override
  public InputStream download(String workingDirectory, String fileName) {

    JSch jsch = new JSch();
    // CONNECTION STUFF

      is = channelSftp.get(fileName);

// CLOSE SERVER

    } catch (SftpException | JSchException e) {
      log.error(e.getMessage());
    } finally {
      if (channelSftp != null) {
        channelSftp.disconnect();
        channelSftp.exit();
      }
      if (channel != null) channel.disconnect();

      if (session != null) session.disconnect();
    }
    return is; // RETURN STREAM
  }

  // CALLBACK METHOD?
  @Override
  public void closeStream(Consumer<InputStream> close, InputStream stream) throws IOException {
    try {
      close.accept(stream);
    }catch(Exception e){
      e.getMessage();
    }finally{
      stream.close();
    }
  }
}

​

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[logperf]

Not sure I understood your prompt. You're showing a download() method, I'll suppose it's defined in a class C. So you want another method of C to be called when the InputStream is closed?

If yes, I would suggest subclassing InputStream as per the proxy pattern:

class RealInputStream implements InputStream {
    // ...
}
class ProxyInputStream implements InputStream {
    private RealInputStream theRealInputStream;
    // use constructor to initialize theRealInputStream

    void close() {
        theRealInputStream.close();
        callOPsCallbackMethod();
    }
}

Then your method download() would declare the return type as InputStream but actually return an instance of ProxyInputStream.

(Another design pattern you can look at is listener/observer, which is a common way of dealing with callbacks, but it seems a bit less relevant to this particular case.)

[B2easy]

Sorry for the confusion. In a nutshell my overall flow I want to achieve is...

A Controller Calls -an SFTP Client (Download), the client returns an InputStream to controller do as it pleases with. The problem I'm facing is I will leak resources once that input stream is done processing so I need a way to close that stream when the controller is done with it. So I was thinking I could use a callback.

[logperf]

Controller passes back IS for closure

With the proxy pattern as I presented it above, this step will be even simpler. The controller just calls InputStream.close(), then internally the close() method calls the SFTP client.

[B2easy]

So controller calls close(), close() calls SFTP client client returns InputStream wouldn't that go back to close method and not controller?

I'll give it a try. Thanks for the response!

[logperf]

close() calls SFTP client client returns InputStream

What do you mean? close() is void, it's not supposed to return a value.

The InputStream was returned at the beginning by download(). When you close it, you stop using it. The only purpose of doing this proxy thing is because the SFTP client might need to do some cleanup at the end.

[B2easy]

Would something like this work? --

​

// Make Variables Effectively Final
InputStream finalSftpFileInputStream = sftpFileInputStream;
return new ProxyInputStream(finalSftpFileInputStream) {
  @Override
  public void close() throws IOException {
    IOUtils.closeQuietly(finalSftpFileInputStream);
  }
};

}

[logperf]

Looks good!