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I am trying to understand SocketChannels, and NIO in general. I know how to work with regular sockets and how to make a simple thread-per-client server (using the regular blocking sockets).
So my questions:
A selectable channel for stream-oriented connecting sockets.. What does that mean?I have read the also this documentation, but somehow I am not getting it...
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A SocketChannel is a non-blocking way to read from sockets, so that you can have one thread communicate with a bunch of open connections at once.
Socket channels are safe for use by multiple concurrent threads. They support concurrent reading and writing, though at most one thread may be reading and at most one thread may be writing at any given time.
A selectable channel for stream-oriented connecting sockets. A socket channel is created by invoking one of the open methods of this class. It is not possible to create a channel for an arbitrary, pre-existing socket. A newly-created socket channel is open but not yet connected.
Normally, a server runs on a specific computer and has a socket that is bound to a specific port number. The server just waits, listening to the socket for a client to make a connection request.
A Socket is a blocking input/output device. It makes the Thread that is using it to block on reads and potentially also block on writes if the underlying buffer is full. Therefore, you have to create a bunch of different threads if your server has a bunch of open Sockets.
A SocketChannel is a non-blocking way to read from sockets, so that you can have one thread communicate with a bunch of open connections at once. This works by adding a bunch of SocketChannels to a Selector, then looping on the selector's select() method, which can notify you if sockets have been accepted, received data, or closed. This allows you to communicate with multiple clients in one thread and not have the overhead of multiple threads and synchronization.
Buffers are another feature of NIO that allows you to access the underlying data from reads and writes to avoid the overhead of copying data into new arrays.
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By now NIO is so old that few remember what Java was like before 1.4, which is what you need to know in order to understand the "why" of NIO.
In a nutshell, up to Java 1.3, all I/O was of the blocking type. And worse, there was no analog of the select() system call to multiplex I/O. As a result, a server implemented in Java had no choice but to employ a "one-thread-per-connection" service strategy.
The basic point of NIO, introduced in Java 1.4, was to make the functionality of traditional UNIX-style multiplexed non-blocking I/O available in Java. If you understand how to program with select() or poll() to detect I/O readiness on a set of file descriptors (sockets, usually), then you will find the services you need for that in NIO: you will use SocketChannels for non-blocking I/O endpoints, and Selectors for fdsets or pollfd arrays. Servers with threadpools, or with threads handling more than one connection each, now become possible. That's the "extra".
A Buffer is the kind of byte array you need for non-blocking socket I/O, especially on the output/write side. If only part of a buffer can be written immediately, with blocking I/O your thread will simply block until the entirety can be written. With non-blocking I/O, your thread gets a return value of how much was written, leaving it up to you to handle the left-over for the next round. A Buffer takes care of such mechanical details by explicitly implementing a producer/consumer pattern for filling and draining, it being understood that your threads and the JVM's kernel will not be in sync.
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