9， Java NIO SocketChannel

A Java NIO SocketChannel is a channel that is connected to a TCP network socket. It is Java NIO's equivalent of Java Networking's Sockets. There are two ways a  SocketChannel  can be created:

You open a  SocketChannel  and connect to a server somewhere on the internet.

A  SocketChannel  can be created when an incoming connection arrives at a  ServerSocketChannel .

Opening a SocketChannel
Here is how you open a  SocketChannel :

SocketChannel socketChannel = SocketChannel.open();
socketChannel.connect(new InetSocketAddress("http://jenkov.com", 80));

Closing a SocketChannel
You close a  SocketChannel  after use by calling the  SocketChannel.close()  method. Here is how that is done:

socketChannel.close();

Reading from a SocketChannel
To read data from a  SocketChannel  you call one of the  read()  methods. Here is an example:

ByteBuffer buf = ByteBuffer.allocate(48);

int bytesRead = socketChannel.read(buf);

First a  Buffer  is allocated. The data read from the  SocketChannel  is read into the  Buffer .
Second the  SocketChannel.read()  method is called. This method reads data from the  SocketChannel  into the Buffer . The  int  returned by the  read()  method tells how many bytes were witten into the  Buffer .
 If -1 is returned, the end-of-stream is reached (the connection is closed).
Writing to a SocketChannel
Writing data to a  SocketChannel  is done using the  SocketChannel.write()  method, which takes a  Buffer  as parameter. Here is an example:

String newData = "New String to write to file..." + System.currentTimeMillis();

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put(newData.getBytes());

buf.flip();

while(buf.hasRemaining()) {
    channel.write(buf);
}

Notice how the  SocketChannel.write()  method is called inside a while-loop. There is no guarantee of how many bytes the  write()  method writes to the  SocketChannel . Therefore we repeat the  write()  call until the  Buffer has no further bytes to write.
Non-blocking Mode
You can set a  SocketChannel  into non-blocking mode. When you do so, you can call  connect() ,  read()  and  write()  in asynchronous mode.
connect()
If the  SocketChannel  is in non-blocking mode, and you call  connect() , the method may return before a connection is established. To determine whether the connection is established, you can call the  finishConnect()  method, like this:

socketChannel.configureBlocking(false);
socketChannel.connect(new InetSocketAddress("http://jenkov.com", 80));

while(! socketChannel.finishConnect() ){
    //wait, or do something else...    
}

write()
In non-blocking mode the  write()  method may return without having written anything. Therefore you need to call the  write()  method in a loop. But, since this is already being done in the previous write examples, no need to do anything differently here.
read()
In non-blocking mode the  read()  method may return without having read any data at all. Therefore you need to pay attention to the returned  int , which tells how many bytes were read.
Non-blocking Mode with Selectors
The non-blocking mode of  SocketChannel 's works much better with  Selector 's. By registering one or more SocketChannel 's with a  Selector , you can ask the  Selector  for channels that are ready for reading, writing etc. How to use  Selector 's with  SocketChannel 's is explained in more detail in a later text in this tutorial.