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Non Blocking IO in Java

Non-blocking I/O

https://en.wikipedia.org/wiki/Non-blockingI/O(Java)

The APIs of NIO were designed to provide access to the low-level I/O operations of modern OS

NIO

https://docs.oracle.com/javase/10/docs/api/java/nio/package-summary.html

  • New Input Output API package
  • java.nio : buffer class
  • java.nio.channels : file channel, tcp channel, udp channel
  • channel (양방향) ↔  stream (단방향-기존 IO)
  • 동기, 비동기 모두 지원
  • 블로킹, 넌블로킹 모두 지원

Non Blocking operation

  • connect(), accept(), read(), write() 즉시 리턴됨으로 작업 준비가 된 채널만 선택해서 처리해야함

  • Selector 필요 → Channel 작업 처리가 준비된 채널 선택

    • Selector : event listener
    • channel 작업 처리 준비 → selector 에 통보
    • selector → 통보받은 channel 선택
  • 작업 스레드가 블로킹되지 않기 때문에 selector가 선택한 채널을 메소드 호출 후 리턴한 하나의 스레드에서 다시 처리함
  • 스레드풀을 사용하는 경우, 적은 수의 스레드로 많은 양의 작업 처리

Selector operation

  1. 채널은 자신의 작업 유형을 키(Selection Key)로 생성

    selector.register() → selectionKey 생성한 후 관심키셋에 저장

  2. register the selection key to selector’s interest-set
  3. selector detects key’s ready channel in interest-set, update it to selected-key set

  4. working threads operate the key in selected-key set (작업 스레드가 채널 작업을 처리할 때 SelectionKey필요)

    → with SelectionKey, 작업유형 변경, 첨부 객체 저장, 채널 등록 취소

    selector

SelectionKey

  • selector.register() 로 생성됨
  • 작업 유형 정보를 가짐(읽기, 쓰기,)
  • 작업 스레드가 채널작업을 할 때 필요로함
  • 채널의 keyFor() 메소드로 얻을 수 있음

Selector creation

try {
	Selector selector = Selector.open();
} catch (IOException e)

Selector registration

// 어떤 작업에 관심을 둘지 ops
SelectionKey selectionKey = serverSocketChannel.register(Selectot sel, int ops);
SelectionKey selectionKey = socketChannel.register(Selector sel, int ops);

IO Stream Blocking

  • input stream → read() → block thread before the data input
  • output stream → write() → block thread before the data out
  • no interrupt allowed
  • large and sequential data

Selectors

  • provides a mechanism for waiting on channels and recognizing when one or more become available for data transfer.
  • When a number of channels are registered with the selector, it enables blocking of the program flow until at least one channel is ready for use, or until an interruption condition occurs. → select() returns
  • selector.wakeup(): 작업유형을 변경하고(selectionKey) selector 에 알려주기 위해 사용
  • Although this multiplexing behavior could be implemented with threads, the selector can provide a significantly more efficient implementation using lower-level operating system constructs.
  • multiplexor of SelectableChannel objects
  • created by invoking open method(use the system’s default selector provider to create a new selector)
  • remains open until close method

  • three sets of selection keys

    • key set : the current registrations of this selector returned by key method

      added to a selectors’ key set as a side effect of registering a channel via the channel’s register method

    • selected-key set : each key’s channel was detected to be ready for at least one of the operations identified in the key’s interest set during a prior selection operation that adds keys or updates keys in the set returned by selectedKeys method

      removed by invoking the remove method

    • cancelled-key : the set of keys that have been cancelled but whose channels have not yet been deregistered.

      added to selector’s cancelled-key set when it is canceled by closing its channel or invoking its cancel method

Channel Operation in working threads

  1. selected-key set 에서 selectionKey를 하나씩 꺼내 작업 유형 확인

  2. 작업 유형별(the selectionKey)로 채널 객체 얻기 

    ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();

    selectionKey 의 첨부객체를 얻기도 함

Reference

Published May 7, 2022

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