Asynchronous network io

Question

I read java doc (http://java.sun.com/javase/6/docs/api/java/nio/channels/SocketChannel.html) saying that e.g. socketchannel is non-blocking. After searching on the internet, some say java nio is still a based on syncrhonous style. If one needs asynchronous io, additonal frameworks e.g. netty, mina, etc are required; or one has to wait the relaased of jdk7.
I am confused because, from the javadoc it seems that some java nio methods can use asynchronous operation. Isn't this asynchronous approach? Or is there any resrouce/ doc detailing about this?
Many thanks

Answer 1

I am learning to build a distributed system which has assumption with asychronous networking. And I read some articles on the internet saying that though nio (e.g. SocketChannel) is non-blocking, somehow it is still synchronous.There are three cases:
1. Blocking I/O (java.net or blocking java.nio): if you read and the socket receive buffer is empty, you block until some data arrives; if you write and the socket send buffer is full, you block until some room is created (by the TCP stack writing out to the network).
2. Non-blocking I/O (java.nio in non-blocking mode): if you read and the socket buffer is empty, it returns zero immediately, otherwise you block for a short period of time while data is shifted from the socket receive buffer into your application; similarly, if you write and the socket send buffer is full, it returns zero immediately, otherwise you block for a short period of time while data is shifted from your application into the socket send buffer.
3. Asynchronous I/O (JDK 7): whether you read or write, the method returns immediately under all circumstances to your code and you are given a Future object via which you can interrogate status later on..
You can see that for your purposes (2) and (3) are both 'asynchronous' in your sense.
What happens on the network is always asynchronous.
My understanding is that in asynchronous networking enviroment, a node P1 may send a message to another node P2, and perhaps the network connection may be very slow or even disconnect, but nodes P1 and P2 are still alive. In this case, if using synchronous api, P1 may assume that P2 crashes (but P2 does not crash), which is not correct.The only way that can happen via TCP is if you set a read timeout or the keepalive feature. Which of the 3 APIs you use makes no difference to this.
Using asynchronous api would prevent such issueNo it wouldn't.